7.7. PROBLEMS 353
VS/2
VS
WC/2 WC
VS/2
VS
WC/2 WC
Figure 7.27: Figure for problem 7.17
1000 A 3500 A
Emitter Base Collector
Figure 7.28: Figure for problem 7.18.
- Next, I grade the InGaAs to InP in the collector, rather than in the base. Assume the
doping in the collector to beND=10^16 cm−^3. What is the length of the parabolic
grade necessary to eliminate the barrier atVBC=0Why do you want the grading
distance to be minimum?
- How can you shorten the grading distance in the collector by half? What is the
penalty you pay?
- Calculate the collector delay time for the abrupt and graded cases.
For the graded case, assume that the velocity profile is as shown in figure 7.29. For
the abrupt case, assume a saturation velocityvs 1. In the figure,t 0 is the length of the
grade calculated in part (b),vs 2 =4. 107 cm/s,andvs 1 =10^7 cm/s.Useacollector
width of 3500A. ̊
- How do you expect the Kirk current threshold in the graded case to compare with the
ungraded one? Assume the same velocity profiles as above.
Problem 7.19The base-collector junction in a bipolar transistor has the structure shown
in figure 7.30. A p-type doping sheet is added at the i-n junction to create a 0.3 eV drop
across the intrinsic region. Calculate the density of this doping sheet. This is a ballistic
