Chapter 8
FIELD EFFECT TRANSISTORS
8.1 INTRODUCTION .................................
In this and the next chapter we will examine the field effect transistor (FET) and Metal-Oxide-
Semiconductor FETs (MOSFETs). These simple devices are majority carrier devices which are
relatively simple to fabricate and are extremely versatile. FETs are now made from a wide variety
of materials (Si, SiGe, GaAs, InGaAs, GaN, SiC, etc.). Figure 8.1 shows a GaAs-based Metal
Semiconductor FET or MESFET.
The basic concept behind the FET is quite simple and is illustrated in figure 8.2. The device
consists of an active channel through which electrons (or holes) flow from the source to the drain.
The source and drain contacts are ohmic contacts. The conductivity of the channel is modulated
by a potential applied to the gate. This results in the modulation of the charge density flowing in
the channel.Itisimportanttoisolatethegatefromthechannelsothatnocurrentflowsintothe
gate. The gate isolation is done in a variety of ways, leading to a number of different devices. In
the MOSFET, the gate is isolated from the channel by an oxide. This is the basis of the silicon
devices. In the metal-semiconductor FET or MESFET, the gate forms a Schottky barrier with the
semiconductor and the gate current is small in the useful range of gate voltages. In the junction
FET or JFET, ap-njunction is used in reverse bias to isolate the gate. Heterojunction field effect
transistors (HFETs) or modulation doped FETs (MODFETs) use a large bandgap semiconductor
to isolate the gate from the active channel. In this chapter we will examine the MESFET or JFET
devices. In the next chapter we examine MOSFETs.
8.2 JFETANDMESFET:CHARGECONTROL ...................
The operation of JFETs and MESFETs are similar. The key difference is that in a JFET a
p+−nstructure is used to create a barrier between the metal gate and the semiconductor while
in a MESFET the Schottky barrier height is used. We have seen from chapter 5that the reverse
current in a Schottky junction is much larger than that in ap-njunction.AsaresulttheJFETis
especiallyusedinmaterialsforwhichitisdifficulttoproducealargeSchottkybarrier.