SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

8.2. JFET AND MESFET:CHARGE CONTROL 361

VDS =VDS1

VDS =VDS2>VDS1

ID

ID

ID

VGS = V

VGS = V

VDS

VDS

VDS

VDS =VDS3>VDS2

VGS = V

S

Pinch-off at drain

(a)

(b)

(c)

Current saturates

D

S D

S D

Figure 8.5: The effect of increased drain bias at a fixed gate bias. (a) the drain bias is small;
(b) the drain bias is increased and the channel is constricted near the drain; (c) the drain bias is
increased to the point that the channel is pinched off at the drain side. The drain current saturates
as shown.

You may use the following parameters:

Dp =20cm^2 /s

Lp =1. 0 μm

A∗ =8Acm−^2 K−^2

The gate current under normal operation is just the reverse-bias current of the junction

between the gate and the semiconductor. For the Schottky case we have (see chapter 5)

Js=A∗T^2 exp

(

−

eφb

kBT

)