Transistor

I

Introduction

Transistor, any of various electronic devices used as amplifiers or oscillators in communications, control, and computer systems (see Electronics). Since its advent in 1948 the transistor has largely replaced thermionic 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 semiconductor, 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.

The transistor was developed at Bell Laboratories by the American physicists Walter Houser Brattain, John Bardeen, and William Bradford Shockley. For this achievement and their pioneering research on semiconductors, the three shared the 1956 Nobel Prize for Physics. Shockley is noted as the initiator and director of the research programme 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. Each atom in the crystal has four valence electrons, each of which interacts with the electron of a neighbouring atom to form a 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 one 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 bonding with the germanium or silicon atoms. Thus, when an electronic field is applied, the remaining electrons in donor impurities are 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 neighbouring atoms to satisfy the deficiency in valence-bond structure. The resultant deficiencies, or so-called holes, in the structure of neighbouring atoms, in turn, are filled by other electrons and so on. 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 also be produced 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 (figure 1), 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, current does not flow.