8.10. PROBLEMS 423
Problem 8.2By drawing the band profile of a MESFET, discuss the restrictions on the
gate bias values that can be allowed.
Problem 8.3Consider ann-channel Si JFET at 300 K with the following parameters:
p+-doping, Na =5× 1018 cm−^3
n-doping, Nd =10^17 cm−^3
Channel thickness, h =0. 5 μm
(a) Calculate the internal pinch-off for the device. (b) Calculate the gate bias required to
make the width of the undepleted channel 0.25μm.
Problem 8.4Consider a GaAs JFET with the same characteristics as those of the Si
device in problem 8.3. Repeat the calculations for this GaAs device.
Problem 8.5Ann-type In 0. 53 Ga 0. 47 As epitaxial layer doped at 1016 cm−^3 is to be used
as a channel in a FET. A decision is to be made whether the JFET or MESFET technology
is to be used for the device. In the JFET technology ap+region can be made with a doping
of 5 × 1017 cm−^3. In the MESFET technology a Schottky barrier with a height of 0.4 V is
available. Which technology will you use? Give reasons considering gate isolation issues.
(R∗=5Acm−^2 K−^2 ;Dp=20cm^2 /s;Dn=50cm^2 /s;Ln=5μm;Lp=5μm.)
Problem 8.6Consider ap-channel Si JFET with the following parameters:
p-doping, Na =5× 1016 cm−^3
n+-doping, Nd =5× 1018 cm−^3
Channel depth, h =0. 25 μm
(a) Calculate the internal pinch off for the device as well as the gate bias needed for pinch
off.
(b) Calculate the width of the undepleted channel for gate biases ofVGS=1VandVGS=
2VforVDS=0.
Problem 8.7Design an AlInAs/GaInAs HEMT for maximumgmsuch that charge in the
channel is 3 × 1012 cm−^2. Assume the doping in the AlInAs is 5 × 1012 cm−^2. Assume
the surface barrier is 0.8 V andΔEC=0. 55 eV. Also, assume that the substrate is GaInAs
and is doped p-type such thatEF=EVin the substrate. Assume the buffer is 0. 5 μm
thick. Also assume the minimum spacer allowed is 2 nm. What is the current available
from the device. At zero gate bias assuimng a gate length of 1 μm. The velocity-field wave
is shown in figure 8.48.
Problem 8.8Consider ann-channel GaAs MESFET with the following parameters:
Schottky barrier height, φb =0.8V
Channel doping, Nd =5× 1016 cm−^3
Channel width, h =0. 8 μm
