Electrical Articles: TRANSISTORS

INTRODUCTION TO TRANSISTORS

The discovery of the first transistor in 1948 by a team of physicists at the Bell Telephone Laboratories

sparked an interest in solid-state research that spread rapidly. The transistor, which began as a simple

laboratory oddity, was rapidly developed into a semiconductor device of major importance. The transistor

demonstrated for the first time in history that amplification in solids was possible. Before the transistor,

amplification was achieved only with electron tubes. Transistors now perform numerous electronic tasks

with new and improved transistor designs being continually put on the market. In many cases, transistors are

more desirable than tubes because they are small, rugged, require no filament power, and operate at low

voltages with comparatively high efficiency. The development of a family of transistors has even made

possible the miniaturization of electronic circuits. Figure 2-1 shows a sample of the many different types of

transistors you may encounter when working with electronic equipment.

have infiltrated virtually every area of science and industry, from the family car to satellites.

Even the military depends heavily on transistors. The ever increasing uses for transistors have created an

urgent need for sound and basic information regarding their operation.

From your study of the PN-junction diode in the preceding chapter, you now have the basic knowledge

to grasp the principles of transistor operation. In this chapter you will first become acquainted with the basic

types of transistors, their construction, and their theory of operation. You will also find out just how and why

transistors amplify. Once this basic information is understood, transistor terminology, capabilities,

limitations, and identification will be discussed. Last, we will talk about transistor maintenance, integrated

circuits, circuit boards, and modular circuitry.

TRANSISTOR FUNDAMENTALS

The first solid-state device discussed was the two-element semiconductor diode. The next device on our

list is even more unique. It not only has one more element than the diode but it can amplify as well.

Semiconductor devices that have-three or more elements are called TRANSISTORS. The term transistor

was derived from the words TRANSfer and resISTOR. This term was adopted because it best describes the

operation of the transistor - the transfer of an input signal current from a low-resistance circuit to a high-

resistance circuit. Basically, the transistor is a solid-state device that amplifies by controlling the flow of

current carriers through its semiconductor materials.

There are many different types of transistors, but their basic theory of operation is all the same. As a

matter of fact, the theory we will be using to explain the operation of a transistor is the same theory used

earlier with the PN-junction diode except that now two such junctions are required to form the three

elements of a transistor. The three elements of the two-junction transistor are (1) the EMITTER, which gives

off, or emits," current carriers (electrons or holes); (2) the BASE, which controls the flow of current carriers;

and (3) the COLLECTOR, which collects the current carriers.

CLASSIFICATION

Transistors are classified as either NPN or PNP according to the arrangement of their N and P

materials. Their basic construction and chemical treatment is implied by their names, "NPN" or "PNP." That

2-3

is, an NPN transistor is formed by introducing a thin region of P-type material between two regions of

N-type material. On the other hand, a PNP transistor is formed by introducing a thin region of N-type

material between two regions of P-type material. Transistors constructed in this manner have two PN

junctions, as shown in figure 2-2. One PN junction is between the emitter and the base; the other PN

junction is between the collector and the base. The two junctions share one section of semiconductor

material so that the transistor actually consists of three elements.