8.10. PROBLEMS 429
GaAs
GaAs
5 nm
30 nm
2 nm δ -Doping
AlGaAs
15 nm
AlGaAs
Triangular QW Flat QW
m* = 0.2m 0
m* = 0.64m 0
Figure 8.50: Figure for problem 8.27.
Problem 8.28Consider the AlInAs/GaInAs HEMT, shown in figure 8.51 where the
AlInAs is delta-doped with Si to the level of 5 × 1012 cm−^2. The spacer layer thickness is
5nm. You may assume that the Schottky barrier height is determined by Fermi level
pinning of the surface and is 1 eV. Next, I consider the same structure grown onp+
GaInAs(EFp≈EV), where the thickness of the buffer is 1 μmto enable threshold voltage
adjustment. What is the sheet charge in this structure compared to the structure grown on
undoped GaInAs? Last, but not least consider a forward bias of 0.8 eV applied to the
conventional HEMT structure. Assuming an effective mass of 0. 5 m 0. Assume tunneling
as the transport mechanism. Calculate the position of the Fermi level around the donor by
balancing the current in with that out and linking the resident electron concentration to
EF.UseEg(GaInAs)=0. 7 eVandEg(AlInAs)=1. 4 eV,andΔEC=0. 5 eV.
Problem 8.29In this problem we will consider the effect of the source resistance on the
device transconductance. Consider ann-channel GaAs MESFET with the following
