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Transistor

The history of the transistor

Silicon and germanium

Doping

p-n junction diodes

Transistor action

Common base, common emitter, and common-collector configurations

Field-effect transistors (fets)

Integrated circuits

Application-specific integrated circuits

Complementary metal-oxide semiconductors

The significance and future of the transistor

Resources

A transistor is a small, solid device capable of amplifying electrical signals or switching them on and off. Most transistors are made using the unique semi-conducting properties of silicon or germanium (most often silicon). To make a transistor, a semiconducting crystal is selectively contaminated with other elements, such as arsenic, phosophorus, boron, or gallium.

A transistor is controlled by voltages communicated to it through three or more metallic contacts. Transistors are active devices, meaning that they must be supplied with power to function. Virtually all electronic devices contain transistors, from a handful to hundreds of millions. To the extent that our civilization has come to depend on computers and electronic communications, therefore, it depends on the transistor. The term transistor is a shortening of TRANSfer ResISTOR.

A transistor can perform a variety of useful electrical tasks because its resistance (the ease with which an electrical current flows through it) can be adjusted using a low-power control signal applied to one of a transistors three metallic terminals. The resulting change in resistance between the other two terminals of the transistorthrough one of which current enters the transistor, leaving through the otherchanges the current passed through the transistor. This current can in turn be converted into a voltage by passing it through a resistor (a passive or unpowered circuit device that simply dissipates energy); the change in voltage across this load resistor can be many times greater than the change in voltage that was used to alter the resistance of the transistor itself. This increase in amplitude or strength is called amplification, one of the most basic processes in electronics.

Transistor amplification can be compared to controlling a powerful flow of water through a pipe by turning a valve: in this analogy, the force applied to the valve represents the transistors control voltage, while the water flowing through the pipe represents its output current. A small, varying force applied to the valvea back-and-forth wigglecauses matched variations in the greater force carried by the water passing through the pipe; a small signal thus generates another that varies identically in time but is larger in amplitude. Vacuum tubes, which were developed before transistors but perform the same functions, are termed valves in British usage in reference to this analogy.

When a transistor acts as an amplifier it does not create the additional energy appearing in its output signal, just as a valve does not create water it dispenses. Rather, a transistor modulates the energy flowing from a battery or power supply in a way that is similar to a valve adjusting the flow rate from a source of pressurized water.

It is also clear from the valve analogy that instead of wiggling the valve one might choose instead to operate it in only two positions, open and shut, avoiding partial settings completely. Vacuum tubes and transistors can also be operated in this way, switching an electrical current on and off in response to a two-valued control signal. The on-off signals generated by this technique, switching, are the basis of digital electronics; the constantly-varying signals involved in amplification are the basis of analog electronics. Both rely on transistors, which are cheaper, smaller, and more reliable than other devices that can perform these functions.

The history of the transistor

Discovery of the transistor was publicly announced in 1948. Before this time, electronics had depended almost exclusively upon vacuum tubes for amplification and switching actions. Vacuum tubes are relatively bulky, short-lived, and wasteful of power; transistors are smallfrom peanut-size to only a few molecules acrosslong-lived, and dissipate far less power. Transistors are also resistant to mechanical shocks and can be manufactured by the millions on tiny semiconductor crystals (chips) using optical techniques. Transistors and related solid-state (i.e., entirely solid) devices have replaced vacuum tubes except for specialized applications, especially those involving high power.

Silicon and germanium

The first transistors were made from germanium, but now most transistors are made from silicon. Silicon (Si) and germanium (Ge) form similar crystal structures with similar physical properties, but silicon is preferred over germanium because of silicons superior thermal characteristics. Crystals of Si and Ge are neither good electrical insulators nor good electrical conductors, but conduct electricity at a level midway between metallic conductors (which have very low resistance to electric current) and nonmetallic insulators such as glass (which have a very high resistance to electric current). Transistor action is made possible by semiconduction.

Each atom in a silicon or germanium crystal lattice has four atoms as close neighbors. That is, each atom is held in its place in the crystals orderly structure because each atom shares its four outermost electrons with the outermost electrons of four nearby atoms. This sharing holds the atoms together by the process termed covalent bonding. Covalent bonding also prevents these outermost electrons from moving through the crystal (i.e., flowing as an electric current) as easily as do the conduction electrons in metals. They are not bound too tightly to break loose if given a small amount of extra energy, but cannot wander easily through the crystal. Heat, light, or ionizing radiation may all increase the semiconductors electrical conductivity by liberating these electrons to support current. Usually these effects are unwanted, because one does not want the properties of a circuit to vary with temperature. Ionizing radiation, furthermore, may cause transistors to fail by permanently altering the crystal structure. The first active communications satellite placed in orbit by the United States, Telstar (1962), failed when its transistors were exposed to unexpected levels of ionizing radiation. (Early satellites such as Echo had been passive radio reflectors, containing no amplifying transistor circuits to be affected by radiation.)

Doping

a pure or intrinsic silicon crystal contains about one non-silicon impurity atom for every 100 million or so silicon atoms. These impurity atoms are implanted in the crystal by a process termed doping. They are located in the crystal lattice as if they were themselves silicon atoms, but change the properties of the lattice radically because of their distinct properties.

When doping adds impurity atoms with five electrons in their outermost (valence) orbit, the result is termed an n -type semiconductor. Arsenic, for example, has five valence electrons and is often used to produce n -type semiconductor. Pentavalent impurity atoms share only four of their five valance electrons with their four closest silicon neighbors; the fifth is free to move through the crystal in response to any electric field that may be present, almost like a conduction electron in an ordinary metal. An n -type semiconductor thus conducts electricity more easily than an intrinsic semiconductor.

If an impurity with only three valence electrons (e.g., boron, aluminum, gallium, and indium) is used, p-type semiconductor results. These atoms are short one of the electrons needed to establish a covalent bond with all four of its silicon neighbors and so introduce a defect into the crystal lattice, a positively-charged location where a negatively-charged electron would be found if a silicon atom had not been displaced by the impurity atom. This defect, termed a hole, can move when a neighboring electron slips into the hole, leaving a new hole behind. The hole will have moved from one location to another within the crystal, behaving much like a positive counterpart of an electron.

Holes travel somewhat more slowly than electrons within a an electrical field of given strength, but this difference in speed is usually not important in practice. Both the excess electrons donated in n -type semiconductor by pentavalent impurity atoms and the holes created in p-type semiconductor by trivalent impurity atoms increase the conductivity of the semiconductor; for example, at 86° F (30° C) the conductivity of n -type silicon with one pentavalent impurity atom per 100 million silicon atoms is 24,100 times greater than that of intrinsic silicon.

p-n junction diodes

A useful electrical property results at a boundary where p-type material abuts on n -type material in the same semiconductor crystal. The result is termed a p-n junction diode (or simply junction diode). A junction diode may be thought of as a one-way valve for electricity; it will carry current in one direction much more easily than in the opposite direction. Understanding of the transistorespecially that type termed the bipolar junction transistor or BJTbegins with knowledge of the p-n junction diode. A BJT is, in effect, a back-to-back pair of p-n diodes within a single crystal.

In either p -type or n -type semiconductors, there are two types of charge carriers that carry current: majority carriers and minority carriers. Electrons are the majority carriers in n -type material, due to the extra electrons donated by pentavalent impurity atoms, and holes are the majority carriers in p -type semiconductors. The minority carriers are the relatively few, oppositely charged carriers, electrons in p-type and holes in n -type semiconductor, which cannot be eliminated entirely. Heat, ionizing radiation, and unintended impurities in the original intrinsic crystal produce minority carriers. Practical diodes do not behave ideally because minority carriers allow a small reverse current, that is, a trickle of charges leaking backward through a diode, whereas an ideal diode would present a total block to current in that direction. Leakage current occurs in transistors as well as diodes, and these currents can have important consequences for circuit performance.

If voltage is applied across a p -n junction diode with polarity that causes the p region of the diode to be more positive than the n region, the majority carriers in both p and n regions will be pushed toward each other, meeting at the boundary. A diode polarized in this way is said to be forward biased. A forward-biased diode conducts quite well. If the voltage polarity is reversed causing the n -type material to be more positive than the p -type material, the two types of majority carriers will be pulled away from each other. This condition is called reverse bias or back bias. The small current leak through a back-biased diode is the result of minority carriers moving in the opposite direction compared to majority carriers.

There is a very thin volume at the boundary where n -type semiconductor materials interfaces with p -type material, termed the depletion region. In the depletion region electrons tend to fill adjacent holes, depleting the crystal of carriers. When majority carriers from each region are pushed toward each other, hole-electron pairs continually annihilate each other. As each hole is filled by an electron, a new hole and a new electron will be injected into the crystal at the ohmic connections to the crystal (i.e., those places where metal contacts are applied). In this way current can continue to flow through the diode as long as the circuit is energized.

If the reverse-biasing voltage across a diode increases above a critical threshold the diode will suddenly break into heavy conduction when the electric field in the depletion region between the n and p materials is so strong that electrons are torn from their bonding roles. This condition is called Zener breakdown. Usually transistors are operated at voltages low enough so that this type of breakdown doesnt take place. Unless the breakdown current is limited by the external circuitry the transistor or diode may easily destroyed when excess voltages or voltages with the wrong polarity are applied.

Bipolar junction transistors

If the same crystal is doped so that each end is n -type and the very thin slice in the center is p -type, the resulting sandwich forms a bipolar junction transistor or n-p-n transistor. In an n-p-n transistor one of the n -type regions is termed the collector, the other n -type region is termed the emitter. (The emitter emits majority charge carriers, and the collector collects them.) The very thin slice of p-type material in the center is termed the base of the transistor. In a p-n-p transistor the collector and emitter regions are made from p-type semiconductor and the base has the characteristics of n-type material. Both n-p-n and p-n-p transistors are in common use but these two transistor types are not directly interchangeable since they require different power-supply polarities. Many circuits employ both n-p-n and p-n-p transistors, but the circuitry must supply the correct voltages. It is common to connect the two types of transistors together in an arrangement that is called complementary symmetry.

Transistor action

Transistors are able to amplify signals because their design permits the supply of charge carriers to be adjusted electrically. A transistor will have a high electrical resistance when it is starved for charge carriers but it will conduct quite well when a control signal injects extra carriers that can be used to support increased current.

Common base, common emitter, and common-collector configurations

There are three ways to connect a bipolar junction transistor into a working circuit, depending upon which of the three transistor elements is chosen as the common reference for the other two elements. These variations, called common base, common emitter, and common collector, produce different circuit actions each with unique characteristics. An n-p-n transistor configured as a common-emitter amplifier, where both the base and the collector circuits are referenced to the emitter, is normally connected with a positive voltage on the collector, as referenced to the emitter. The collector-base diode and the base-emitter diode appear to be in series, connected back-to-back. The collector-base diode is reverse biased so that almost no current will flow unless the base-emitter diode is forward biased. The very small current in the collector circuit under these conditions is because the p-type material in the base is starved for the n-type majority carriers that the collector circuit requires if it is to conduct a significant current. When the base-emitter junction is forward biased, the carriers needed for current in the collector circuit find their way into the collector.

The base-emitter diode in the transistor offers a very low resistance to current flow when it is forward biased. It is therefore very easy to cause current in the transistors input circuit. Since the base region is made very thin, most of the majority carriers that flow from the emitter will be caught by the strong electric field in the collector base junction before they can exit through the base connection. It takes only a small amount of power to cause current in the transistors forward-biased base-emitter input circuit yet almost all this easily forced input current appears in the collector circuit. A low-powered signal becomes a higher-powered signal when the input current caused by a low voltage appears almost undiminished in the collector circuit, but at a higher voltage.

Field-effect transistors (fets)

Field-effect transistors (FETs) are solid-state active devices based on a different principle than BJTs but producing much the same result. FETs are three-terminal devices, just as are BJTs. The input terminal of an FET is termed its gate and constitutes one of the electrodes of a reverse-biased diode. FETs achieve current control by channeling current through a narrow n-type or p-type pathway whose conductivity is adjusted by the input signal. The output current controlled by an FET passes between the two remaining terminals called a source and a drain. The current through an FET must find its way through a narrow channel formed by the input-diode junction. Since this input diode is reverse biased, this channel tends to have few charge carriers. The input signal to the FET can deplete or enhance the number of available charge carriers in this channel, regulating the current in the drain circuit. Because the input diode is reverse biased, the FET demands almost no current from the signal source, therefore almost no power must be supplied. The power gain commonly achieved in an FET amplifier is very high.

A particular type of FET called a MOSFET (metal oxide semiconductor field-effect Transistor) can have an input resistance as high as 1018 ohms. Because of their very high input resistance, FETs are instantly destroyed if they receive even a small static-electric charge from careless handling. Sliding across a plastic chair may impart enough charge to a technicians body to destroy a field-effect transistors input diode at the first touch. FETs must be handled only by persons who ground themselves before touching these devices to first dissipate static charges.

FETs are particularly useful as amplifiers of very weak signals such as those produced by high-quality microphones. FETs have more desirable overload characteristics than BJTs, so that FETs are able to handle many signals simultaneously, some strong and some weak, without suffering troublesome distortion. Before FETs were used in automobile receivers, these radios were easily overloaded by strong signals; the introduction of FETs made a tremendous improvement in automobile-radio receiver performance.

Integrated circuits

One of technologys most significant breakthroughs has been the discovery that many interconnected transistors can be created simultaneously on a single, small chip of semiconductor material. The techniques used to create individual transistors could also connect these devices to form microscopic circuits that are unified or integrated into a single solid object. The first integrated circuits (ICs) were primitive arrangements utilizing just a few transistors or diodes, but now it is common for single chips to contain millions of transistors.

Application-specific integrated circuits

Special integrated circuits are developed as application-specific integrated circuits, or ASICs, which are single chips performing a specific set of tasks, useful for only one job. For example, almost all the circuitry needed for an AM-FM radio receiver is routinely produced on a single ASIC chip that replaces the hundreds of individual components. TV receivers have also become increasingly dependent upon ASICs. ICs may also take the form of generalized modules that were intended to be used in a broad range of applications. Many of the first generalized ICs were designed as operational amplifiers, a general-purpose amplifier made from many transistors.

KEY TERMS

Capacitor Passive circuit component used to introduce capacitance.

Covalent bond A chemical bond formed when two atoms share a pair of electrons with each other.

Crystal Ordered three dimensional array atoms.

Dopant A chemical impurity which is added to a pure substance in minute quantities in order to alter its properties.

Doping Adding impurities to change semiconductor properties.

Inductor A component designed to introduce inductance.

Intrinsic Semiconductor material containing very few impurities.

Pentavalent An element with five valence electrons.

Photoconducive A better conductor of electricity when illuminated.

Resistor An electric circuit component that opposes the flow of current.

Tetravalent Element with four valence electrons.

Trivalent Element with four valence electrons.

Valence electrons The electrons in the outermost shell of an atom that determine an elements chemical properties.

Complementary metal-oxide semiconductors

Complementary metal-oxide semiconductors or CMOS devices are coupled complementary MOSFETS in series, configured so that either MOSFET will conduct when the other is turned off. CMOS devices are frequently used as on-off switches in computer logic and memory circuits. CMOS devices use so little power that they allow electronic watches to operate for five years without battery replacement. CMOS devices only require a significant current when the two complementary FETs in series change their conduction state, i.e., a quick pulse of current flows only during each switching action. CMOS chips are commonly used in computer and calculator circuits, and CMOS is equally as common in consumer-type entertainment equipment.

The significance and future of the transistor

Perhaps the principal contribution of transistors has been the feasibility of highly complex yet miniature electronic equipment; they have made it possible to hold more electronics in ones hand than could be contained in a large building in the days when vacuum tubes were the only active devices available. This in turn has made it possible to pack complex functionality into packages of manageable sizecomputers, cell phones, automobile engine controllers, and a host of other tools. Transistors continue to be intensively researched around the world, for decreasing the size and power consumption of the individual transistor on a chip offers immediate profits. Researchers have already demonstrated, in the laboratory, extremely small transistors made out of only a few molecules even a transistor employing only a single electron. They have also demonstrated the practicality of transistors made out of plastic, which could be even cheaper and more shock-resistant than conventional devices. However, it may be years before such exotic developments see commercial application. In the near future, increased transistor densities on chips are likely to be achieved by improving fabrication techniques for traditional semiconductor devices.

Resources

BOOKS

Grabinski, Wladyslaw, et al. eds. Transistor Level Modeling for Analog/ RF IC Design. New York: Springer, 2006.

Hoffman, Kurt. System Integration: From Transistor Design to Large Scale Integrated Circuits. New York: John Wiley & Sons, 2004.

Kasap, Safa O. Principles of Electronic Materials and Devices. New York: McGraw-Hill Science, 2005.

PERIODICALS

Bachtold, Adrian, et al. Logic Circuits with Carbon Nanotube Transistors. Science (November 9, 2001): 1317-1320.

Markoff, John. Xerox Says New Material Will Allow Plastic Transistors. New York Times. (December 3, 2002).

Shimoda, Tatsuya. Solution-Processed Silicon Films and Transistors. Nature. 440 (2006): 783-386.

Donald Beaty

Larry Gilman

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Transistor

A transistor is a small, solid device capable of amplifying and switching electrical signals. A transistor can be manufactured using a wide variety of materials; most transistors utilize the unique semiconducting properties of silicon or germanium that has been selectively contaminated with other elements (e.g., arsenic, phosophorus, boron, gallium). A transistor is controlled by voltages communicated to it through three or more metallic contacts. Transistors are active devices, meaning that they must be supplied with power to function. Virtually all electronic devices contain transistors, from a handful to many millions; to the extent that our civilization has come to depend on computers and electronic communications, therefore, it depends on the transistor. The term transistor is a shortening of TRANSfer ResISTOR.

A transistor can perform a variety of useful electrical tasks because its resistance (the ease with which an electrical current flows through it) can be adjusted using a low-power control signal applied to one of a transistor's three metallic terminals. The resulting change in resistance between the other two terminals of the transistor—through one of which current enters the transistor, leaving through the other—changes the current passed through the transistor. This current can in turn be converted into a voltage by passing it through a resistor (a passive or unpowered circuit device that simply dissipates energy ); the change in voltage across this "load" resistor can be many times greater than the change in voltage that was used to alter the resistance of the transistor itself. This increase in amplitude or strength is called amplification, one of the most basic processes in electronics .

Transistor amplification can be compared to controlling a powerful flow of water through a pipe by turning a valve: in this analogy, the force applied to the valve represents the transistor's control voltage, while the water flowing through the pipe represents its output current. A small, varying force applied to the valve—a back-and-forth wiggle—causes matched variations in the greater force carried by the water passing through the pipe; a small signal thus generates another that varies identically in time but is larger in amplitude. Vacuum tubes, which were developed before transistors but perform the same functions, are termed "valves" in British usage in reference to this analogy.

When a transistor acts as an amplifier it does not create the additional energy appearing in its output signal, just as a valve does not create water it dispenses. Rather, a transistor modulates the energy flowing from a battery or power supply in a way that is similar to a valve adjusting the flow rate from a source of pressurized water.

It is also clear from the valve analogy that instead of wiggling the valve one might choose instead to operate it in only two positions, open and shut, avoiding partial settings completely. Vacuum tubes and transistors can also be operated in this way, switching an electrical current on and off in response to a two-valued control signal. The on-off signals generated by this technique, switching, are the basis of digital electronics; the constantly-varying signals involved in amplification are the basis of analog electronics. Both rely on transistors, which are cheaper, smaller, and more reliable than other devices that can perform these functions.


The history of the transistor

Discovery of the transistor was publicly announced in 1948. Before this time, electronics had depended almost exclusively upon vacuum tubes for amplification and switching actions. Vacuum tubes are relatively bulky, short-lived, and wasteful of power; transistors are small—from peanut-size to only a few molecules across—long-lived, and dissipate far less power. Transistors are also resistant to mechanical shocks and can be manufactured by the millions on tiny semiconductor crystals (chips) using optical techniques. Transistors and related solid-state (i.e., entirely solid) devices have replaced vacuum tubes except for specialized applications, especially those involving high power.

Silicon and germanium

The first transistors were made from germanium, but now most transistors are made from silicon. Silicon (Si) and germanium (Ge) form similar crystal structures with similar physical properties, but silicon is preferred over germanium because of silicon's superior thermal characteristics. Crystals of Si and Ge are neither good electrical insulators nor good electrical conductors, but conduct electricity at a level midway between metallic conductors (which have very low resistance to electric current ) and nonmetallic insulators such as glass (which have a very high resistance to electric current). Transistor action is made possible by semiconduction.

Each atom in a silicon or germanium crystal lattice has four atoms as close neighbors. That is, each atom is held in its place in the crystal's orderly structure because each atom shares its four outermost electrons with the outermost electrons of four nearby atoms. This sharing holds the atoms together by the process termed covalent bonding. Covalent bonding also prevents these outermost electrons from moving through the crystal (i.e., flowing as an electric current) as easily as do the conduction electrons in metals. They are not bound too tightly to break loose if given a small amount of extra energy, but cannot wander easily through the crystal. Heat , light , or ionizing radiation may all increase the semiconductor's electrical conductivity by liberating these electrons to support current. Usually these effects are unwanted, because one does not want the properties of a circuit to vary with temperature . Ionizing radiation , furthermore, may cause transistors to fail by permanently altering the crystal structure. The first active communications satellite placed in orbit by the United States, Telstar (1962), failed when its transistors were exposed to unexpected levels of ionizing radiation. (Early satellites such as Echo had been passive radio reflectors, containing no amplifying transistor circuits to be affected by radiation.)


Doping

An pure or "intrinsic" silicon crystal contains about one non-silicon impurity atom for every 100 million or so silicon atoms. These impurity atoms are implanted in the crystal by a process termed doping. They are located in the crystal lattice as if they were themselves silicon atoms, but change the properties of the lattice radically because of their distinct properties.

When doping adds impurity atoms with five electrons in their outermost (valence ) orbit, the result is termed an n-type semiconductor. Arsenic, for example, has five valence electrons and is often used to produce n-type semiconductor. Pentavalent impurity atoms share only four of their five valance electrons with their four closest silicon neighbors; the fifth is free to move through the crystal in response to any electric field that may be present, almost like a conduction electron in an ordinary metal . An n-type semiconductor thus conducts electricity more easily than an intrinsic semiconductor.

If an impurity with only three valence electrons (e.g., boron, aluminum , gallium, and indium) is used, p-type semiconductor results. These atoms are short one of the electrons needed to establish a covalent bond with all four of its silicon neighbors and so introduce a defect into the crystal lattice, a positively-charged location where a negatively-charged electron would be found if a silicon atom had not been displaced by the impurity atom. This defect, termed a hole, can move when a neighboring electron slips into the hole, leaving a new hole behind. The hole will have moved from one location to another within the crystal, behaving much like a positive counterpart of an electron.

Holes travel somewhat more slowly than electrons within a an electrical field of given strength, but this difference in speed is usually not important in practice. Both the excess electrons donated in n-type semiconductor by pentavalent impurity atoms and the holes created in p-type semiconductor by trivalent impurity atoms increase the conductivity of the semiconductor; for example, at 86°F (30°C) the conductivity of n-type silicon with one pentavalent impurity atom per 100 million silicon atoms is 24,100 times greater than that of intrinsic silicon.


p-n junction diodes

A useful electrical property results at a boundary where p-type material abuts on n-type material in the same semiconductor crystal. The result is termed a p-n junction diode (or simply junction diode). A junction diode may be thought of as a one-way valve for electricity; it will carry current in one direction much more easily than in the opposite direction. Understanding of the transistor—especially that type termed the bipolar junction transistor or BJT—begins with knowledge of the p-n junction diode. A BJT is, in effect, a back-to-back pair of p-n diodes within a single crystal.

In either p-type or n-type semiconductors, there are two types of charge carriers that carry current: majority carriers and minority carriers. Electrons are the majority carriers in n-type material, due to the extra electrons donated by pentavalent impurity atoms, and holes are the majority carriers in p-type semiconductors. The minority carriers are the relatively few, oppositely charged carriers, electrons in p type and holes in n-type semiconductor, which cannot be eliminated entirely. Heat, ionizing radiation, and unintended impurities in the original intrinsic crystal produce minority carriers. Practical diodes do not behave ideally because minority carriers allow a small reverse current, that is, a trickle of charges leaking backward through a diode, whereas an ideal diode would present a total block to current in that direction. Leakage current occurs in transistors as well as diodes, and these currents can have important consequences for circuit performance.

If voltage is applied across a p-n junction diode with polarity that causes the p region of the diode to be more positive than the n region, the majority carriers in both p and n regions will be pushed toward each other, meeting at the boundary. A diode polarized in this way is said to be forward biased. A forward-biased diode conducts quite well. If the voltage polarity is reversed causing the n-type material to be more positive than the p-type material, the two types of majority carriers will be pulled away from each other. This condition is called reverse bias or back bias. The small current leak through a back-biased diode is the result of minority carriers moving in the opposite direction compared to majority carriers.

There is a very thin volume at the boundary where n-type semiconductor materials interfaces with p-type material, termed the depletion region. In the depletion region electrons tend to fill adjacent holes, depleting the crystal of carriers. When majority carriers from each region are pushed toward each other, hole-electron pairs continually annihilate each other. As each hole is filled by an electron, a new hole and a new electron will be injected into the crystal at the ohmic connections to the crystal (i.e., those places where metal contacts are applied). In this way current can continue to flow through the diode as long as the circuit is energized.

If the reverse-biasing voltage across a diode increases above a critical threshold the diode will suddenly break into heavy conduction when the electric field in the depletion region between the n and p materials is so strong that electrons are torn from their bonding roles. This condition is called Zener breakdown. Usually transistors are operated at voltages low enough so that this type of breakdown doesn't take place. Unless the breakdown current is limited by the external circuitry the transistor or diode may easily destroyed when excess voltages or voltages with the wrong polarity are applied.


Bipolar junction transistors

If the same crystal is doped so that each end is n-type and the very thin slice in the center is p-type, the resulting sandwich forms a bipolar junction transistor or n-p-n transistor. In an n-p-n transistor one of the n-type regions is termed the collector, the other n-type region is termed the emitter. (The emitter emits majority charge carriers, and the collector collects them.) The very thin slice of p-type material in the center is termed the base of the transistor. In a p-n-p transistor the collector and emitter regions are made from p-type semiconductor and the base has the characteristics of n-type material. Both n-p-n and p-n-p transistors are in common use but these two transistor types are not directly interchangeable since they require different power-supply polarities. Many circuits employ both n-p-n and p-n-p transistors, but the circuitry must supply the correct voltages. It is common to connect the two types of transistors together in an arrangement that is called complementary symmetry .

Transistor action

Transistors are able to amplify signals because their design permits the supply of charge carriers to be adjusted electrically. A transistor will have a high electrical resistance when it is starved for charge carriers but it will conduct quite well when a control signal injects extra carriers that can be used to support increased current.


Common base, common emitter, and common-collector configurations

There are three ways to connect a bipolar junction transistor into a working circuit, depending upon which of the three transistor elements is chosen as the common reference for the other two elements. These variations, called common base, common emitter, and common collector, produce different circuit actions each with unique characteristics. An n-p-n transistor configured as a common-emitter amplifier, where both the base and the collector circuits are referenced to the emitter, is normally connected with a positive voltage on the collector, as referenced to the emitter. The collector-base diode and the base-emitter diode appear to be in series, connected back-to-back. The collector-base diode is reverse biased so that almost no current will flow unless the base-emitter diode is forward biased. The very small current in the collector circuit under these conditions is because the p-type material in the base is starved for the n-type majority carriers that the collector circuit requires if it is to conduct a significant current. When the base-emitter junction is forward biased, the carriers needed for current in the collector circuit find their way into the collector.

The base-emitter diode in the transistor offers a very low resistance to current flow when it is forward biased. It is therefore very easy to cause current in the transistor's input circuit. Since the base region is made very thin, most of the majority carriers that flow from the emitter will be caught by the strong electric field in the collector base junction before they can exit through the base connection. It takes only a small amount of power to cause current in the transistor's forward-biased base-emitter input circuit yet almost all this easily forced input current appears in the collector circuit. A low-powered signal becomes a higher-powered signal when the input current caused by a low voltage appears almost undiminished in the collector circuit, but at a higher voltage.


Field-effect transistors (FETs)

Field-effect transistors (FETs) are solid-state active devices based on a different principle than BJTs but producing much the same result. FETs are three-terminal devices, just as are BJTs. The input terminal of an FET is termed its gate and constitutes one of the electrodes of a reverse-biased diode. FETs achieve current control by channeling current through a narrow n-type or p-type pathway whose conductivity is adjusted by the input signal. The output current controlled by an FET passes between the two remaining terminals called a source and a drain. The current through an FET must find its way through a narrow channel formed by the input-diode junction. Since this input diode is reverse biased, this channel tends to have few charge carriers. The input signal to the FET can deplete or enhance the number of available charge carriers in this channel, regulating the current in the drain circuit. Because the input diode is reverse biased, the FET demands almost no current from the signal source, therefore almost no power must be supplied. The power gain commonly achieved in an FET amplifier is very high.

A particular type of FET called a MOSFET (metal oxide semiconductor field-effect Transistor) can have an input resistance as high as 1018 ohms. Because of their very high input resistance, FETs are instantly destroyed if they receive even a small static-electric charge from careless handling. Sliding across a plastic chair may impart enough charge to a technician's body to destroy a field-effect transistor's input diode at the first touch . FETs must be handled only by persons who ground themselves before touching these devices to first dissipate static charges.

FETs are particularly useful as amplifiers of very weak signals such as those produced by high-quality microphones. FETs have more desirable overload characteristics than BJTs, so that FETs are able to handle many signals simultaneously, some strong and some weak, without suffering troublesome distortion. Before FETs were used in automobile receivers, these radios were easily overloaded by strong signals; the introduction of FETs made a tremendous improvement in automobile-radio receiver performance.


Integrated circuits

One of technology's most significant breakthroughs has been the discovery that many interconnected transistors can be created simultaneously on a single, small chip of semiconductor material. The techniques used to create individual transistors could also connect these devices to form microscopic circuits that are unified or integrated into a single solid object. The first integrated circuits (ICs) were primitive arrangements utilizing just a few transistors or diodes, but now it is common for single chips to contain millions of transistors.

Application-specific integrated circuits

Special integrated circuits are developed as application-specific integrated circuits, or ASICs, which are single chips performing a specific set of tasks, useful for only one job. For example, almost all the circuitry needed for an AM-FM radio receiver is routinely produced on a single ASIC chip that replaces the hundreds of individual components. TV receivers have also become increasingly dependent upon ASICs. ICs may also take the form of generalized modules intended to be used in a broad range of applications. Many of the first generalized ICs were designed as operational amplifiers, a general-purpose amplifier made from many transistors.


Complementary metal-oxide semiconductors

Complementary metal-oxide semiconductors or CMOS devices are coupled complementary MOSFETS in series, configured so that either MOSFET will conduct when the other is turned off. CMOS devices are frequently used as on-off switches in computer logic and memory circuits. CMOS devices use so little power that they allow electronic watches to operate for five years without battery replacement. CMOS devices only require a significant current when the two complementary FETs in series change their conduction state, i.e., a quick pulse of current flows only during each switching action. CMOS chips are commonly used in computer and calculator circuits, and CMOS is equally as common in consumer-type entertainment equipment.


The significance and future of the transistor

Perhaps the principal contribution of transistors has been the feasibility of highly complex yet miniature electronic equipment; they have made it possible to hold more electronics in one's hand than could be contained in a large building in the days when vacuum tubes were the only active devices available. This in turn has made it possible to pack complex functionality into packages of manageable size—computers, cell phones, automobile engine controllers, and a host of other tools. Transistors continue to be intensively researched around the world, for decreasing the size and power consumption of the individual transistor on a chip offers immediate profits. Researchers have already demonstrated, in the laboratory, extremely small transistors made out of only a few molecules—even a transistor employing only a single electron. They have also demonstrated the practicality of transistors made out of plastic, which could be even cheaper and more shock-resistant than conventional devices. However, it may be years before such exotic developments see commercial application. In the near future, increasing transistor densities on chips (which are greatly desired by manufacturers) are likely to be achieved by improving fabrication techniques for traditional semiconductor devices.


Resources

periodicals

Bachtold, Adrian, et al. "Logic Circuits with Carbon Nanotube Transistors." Science (November 9, 2001): 1317-1320.

Markoff, John. "Xerox Says New Material Will Allow Plastic Transistors." New York Times December 3, 2002.


Donald Beaty Larry Gilman

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Capacitor

—Passive circuit component used to introduce capacitance.

Covalent bond

—A chemical bond formed when two atoms share a pair of electrons with each other.

Crystal

—Ordered three dimensional array atoms.

Dopant

—A chemical impurity which is added to a pure substance in minute quantities in order to alter its properties.

Doping

—Adding impurities to change semiconductor properties.

Inductor

—A component designed to introduce inductance.

Intrinsic

—Semiconductor material containing very few impurities.

Pentavalent

—An element with five valence electrons.

Photoconducive

—A better conductor of electricity when illuminated.

Resistor

—An electric circuit component that opposes the flow of current.

Tetravalent

—Element with four valence electrons.

Trivalent

—Element with four valence electrons.

Valence electrons

—The electrons in the outermost shell of an atom that determine an element's chemical properties.

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Transistor

A transistor is a solid-state electronic device used to control the flow of an electric current. The term solid-state refers to devices that take advantage of special properties of solids. (It usually refers to devices made of semiconducting materials.) Since they were invented in the 1940s, transistors have come to revolutionize modern communications. They are found in an enormous variety of electrical devices, ranging from popular consumer items such as home computer games, pocket calculators, and portable stereos to the complex electronic systems used by business and industry.

Until World War II (193945), most systems of communication used vacuum tubes for the amplification and control of electrical current. However, vacuum tubes have a number of serious disadvantages. They are bulky and fragile, they consume a lot of power, and they have a tendency to overheat. The demands of radar in particular during the war encouraged scientists to look for another method of amplifying and controlling electric current in communication devices.

Semiconductors

The discovery of the transistor was announced in 1948 by three scientists from the Bell Telephone Laboratories: William Shockley (19101989), John Bardeen (19081991), and Walter Brattain (19021987). The key to this discovery is a class of materials known as semiconductors. Semiconductors are substances that conduct an electric current only very poorly. They fall somewhere between true conductors (such as silver, aluminum, and copper) and nonconductors (such as wool, cotton, paper, air, wood, and most plastics). The two most commonly used semiconducting elements are silicon and germanium. Some important semiconducting compounds include cadmium selenide, cadmium telluride, and gallium arsenide.

Semiconductors fall into one of two general categories: n-type semiconductors and p-type semiconductors. The former class consists of materials that have a slight excess of electrons, while those in the latter class have a slight deficiency of electrons.

Words to Know

Amplification: Increasing the strength of some signal such as the amount of electrical current passing through a transistor.

Base: The middle slice of a transistor.

Chip: The piece of semiconducting material on which integrated circuits are etched.

Collector: One of the outer slices of a transistor.

Dopant: An impurity added to a semiconducting material.

Doping: The act of adding impurities to change semiconductor properties.

Emitter: One of the outer slices of a transistor.

Integrated circuit (IC): An electronic device that contains thousands or millions of microscopic-sized transistors etched on a single piece (chip) of material.

N-type semiconductor: An element or compound that has a slight excess of electrons.

P-type semiconductor: An element or compound that has a slight deficiency of electrons.

Resistor: Component used to introduce resistance.

Semiconductor: A substance that conducts an electric currentbut only very poorly.

Solid-state: A term used for electronic devices that take advantage of special properties of solids. It usually refers to devices made of semiconducting materials.

The conductivity of both n-type and p-type semiconductors can be enhanced greatly by adding very small amounts of impurities. This process is known as doping and involves the addition of roughly one atom of dopant (such as boron or phosphorus) for each ten million atoms of the base semiconductor (such as silicon or germanium).

Operation of a transistor

A typical transistor looks like a sandwich with one type of semiconductor as the slices of bread and the second type of semiconductor as the filling. For example, a thin slice of a p-type semiconductor might be placed between two thicker slices of an n-type semiconductor. The middle slice of the transistor is known as the base, while the two outer slices are called the collector and the emitter.

Suppose that this transistor is placed into an electric circuit, and current is allowed to flow through it. The current flows into the transistor through the collector, across the base, and out through the emitter.

The flow of this current can be controlled by attaching a second source of electric current to the base itself. The amount of current that flows through the transistor will be determined by this second source of electric current. If a relatively small current is allowed to flow into the base, the transistor does not permit a very large flow of current through it. If a relatively large current is allowed to flow into the base, the transistor allows a much larger flow of current through it.

For example, suppose that a particular transistor typically permits a flow of 0.01 milliamperes when the electrical flow into the base is at a minimum. Then suppose that the flow into the base is increased by a small

amount. That small increase will allow a much larger flow of electric currentsay up to 2.5 milliamperesthrough it. The transistor has been made to act, therefore, like an amplifier.

Many other kinds of transistors have been developed to perform other electronic functions. One of the greatest breakthroughs in transistor research was the invention in 1959 of the integrated circuit (IC). An integrated circuit is an electronic device that contains large numbers (usually thousands or millions) of microscopic-sized transistors etched on a single piece (chip) of material.

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transistor Electronic device made of semiconductor material that can amplify electrical signals. The material, such as silicon or germanium, is ‘doped’ with minute amounts of phosphorus, arsenic or antimony to produce n-type material, in which negative charges (electrons) carry current; or with aluminium, gallium or indium to give a p-type material. Joining together a piece of each produces a diode. Sandwiching one type between two of the other produces a transistor. Transistors were first developed (1948) by US physicists John Bardeen, Walter Brattain, and William Shockley, making possible many advances in technology, especially in computers, portable radios and televisions, satellites, and control systems.

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transistor A semiconductor device having, in general, three terminals that are attached to electrode regions within the device. Current flowing between two of these electrodes is made to vary in response to voltage or current variations imposed on the third electrode. The device is capable of current or voltage amplification depending on the particular circuit implementation employed. It can also be used as a switch by driving it between its maximum and minimum of current flow.

The transistor was invented in 1948 by Shockley, Brattain, and Bardeen at Bell Telephone Labs. As performance and manufacturing techniques improved, the transistor enabled a huge growth in computer technology.

See also bipolar transistor, field-effect transistor, MOSFET.

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tran·sis·tor / tranˈzistər/ • n. a semiconductor device with three connections, capable of amplification in addition to rectification. ∎  (also transistor radio) a portable radio using circuits containing transistors rather than vacuum tubes.

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The Transistor

The transistor allows the miniaturization of electronic equipment and is regarded as the nerve cell of the Information Age. Its first use was in the telephone switching machine in Englewood, New Jersey, in 1952. By the end of the twentieth century, the transistor could be found everywhere, in supercomputers, televisions, radios, toys, greeting cards, and garage door openers.

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