8.10. PROBLEMS 427
10 nm InGaAs
Delta doping
5 x 10^12 cm-2 20 nm
InGaAs buffer
10 nm InAlAs
Spacer=5nm Ideal gm profile
Figure 8.49: Figure for problem 8.21.
tothestrainintheAlGaN)contribute1x10^13 cm−^2 and 5× 1012 cm−^2 electrons to this
charge, respectively.
(a) Draw the band diagram of this structure assuming that the surface pinning is 1.8 eV
and the conduction band discontinuity of 0.7 eV.
(b) When measuring device A, I find the output conductance is high. I therefore grow a
different device on a relaxed AlGaN buffer to reduce substrate injection and grow the
strained 10 nm GaN QW followed by the same AlGaN cap I grew before (ie. 200A ̊).
Draw the band diagram of device B by calculating and showing the relevant voltages and
changes in the system. How much electron charge is available? How would you expect the
output conductance to change?
(c) What is the main problem in device B? Suggest a qualitative solution to this problem.
Problem 8.21I make a HEMT as shown in figure 8.49 and get a gmversus VGScurve
that deviates from the ideal one. Draw the charge, electric field and energy band profiles
for this structure along the line AA’. What is the transconductance curve you measure and
why? The electron velocity in the structure is 2 x 10^7 cm−^2 .AssumeΔEC=0.5eV,
Schottky barrier height,φB=0.8eV,andΔd = 5 nm, whereΔd is the mean distance
between the electron gas and hetero-interface. You may also assume that the transistor
operates in the fully saturated region.
Problem 8.22Consider a GaAsn-channel MESFET operating under conditions such that
one can assume that the field in the channel has a constant value of 5.0 kV/ cm−^1 .The
channel length is 2.0μm. Calculate the transit time for an electron to traverse the channel
if one assumes a constant mobility of 7500 cm^2 /V·s. What would the time be if the correct
velocity-field relations plotted in chapter B were used?
