8.10. PROBLEMS 425
Problem 8.10In the text we used the constant-mobility model to obtain the relation
between the drain current and the gate and drain voltages below pinch-off. Obtain a result
for the depletion regionh−h(x)as a function ofx(distance from source to drain) for a
gate biasVGSand a drain biasVDS. Use the symbols used in the text for other device
parameters.
Problem 8.11Consider the design of twon-channel GaAs MESFETs with the following
parameters:
Schottky barrier height, φb =0.8V; 0 .8V
Channel doping, Nd =2× 1016 cm−^3 ;2× 1017 cm−^3
The first sequence belongs to one device and the second sequence to the other. Calculate
the depths of the channel needed for each device so that the devices are just turned off in
the absence of any gate bias.
Problem 8.12Consider ann-channel GaAs MESFET at 300 K with the following
parameters:
Schottky barrier height, φb =0.8V
Channel thickness, h =0. 25 μm
Calculate the channel doping needed so that the device turns off at a gate bias ofVGS=VT
=0.5V.
Problem 8.13Consider ann-channel Si MESFET at 300 K with the following known
parameters:
Barrier height, φb =0.7V
Channel doping, Nd =10^16 cm−^3
It is found that when a gate bias ofVGS=−0.3 V is applied (VDS= 0), the channel is just
fully depleted. Calculate the channel depthhfor the device.
Problem 8.14Consider a GaAsn-channel MESFET at 300 K with the following
parameters:
Schottky barrier height, φb =0.8V
Electron mobility, μn = 6000 cm^2 /V·s
Channel width, Z =25μm
Channel length, L =1. 0 μm
Channel depth, h =0. 25 μm
Channel doping, Nd =1. 0 × 1017 cm−^3
(a) Calculate the gate biasVGS=VTneeded for the device to just turn off.
(b) CalculateVD(sat)for gate biases ofVGS=−1.5 V andVGS=−3.0 V.
(c) Calculate the saturation drain current for the cases considered in part b.
