TRANSISTOR

I

INTRODUCTION

Transistor, in electronics, common name for a group of electronic devices used as amplifiers or oscillators in communications, control, and computer systems (see Amplifier; Computer; Electronics). Until the advent of the transistor in 1948, developments in the field of electronics were dependent on the use of thermionic vacuum tubes, magnetic amplifiers, specialized rotating machinery, and special capacitors as amplifiers. See Vacuum Tubes.

Capable of performing many functions of the vacuum tube in electronic circuits, the transistor is a solid-state device consisting of a tiny piece of semiconducting material, usually germanium or silicon, to which three or more electrical connections are made. The basic components of the transistor are comparable to those of a triode vacuum tube and include the emitter, which corresponds to the heated cathode of the triode tube as the source of electrons. See Electron.

The transistor was developed at Bell Telephone Laboratories by the American physicists Walter Houser Brattain, John Bardeen, and William Bradford Shockley. For this achievement, the three shared the 1956 Nobel Prize in physics. Shockley is noted as the initiator and director of the research program in semiconducting materials that led to the discovery of this group of devices; his associates, Brattain and Bardeen, are credited with the invention of an important type of transistor.

II

ATOMIC STRUCTURE OF SEMICONDUCTORS

The electrical properties of a semiconducting material are determined by its atomic structure. In a crystal of pure germanium or silicon, the atoms are bound together in a periodic arrangement forming a perfectly regular diamond-cubic lattice (see Crystal). Each atom in the crystal has four valence electrons, each of which interacts with the electron of a neighboring atom to form a divalent bond. Because the electrons are not free to move, the pure crystalline material acts, at low temperatures, as an insulator.

III

FUNCTION OF IMPURITIES

Germanium or silicon crystals containing small amounts of certain impurities can conduct electricity even at low temperatures. Such impurities function in the crystal in either of two ways. An impurity element, such as phosphorus, antimony, or arsenic, is called a donor impurity because it contributes excess electrons. This group of elements has five valence electrons, only four of which enter into divalent bonding with the germanium or silicon atoms. Thus, when an electronic field is applied, the remaining electron in donor impurities is free to move through the crystalline material.

In contrast, impurity elements, such as gallium and indium, have only three valence electrons, lacking one to complete the interatomic-bond structure within the crystal. Such impurities are known as acceptor impurities because these elements accept electrons from neighboring atoms to satisfy the deficiency in valence-bond structure. The resultant deficiencies, or so-called holes, in the structure of neighboring atoms, in turn, are filled by other electrons. These holes behave as positive charges, appearing to move under an applied voltage in a direction opposite to that of the electrons.

IV

N-TYPE AND P-TYPE SEMICONDUCTORS

A germanium or silicon crystal, containing donor-impurity atoms, is called a negative, or n-type, semiconductor to indicate the presence of excess negatively charged electrons. The use of an acceptor impurity produces a positive, or p-type, semiconductor, so called because of the presence of positively charged holes.

A single crystal containing both n-type and p-type regions may be prepared by introducing the donor and acceptor impurities into molten germanium or silicon in a crucible at different stages of crystal formation. The resultant crystal has two distinct regions of n-type and p-type material, and the boundary joining the two areas is known as an n-p junction. Such a junction may be produced also by placing a piece of donor-impurity material against the surface of a p-type crystal or a piece of acceptor-impurity material against an n-type crystal and applying heat to diffuse the impurity atoms through the outer layer.

When an external voltage is applied, the n-p junction acts as a rectifier, permitting current to flow in only one direction (see Rectification). If the p-type region is connected to the positive terminal of a battery and the n-type to the negative terminal, a large current flows through the material across the junction. If the battery is connected in the opposite manner, as shown in the diagram in Fig. 1, current does not flow.

V

TRANSISTOR OPERATION

In the transistor, a combination of two junctions may be used to achieve amplification. One type, called the n-p-n junction transistor, consists of a very thin layer of p-type material between two sections of n-type material, arranged in a circuit as shown in Fig. 2. The n-type material at the left of the diagram is the emitter element of the transistor, constituting the electron source. To permit the forward flow of current across the n-p junction, the emitter has a small negative voltage with respect to the p-type layer, or base component, that controls the electron flow. The n-type material in the output circuit serves as the collector element, which has a large positive voltage with respect to the base to prevent reverse current flow. Electrons moving from the emitter enter the base, are attracted to the positively charged collector, and flow through the output circuit. The input impedance, or resistance to current flow, between the emitter and the base is low, whereas the output impedance between collector and base is high. Therefore, small changes in the voltage of the base cause large changes in the voltage drop across the collector resistance, making this type of transistor an effective amplifier.

Similar in operation to the n-p-n type is the p-n-p junction transistor, which also has two junctions and is equivalent to a triode vacuum tube. Other types with three junctions, such as the n-p-n-p junction transistor, provide greater amplification than the two-junction transistor.

VI

APPLICATIONS

At its present stage of development, the transistor is as effective as a vacuum tube, both of which can amplify to an upper limit of about 1000 megahertz. Among the advantages of the transistor are its small size and very small power requirements. In contrast to the vacuum tube, it does not need power for heating the cathode. Therefore, transistors have replaced most vacuum-tube amplifiers in light, portable electronic equipment, such as airborne navigational aids and the control systems of guided missiles, in which weight and size are prime considerations (see Navigation). Commercial applications include very small hearing aids and compact portable radio and television receivers. In addition, transistors have completely replaced vacuum tubes in electronic computers, which require a great many amplifiers.

Transistors are also used in miniaturized diagnostic instruments, such as those used to transmit electrocardiograph, respiratory, and other data from the bodies of astronauts on space flights (see Space Exploration). Nearly all transmitting equipment used in space-exploration probes employs transistorized circuitry. Transistors also aid in diagnosing diseases. Miniature radio transmitters using transistors can also be implanted in the bodies of animals for ecological studies of feeding habits, patterns of travel, and other factors. A recent commercial application is the transistorized ignition system in automobiles.

During the late 1960s a new electronic technique, the integrated circuit, began to replace the transistor in complex electronic equipment. Although roughly the same size as a transistor, an integrated circuit performs the function of 15 to 20 transistors. A natural development from the integrated circuit in the 1970s has been the production of medium-, large-, and very large-scale integrated circuits (MSI, LSI, and VLSI), which have permitted the building of a compact computer, or minicomputer, containing disk storage units and the communication-control systems on the same frame.

The so-called microprocessor, which came into use in the mid-1970s, is a refinement of the LSI. As a result of further miniaturization, a single microprocessor can incorporate the functions of a number of printed-circuit boards and deliver the performance of the central processing unit of a much larger computer in a hand-held, battery-powered microcomputer.