Chapter 9
FIELD EFFECT TRANSISTORS:
MOSFET
9.1 INTRODUCTION .................................
The basic principles of the field effect transistor have been discussed in chapter 8. A key
requirement for a FET is zero or negligible gate leakage current. To ensure this one needs some
kind of barrier for electron (hole) from the gate to the source, channel and drain. In the devices
in chapter 8 this barrier is provided by a Schottky barrier (orp+−nbuilt-in voltage.) or a large
bandgap semiconductor. Of course one may ask: Why not use an insulator to isolate the gate
from the channel? Obviously an insulator would be an ideal choice but so far only on Si has it
been possible to grow a high quality and reliable insulator. This has led MOSFET technology to
become so dominant. In many ways the MOSFET is an ideal device since a large gate
bias can be applied to “invert” the bands and induce electron (or holes) in a channel without the
concern of gate leakage. An example of a MOSFET today is shown in cross-section in figure 9.2.
Over the last several years steady progress has been made on using the MOSFET concept with
other semiconductors, notably GaAs. Indeed GaAs NMOSFETs have been demonstrated with
channel mobilities much higher than those in NMOS FET based on Si. However, widespread
use of such devices is still not near.
In this chapter we will first discuss the MOS capacitor and examine how mobile charge is
induced in the the MOS structure by “inversion.” It is important to note that in a MOSFET,
unlike the MESFET or JFET, channel charge is induced electrostatically by the gate by using
the gate as a capacitor with the gate metal electrode and the semiconductor being the other plate
of the capacitor without the need for doping, however the addition of dopants in the channel
provides additional control on the charge. Once we discuss the MOS capacitor we will examine
the operation of the MOSFET.