SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

360 CHAPTER 8. FIELD EFFECT TRANSISTORS

ID

ID

ID

VGS = 0

negative VGS

large negative VGS

Linear I-V

VDS

VDS

Suppression

of current

VDS

Device turned

OFF

S D

S D

S D

VDS is small

Figure 8.4: (a) Depletion width and channel in a JFET or MESFET under zero gate bias. The
channel has a large opening. Such a device is called a depletion-mode device; (b) the device
with a negative gate bias showing reduction in the channel opening and current; (c) the gate bias
is large and negative and the channel is pinched off with current in the channel zero.

An important consideration in JFET or MESFET technologies is that the gate current be negli-
gible. the requires that the gate be biased appropriately. This also requires a large built-in voltage
or Schottky barrier height. For small gap semiconductors (e.g. InGaAs, InSb, etc.) this may not
be possible.

Example 8.1Consider ann-MESFET made from GaAs doped at 1017 cm−^3. Calculate

the gate current density under normal operation if:

(i) the gate is made from a Schottky metal with a barrierφb=0. 8 V;

(ii) the gate is made from a heavily dopedp+GaAs.