426 CHAPTER 8. FIELD EFFECT TRANSISTORS
Problem 8.15Consider ann-channel GaAs MESFET at 300 K with the parameters of
problem 8.14. Calculate the transconductance of the device in the saturation region for the
gate biasesVGS=−1.5 V andVGS=−2.0 V. Express the results in terms of mS/mm.
Problem 8.16Consider ann-channel Si MESFET at 300 K. The following parameters
define the MESFET:
Schottky barrier height, φb =0.8V
Channel mobility, μn = 1000 cm^2 /V·s
Channel doping, Nd =5× 1016 cm−^3
Channel length, L =1. 5 μm
Channel depth, h =0. 25 μm
Gate width, Z =25μm
(a) Calculate the turn-on voltageVTfor the structure.
(b) CalculateVDS(sat)at a gate bias ofVGS= 0. Also calculate the device
transconductance.
(c) If the device turn-on voltage is to beVT=−2.0 V, calculate the additional doping
needed for the channel.
Problem 8.17In a MESFET, as the gate length shrinks, the channel doping has to be
increased. Discuss the reasons for this.
Problem 8.18Derive and plot the I-V curves for a GaAs MESFET with
ND=5× 1017 cm−^3 , and a channel thickness of 50 nm. Assume a two-region mobility
model, with a saturated velocityvsat=2× 107 cms. Plot these curves for a gate length of
1 μmand 10 μm, with maximum drain voltage,VDS=2V, and maximum gate voltage,
VGS=0V. Assume the electron mobility in the doped GaAs to be 5000 cm
2
Vs,anda
Schottky barrier height of 1 eV for the gate metal. Normalize the current to unit
with(mA/mm).
Problem 8.19Consider an Al 0. 3 Ga 0. 7 N/GaN HEMT structure. Assume that the Schottky
barrier is 1.7 eV on AlGaN and 0.9 eV on GaN.
(a) How does the sheet charge at the AlGaN/GaN junction vary with the thickness of the
AlGaN barrier? Plot the sheet charge nsfor AlGaN thickness up to 40 nm.
(b) Plot the band diagram of an AlGaN/GaN HEMT with a 30 nm AlGaN cap at zero gate
bias, and at pinch-off. What is the pinch-off voltage?
(c) Now, a 5 nm layer of GaN is addedabovethe AlGaN barrier. Calculate and plot the
band diagram of this structure at zero bias and at pinch-off. What is the effective
Schottky-barrier height in these two cases? Do you expect the gate leakage of this diode to
be different from the AlGaN/GaN structure? Why (not)?
Problem 8.20I grow an AlGaN on GaN HEMT (Device A) where the net polarization
charge, Qπ,NET=1.5x10^13 cm−^2. The spontaneous and piezoelectric polarizations (due
