Download 971 Quiz 01

971 Quiz 01
1. The following figure shows a silicon bar that is subjected to electron injection from
the left and holes injection from the right.
5 1016 carriers
electrons
cm3
Silicon
0m n  1.45 1010 carriers
i
11016 carriers
cm 3
2 m
cm3
a. Determine the minority carrier concentration at x  0 .
p n 0 ( x  0)  nn 0 ( x  0)  ni2
Ans:
(1.45  1010 ) 2
 4.2  10 3 carriers / cm 3
16
5  10
b. Determine the minority carrier concentration at x  2 m .
p n 0 ( x  0) 
p n 0 ( x  2m)  nn 0 ( x  2m)  ni2
Ans:
p n 0 ( x  2m) 
(1.45  1010 ) 2
 2.1  10 4 carriers / cm 3
16
1  10
c. Determine the electron diffusion current density J ndiff ( Dn  32cm 2 / s ).
J ndiff  qDn
Ans:
n ( x  2m)  nno ( x  0m)
dn
 1.6  10 19  32  no
dx
2m
 1.6  10 19  32 
1  1016  5  1016
 102.4  101  1.02  10 3 ( Amp / cm 2 )
2  10 6  100
d. Determine the hole diffusion current density J pdiff ( D p  12cm 2 / s ).
J pdiff  qD p
p ( x  2m)  p no ( x  0 m)
dp
 1.6  10 19  12  no
dx
2 m
2.1  10 4  4.2  10 3
1.68  10 4
19


1
.
6

10

12

2  10 6  100
2  10  4
 1.6  10 10 ( Amp / cm 2 )
Ans:  1.6  10 19  12 
 43  10 11
e. Determine the electric field E (  n  1350cm 2 / V  s  p  480cm 2 / V  s ).
Ans:
dn
dp
 D p )  q[nno ( x  0)  n  p no ( x  0)  p ]E  0
dx
dx
3
10
 1.02  10  (1.6)  10  1.02  10 3
J total  J diff  J drift  q( Dn
J diff
q[nno ( x  0)  n  p no ( x  0)  p ]E  1.02  10 3
E ( x  0m) 
1.02  10 3
1.02  10 3

 94
1.6  10 19  [1350  5  1016  480  4.2  10 3 ] 1.6  10 19  6.75  1019
Or you may also calculate
J total  J diff  J drift  q( Dn
dn
dp
 D p )  q[nno ( x  2m)  n  p no ( x  2m)  p ]E  0
dx
dx
q[nno ( x  2m)  n  p no ( x  2m)  p ]E  1.02  10 3
1.02  10 3
1.02  10 3
E ( x  2m) 

 472
1.6  10 19  [1350  1  1016  480  2.1  10 4 ] 1.6  10 19  [1.350  1019 ]
f. Determine the electron drift current density J ndrift .
Ans: J ndrift  q[nno ( x  0) n ]E  1.6 10 19  [1350  5 1016 ]  94  1.015 103
J ndrift  q[nno ( x  2m) n ]E  1.6 10 19  [1350 11016 ]  472 
g Determine the hole drift current density J pdrift .
Ans: J pdrift  q[ pno ( x  0) p ]E  1.6 10 19  [4.3 103  480]  94 
J pdrift  q[ pno ( x  2m) p ]E  1.6 10 19  [2.110 4  480]  472  7.6 10 10
h. Determine the total current density J .
Ans: The silicon bar do not connect with the other circuit (open circuit) so the total
current should be zero.
i. Which type the silicon bar is?
Ans: The silicon bar only injects electrons, so it becomes N-Type semiconductor.
2. True and false
(1) If we introduce some 3-valence atom into a Si-semiconductor, the majority
carriers of this adopted semiconductor are electrons and it usual denoted by n po .
Ans：3-valence atom into a Si-semiconductor the majority carriers are holes we write
it as p po .
(2) The depletion layer in a p-n junction contains a few numbers of free carriers such
as electrons and holes.
Ans：The depletion layer means no free carries.
(3) There are two types of carriers in a semiconductor. The electrons drift current
direction is opposite to that of the applied electric field. But the holes drift current
is in the same direction of the applied electric field.
Ans：The drift current is always in the same direction of applied electric field.
(4) The drift current in a semiconductor is relational to the concentration gradient.
n
Jn
x
The following electron concentration can drive the drift current in the x direction.
Ans： The drift current is relative with electric field not the concentration gradient.
In this statement, the drift current should be diffusion current.