SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

7.8. DESIGN PROBLEMS 355

V (cm/s)

2 x 10

7

1 x 10^7

.3

Collector Voltage Drop (eV)

Figure 7.31: Figure for problem 7.19.

collector transistor. The velocity field profile for this material is given below. This is

expressed in terms of the velocity versus voltage drop in collector. The reason for the

sudden drop is theΓ−Lintervalley transfer of electrons in GaAs. Of course, this is an

idealized profile to make the problem tractable.

1. Calculate the transit delay for this structure atVCB=0V.


2. I now apply a reverse bias ofVCB=1Von the collector-base junction. Calculate the
   transit delay at this bias. Assume that the depletion thickness is small in the n++ and
   p++ regions, and that the n-region is just fully depleted atVCB=0V.

7.8 DESIGN PROBLEMS ...............................

Problem 7.1Design ap+nSi diode that can be used in a digital system operating at 1

gigabit per second. Assume that the minority carrier lifetime is 107 s. Other parameters

can be obtained from the text. Plot the I-V characteristics of this diode. At what applied

reverse bias would the entiren-region be depleted in this diode?

Problem 7.2ASip+ndiode is to be used in the reverse bias state(VR=5V)as a

high-speed detector. Design the diode so it can operate up to a frequency of 5 GHz. Make

reasonable assumptions for the material parameters.