Figure 2-1.An assortment of different types of transistors.
Transistors 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.
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.
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