Chapter 6: 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 through 5. Although there are important differences between the two types of devices, there are also many similarities, which 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. 6.1a, whereas the JFET transistor is a voltage-controlled device as shown in Fig. 6.1b.
In other words, the current IC in Fig. 6.1a is a direct function of the level of IB. For the FET the current ID will be a function of the voltage VGS applied to the input circuit as shown in Fig. 6.1b. In each case the current of the output circuit is controlled by a parameter of the input circuit—in one case a current level and in the other an applied voltage.
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 indicates 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.

Figure 6.1
(a) Current-controlled and (b) voltage-controlled amplifiers The term field effect in the name deserves some explanation. We are all familiar with the ability of a permanent magnet to draw metal filings to itself without the need for actual contact. The magnetic field of the permanent magnet envelopes the filings and attracts them to the magnet because the magnetic flux lines act so as to be as short as possible. For the FET an electric field is established by the charges present, which controls 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 device with a range of applications similar to one already introduced to compare some of the general characteristics of one to those of the other:
One of the most important characteristics of the FET is its high input impedance.


At a level of 1 M 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 for FETs for the same change in the 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 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.
Three types of FETs are introduced in this chapter: the junction field-effect transistor (JFET), the metal-oxide-semiconductor field-effect transistor (MOSFET), and the metal-semiconductor field-effect transistor (MESFET). 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). The MESFET is a more recent development and takes full advantage of the high-speed characteristics of GaAs as the base semiconductor material. Although currently the more expensive option, the cost issue is often outweighed by the need for higher speeds in RF and computer designs.



Once the FET construction and characteristics have been introduced, the biasing arrangements will be covered in Chapter 7. 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.



Ian Munro Ross and G. C. Dacey were instrumental in the early stages of development of the field-effect transistor. Take particular note of the equipment used in 1955 for their research.

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