SEMICONDUCTOR DEVICE PHYSICS AND DESIGN

290 CHAPTER 6. BIPOLAR JUNCTION TRANSISTORS

6.7 PROBLEMS ....................................

Temperature is 300Kunless otherwise specified.

• Section 6.3

Problem 6.1An npn HBT, shown in figure 6.24, is illuminated leading to an optical

generation of (^1020) cm^13 s.τN=τP=1ns. Assume Shockley boundary conditions.

1. Assume that all the light is absorbed in the collector depletion region. How will
   IB,IC,andIEbe different from the case where there is no illumination?


2. Now assume that all the light is absorbed in the (short) neutral base region. Again
   explain howIB,IC,andIEbe different from the case where there is no
   illumination? Assume that the emitter injection efficiency is 1.

A

Emitter

n

Base

p

Collector

n

B

Emitter

n

Base

p

Collector

n

Figure 6.24: Figure for problem 6.1.

Problem 6.2Consider annpntransistor with the following parameters:

Db=20cm^2 s−^1 De=10cm^2 s−^1

Nde=5× 1018 cm−^3 Nab=5× 1016 cm−^3

Ndc=5× 105 cm−^3 Wb=1. 0 μm

τB=τE=10−^7 s n^2 i=2. 25 × 1020 cm−^6

A=10−^2 cm^2