Field-Effect Transistors

         

 The field-effect transistor (FET) is a three-terminal device used for a variety of applications that match, to a large extent, those of the BJT transistor described in Chapters

3 and 4. Although there are important differences between the two types of devices,

there are also many similarities that will be pointed out in the sections to follow.

          

The primary difference between the two types of transistors is the fact that the

BJT transistor is a current-controlled device as depicted in Fig. 5.1a, while the JFET

transistor is a voltage-controlled device as shown in Fig. 5.1b. In other words, the current

IC in Fig. 5.1a is a direct function of the level of IB. For the FET the current I

will be a function of the voltage VGS applied to the input circuit as shown in Fig. 5.1b.

In each case the current of the output circuit is being controlled by a parameter of the

input circuit—in one case a current level and in the other an applied voltage.

Drs. Ian Munro Ross (front) and 

G. C. Dacey

Just as there are npn and pnp bipolar transistors, there are n-channel and p-channel

field-effect transistors. However, it is important to keep in mind that the BJT transistor

is a bipolar device—the prefix bi- revealing that the conduction level is a function

of two charge carriers, electrons and holes. The FET is a unipolar device

depending solely on either electron (n-channel) or hole (p-channel) conduction.

The term field-effect in the chosen name deserves some explanation. We are all

familiar with the ability of a permanent magnet to draw metal filings to the magnet

without the need for actual contact. The magnetic field of the permanent magnet has

enveloped the filings and attracted them to the magnet through an effort on the part

of the magnetic flux lines to be as short as possible. For the FET an electric field is

established by the charges present that will control the conduction path of the output

circuit without the need for direct contact between the controlling and controlled

quantities.

There is a natural tendency when introducing a second device with a range of applications

similar to one already introduced to compare some of the general characteristics

of one versus the other. One of the most important characteristics of the FET

is its high input impedance. At a level of 1 to several hundred megohms it far exceeds

the typical input resistance levels of the BJT transistor configurations—a very important

characteristic in the design of linear ac amplifier systems. On the other hand,

the BJT transistor has a much higher sensitivity to changes in the applied signal. In

other words, the variation in output current is typically a great deal more for BJTs

than FETs for the same change in applied voltage. For this reason, typical ac voltage

gains for BJT amplifiers are a great deal more than for FETs. In general, FETs are

more temperature stable than BJTs, and FETs are usually smaller in construction than

BJTs, making them particularly useful in integrated-circuit (IC) chips. The construction

characteristics of some FETs, however, can make them more sensitive to handling

than BJTs.

Two types of FETs will be introduced in this chapter: the junction field-effect

transistor (JFET) and the metal-oxide-semiconductor field-effect transistor (MOSFET).

The MOSFET category is further broken down into depletion and enhancement

types, which are both described. The MOSFET transistor has become one of the most

important devices used in the design and construction of integrated circuits for digital

computers. Its thermal stability and other general characteristics make it extremely

popular in computer circuit design. However, as a discrete element in a typical

top-hat container, it must be handled with care (to be discussed in a later

section).

Once the FET construction and characteristics have been introduced, the biasing

arrangements will be covered in Chapter 6. The analysis performed in Chapter 4 using

BJT transistors will prove helpful in the derivation of the important equations and

understanding the results obtained for FET circuits.