SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

358 CHAPTER 8. FIELD EFFECT TRANSISTORS

Gate controlled carrier density in the channel

Source Drain

Gate

Figure 8.2: The physical principle behind the FET involves the use of a gate to alter the charge
in a channel. Depending upon the method used for isolation of gate different FETs arise.

The thickness of the doped active channel ishas shown in figure 8.3 andVbiis the built-
in voltage. The gate bias required to pinch off the channel is simply given by the depletion
approximation

h=

{

2 (Vbi−VGS)

eNd

} 1 / 2

(8.2.1)

It is possible that the built-in voltage may by itself pinch the channel off. In ann-channel
device, if the device is not pinched off byVbi, then a negative gate bias will cause pinch-off.In
ap-channeldevice,apositivebiasisneededforpinch-off.
The pinch-off voltageVp(called the intrinsic pinch-off voltage) is defined by

Vp=

eNdh^2

2 

(8.2.2)

and the gate bias needed for pinch-off for then-channel device is

VT=Vbi−Vp (8.2.3)

whereVTis called the threshold voltage for the device. If the voltageVpis smaller than the
built-in potentialVbi, the device channel is completely depleted in the absence of a gate bias.
A positive gate bias (forn-channel devices) can allow the channel to have free charge and be
conducting. Such devices are said to be enhancement-mode devices. On the other hand, ifVp
is larger thanVbi, the device has free charge in the channel atVG= 0 since the channel is only
partially depleted. A negative gate bias will then turn the device off, i.e., deplete the channel.
Such devices are said to operate in the depletion mode. Electronic circuits may use enhancement-
or depletion-mode devices or even combinations of them, depending upon the application.