ELEC421/6221 Submission deadline October 29, 2014
Assignment 4
1. A uniformly doped silicon npn transistor is to be biased in the forward-active mode with the BC junction reverse biased by 3V. The metallurgical base width is 1.10 µm. The transistor dopings are
N
E

10
17 cm

3
, N
B

10
16 cm

3 , and
N
C

10
15 cm

3 . (a) For T = 300K, calculate the BE voltage at which the minority carrier electron concentration at
x
= 0 is
10 percent of the majority carrier hole concentration. (b) At this bias, determine the minority carrier hole concentration at x’ = 0. (c) Determine the neutral base width for this bias.
2. Consider a uniformly doped npn bipolar transistor at T = 300 K with the following parameters:
N
E

10
18 cm

3
D
E

E 0

8 cm
2
/ s

10

8 s x
E

0 .
8
 m
N
B

5

10
16 cm

3
D
B

B 0

15 cm
2
/ s

5

10

8 s x
B

0 .
7
 m
N
C

10
15 cm

3
D
C

12 cm
2

C 0
J r 0

10

7

3

10

8 s
/ s
A / cm
2
For
V
BE

0 .
60 V
and
V
CE

5 V
, calculate (a) the currents
J nE
, J pE
, J nC
, and J
R
and
(b) the current gain factors 
,

T
,

,

, and
 .
3. (a) The recombination current density,
J
, in a npn silicon bipolar transistor at T = 300 r 0
K is
J r 0

5

10

8
A / cm
2 . The uniform dopings are
N
E

10
18 cm

3
, N
B

5

10
16 cm

3 , and
N
C

10
15 cm

3 .
Other parameters are
D
E

10 cm
2
/ s , D
B

25 cm
2
/ s ,

E 0

10

8 s , and

B 0

10

7 s
.
Determine the neutral base width so that the recombination factor  
0 .
995
when
V
BE

0 .
55 V
.
(b) If
J
remains constant with temperature, what is the value of r 0
 when
V
BE

0 .
55 V
for the case when the temperature T = 400 K? Use the value of x
B
determined in part (a).