Transistors

Type of physical science: Classical physics

Field of study: Electromagnetism

Transistors, semiconducting circuit elements, opened the tremendously powerful possibilities offered by solid-state electronics. The transistor provides amplification of voltage or current without many of the disadvantages of vacuum tubes.

Overview

Negatively charged particles, electrons, are the current carriers in vacuum tubes. In semiconducting devices, such as transistors, the concept of conduction (via negative charge carriers) must be modified to include the contribution of positive charge carriers, called holes, to current flow. Holes are basically the absence of electrons necessary to maintain electrical neutrality in the crystalline lattice. Holes can be considered to have mass, they are mobile, and they can propagate through the lattice. Their movement, thus, constitutes a current. (In physics, a current is defined in terms of the movement of positive charge, a throw-back to the time when it was believed that positive charge was responsible for conduction in ordinary metals. Thus, a negative charge moving to the left is equivalent to a positive charge moving to the right in the presence of an electric field. Both charge motions contribute to a total current moving to the right in this simple example.)

Semiconducting materials that are relatively pure are not particularly useful as circuit elements. Silicon and germanium are the most commonly used semiconducting materials in solid-state devices. To make a semiconducting component, impurities are purposely introduced into the crystalline lattice in a process known as doping. Commonly used impurities are indium, gallium, arsenic, and antimony. These materials alter the conduction of silicon and germanium by changing the number of negatively charged free electrons. Silicon and germanium are group IV elements. Antimony and arsenic are group V elements, and indium and gallium are group III elements. Thus, introduction of antimony or arsenic in large concentrations makes a semiconducting material enriched in free electrons. Such a material is called n type.

Introduction of indium or gallium would increase a deficit of free electrons (hence an excess of holes). Such a material is called p type. In an n-type material, free electrons are the majority carriers and mobile holes minority carriers. In a p-type material, mobile holes are the majority carriers and free electrons the minority carriers. Holes and free electrons have an attraction for one another and can recombine to restore charge neutrality to the lattice locally. In electron-hole recombination, both the hole and free electron are lost as contributions to the current flow. While this occurs at one place in a semiconductor, however, new current carriers are formed at other places in the semiconductor to replenish them. Random movements of mobile charge carriers can be controlled by the application of an external voltage source. This sets up an electric field within the material and dictates a direction of flow for hole and electron movement.

If a semiconductor crystal is doped with group III impurities in part and then doping is abruptly changed to group V elements, a p-n junction diode is created. A diode will allow conduction current flow in one direction and only minority carrier flow in the other. Thus, a diode can restrict current flow direction and act as a rectification element.

A transistor consists of three layers of impurity-doped semiconducting material. The middle layer is doped differently from the two ends. A single transistor, thus, is either a p-n-p or n-p-n type. (All discussions that follow will refer to p-n-p transistors. The n-p-n transistors operate similarly, but holes and electrons reverse roles in the amplification process.) The middle section of a transistor is called the base. Its boundaries with the two outside layers, called the emitter and collector, are very abrupt. If the change in doping were gradual, a transistor would not function properly. The collector portion is somewhat larger than the emitter and is thermally attached to the transistor housing for power dissipation. Excess heat inside the semiconducting crystal could degrade its amplification capability and provide enough energy for impurity atoms to become mobile and destroy the abrupt boundaries between the emitter and base and the collector and base.

At a p-n junction, current will flow in the forward-biased direction but will be suppressed in the reverse direction. A p-n junction, however, is not an ohmic conductor, meaning the voltage applied is not proportional to the resultant current flow. In a p-n junction that is forward-biased, the current increases exponentially as the voltage is increased. A transistor has two p-n junctions. For a p-n-p transistor to function properly, one junction is forward-biased (the base-emitter junction) and the other junction is reverse-biased (the base-collector junction). With this arrangement, the p type material is positive in voltage with respect to the n type.

A large current will flow into the base from the emitter. Little flow of current goes from the collector into the base.

The reverse-biased junction between the base and the collector forms a depletion zone with an electric field pointing from the base to the collector. The reverse-biasing indicates that the depletion field sweeps majority carriers from the base and collector away from the base-collector junction rather than toward the junction. Outside this depletion zone, there is no significant electric field within the semiconductor crystal. Thus, except at the depletion zone, the motion of charge carriers (holes and electrons) is determined exclusively by diffusion.

Charge is a conserved quantity; therefore, by one of Kirchhoff's laws of circuit analysis, the current flowing into the emitter must equal the current leaving the base plus the current going into the collector. Holes, remember, are the majority carrier in the p-type emitter. Three fates are possible for holes injected into the base through the forward-biased emitter-base junction: They can recombine with mobile majority carrier electrons in the n-type base; they can diffuse through the base and recombine with free electrons injected into the base from the external circuitry; or they can diffuse across the base into the depletion zone, where they will be swept across the reverse-biased base-collector junction into the p-type collector. Once there, they diffuse until recombining with free electrons injected into the collector from external circuitry.

It is the thinness of the base that makes a transistor a useful device for amplification. If the base is sufficiently thin, the most probable fate for holes injected into the base is that they will diffuse through the base into the depletion zone. Most of the holes injected into the base are transported into the collector, usually 98 to 99 percent in a typical low-cost transistor. The ratio of collector current to emitter current--the α rating--is thus 0.98 to 0.99 typically. Often, transistors are specified by their β rating, defined as the α rating divided by one minus the α rating. Typical β ratings are 20 to 200 or more.

One other typical feature of transistor construction is that doping of the emitter is more concentrated than that of the base. Therefore, the concentration of holes injected into the base from the emitter is higher than the concentrations of mobile electron majority carriers in the base.

That ensures high values for both the α and β ratings.

Applications

One of the most important uses of a transistor is amplification. Amplification occurs when the voltage output is greater than the voltage input. The gain is defined as the ratio of output voltage to input voltage.

In a typical p-n-p transistor, the base current represents the input and the collector current represents the output. (The discussion will focus on what is called a common emitter amplifier configuration.) Since a transistor has three elements, one must be common to both input and output. Thus, there is a common base, common collector, and common emitter configuration. Each has its own particular utilities, but a common emitter is typically used for voltage amplification. For example, the common collector, often referred to as the emitter-follower configuration, has a voltage gain of approximately one, but can have a sizable current gain. The common base configuration has a current gain that is approximately one, but can have a sizable voltage gain. (It must be emphasized that it is not possible to have both a high voltage and high current gain from the same transistor. That would violate conservation of energy. Power is the product of current and voltage and is equal to the rate at which electrical energy is delivered. Thus, if voltage is high, current must be low, and vice versa.)

One must properly bias and set an operating point for the transistor. The base-emitter junction must remain forward-biased by at least 0.5 volt for silicon and 0.2 volt for germanium.

Since the base-collector junction is reverse-biased, the voltage across this junction is essentially equal to the voltage from collector to emitter. A base current--typically in the range of 10 to 100 microamperes--is selected to set operating conditions for the transistor. With the base current constant, the collector current is essentially only a function of the voltage across the collector to emitter. A power supply or voltage divider often is used to set that negative voltage. The operating point needs to be set at a point somewhere in the middle of a line of constant base current in a graph of collector current versus voltage across collector to emitter. This is necessary for two reasons. First, it permits alternating current (AC) input signals to swing negatively and positively equally about the transistor's operating point. Second, it keeps the transistor's operation away from the forbidden zone, where the product of collector current and voltage across the collector to emitter would exceed the maximum power rating of the transistor. Entry into the forbidden zone for even a short time can cause irreparable damage to the transistor crystal and shorten useful lifetime.

A transistor amplifier must be made stable against thermal runaway. Any rise in temperature will cause an increase in collector current as a result of increased hole mobility.

That, in turn, will raise the temperature of the collector by joule heating of dissipated energy, thereby increasing collector current in a repetitious cycle that will eventually destroy the transistor. To prevent this from happening, a resistor in parallel with a capacitor is placed between the emitter and electrical ground. With the emitter resistor present, if the emitter voltage goes negative, the base voltage will also go negative by the same amount, thereby maintaining the forward voltage of the base-emitter junction constant, regardless of the nature of the induced changes in the base and emitter voltages. The presence of the emitter resistor will decrease the amplifier gain unless a capacitor is attached in parallel to it. This emitter capacitor provides an AC path to ground if it has a smaller capacitative reactance than the resistance of the emitter resistor. Since capacitative reactance depends inversely on the frequency of the signal, the presence of the emitter capacitor sets a lower limit on the frequency of signals that can be effectively amplified by the transistor when operating properly biased and stably against temperature and input voltage changes.

Transistors can be used in multiple stages, one transistor's output serving as the input for the next. In this way, an extremely large effective gain can be accomplished. The net gain of the multistage amplifiers would be the product of the gains of each individual stage.

Context

Many of the functions of transistors in circuits were possible prior to the development of transistor units. Although quite novel in their approach, the early transistor circuit did not represent the development of new electronic capability. Early transistors had advantages over vacuum tubes that made them more attractive as rectifiers, amplifiers, and detectors of electrical signals and as oscillators and switching elements. All those functions were possible previously, using vacuum tube technology. Because of the advantages transistors offered, transistor development proceeded rapidly from the early crude working models to the miniaturized versions available in the 1990's.

The first transistor was developed at Bell Telephone Laboratories, invented by John Bardeen and Walter H. Brattain. Although years of development were involved in the effort, the invention of the transistor is often historically placed in 1948. Compared to modern versions, this first point-contact transistor was quite bulky.

The advantages of transistors over vacuum tubes are numerous and their disadvantages few. Vacuum tubes can handle higher currents flowing through them and larger voltages applied across them than transistors. Vacuum tubes could, in principle, be repaired, whereas transistors must be replaced if they go bad. Nevertheless, that quality is not much of an advantage considering the low cost of most transistors.

The advantages of transistors over vacuum tubes include the following: Semiconducting elements are small and lightweight, permitting miniaturization of electronic equipment. Second, microminiaturization on integrated circuit chips can contain circuits composed of transistors and other elements and their leads that do the work of vacuum tubes and other components that are 100 to 100,000 times larger in size. Third, semiconducting elements, such as transistors, are solids and therefore relatively vibration-free; element vibration in vacuum tubes was the origin of troublesome microphonics in amplifiers. Fourth, transistors dissipate far less thermal energy than vacuum tubes and do not need to warm up; they operate as soon as power is applied to them. Fifth, semiconductor elements, such as transistors, are rugged and can be easily shielded from external environmental conditions. Finally, semiconductor elements, such as transistors, do not experience chemical deterioration of elements; vacuum-tube cathode deterioration will degrade tube performance and will eventually lead to early replacement of expensive components. As a result, the development of the transistor quickly led to an explosion of technological innovation in the realm of electronics.

Principal terms

ACCEPTOR: an impurity atom added to a crystalline lattice to accept electrons

BASE: the thinnest of three parts of a transistor that is doped opposite to the other two parts; it acts as the input portion of the transistor

COLLECTOR: one of three parts of a transistor, doped opposite to the base part and usually connected to the chassis for thermal energy dissipation

DONOR: an impurity atom specifically introduced into a crystal lattice to donate electrons

DOPING: the process of intentionally introducing specific impurities into a semiconducting material to alter its conducting characteristics

EMITTER: one of three parts of a transistor, doped similar to the collector

HOLE: the moving absence of an electron that would make a given lattice atom neutral with respect to the crystal lattice

MAJORITY CARRIER: the carrier of electric charge, positive or negative, that is in the majority in a portion of a semiconductor

MINORITY CARRIER: the carrier of electric charge, positive or negative, that is in the minority in a portion of a semiconductor

Bibliography

Brophy, James J. BASIC ELECTRONICS FOR SCIENTISTS. 5th ed. New York: McGraw-Hill, 1990. Despite the title, this is an excellent text on basic electronics for anyone with a decent command of high school mathematics and physics. Higher-order mathematics sections are well explained to be understandable for the layperson. Filled with practical circuits.

Fry, Jim. ELECTRONIC CIRCUITS. Benton Harbor, Mich.: Heath, 1987. Excellent for the electronics hobbyist and person beginning to investigate electronic circuits involving solid-state semiconducting components such as diodes, transistors, and integrated circuits. Includes a workbook.

Halliday, David, and Robert Resnick. FUNDAMENTALS OF PHYSICS. 3d ed. New York: John Wiley & Sons, 1988. A classic for introductory physics instruction. Contains descriptions of breaking areas of modern physics. The section on electricity and basic electrical components is thorough and easy to understand. Good text for self-study.

Murr, Lawrence E. SOLID-STATE ELECTRONICS. New York: Marcel Dekker, 1978. Highly descriptive and readable, well-illustrated discussion of the theoretical physics of semiconductors. Explains energy bands, crystal structure, conduction processes, and defects in solids.

Simpson, Robert E. INTRODUCTORY ELECTRONICS FOR SCIENTISTS AND ENGINEERS. Boston: Allyn & Bacon, 1975. Presents basic electronics and transistor circuits in a manner understandable for the introductory college student. Much of the book is readable by the average reader. Practical circuits are useful to the electronics hobbyist.

Sprott, Julian C. INTRODUCTION TO MODERN ELECTRONICS. New York: John Wiley & Sons, 1981. Complete treatise on basic electronic circuits. Particularly strong on discussion of basic transistor design, characteristics, and amplification properties. Most sections, including problems, are accessible with high school mathematics.

Suprynowicz, V. A. ELECTRICAL AND ELECTRONICS FUNDAMENTALS: AN APPLIED SURVEY OF ELECTRICAL ENGINEERING. St. Paul, Minn.: West, 1987. Most sections are understandable with high school mathematics. Lucid explanation of electrical conduction, transistor circuits, amplification, and feedback. Provides practical circuits and thorough description of electrical instrumentation, such as oscilloscopes.

Conductors and Resistors

Forces on Charges and Currents

Insulators and Dielectrics

Defects in Solids

Electrical Properties of Solids