ece305#homework#solutions:week#8#week8hwsolutions_s15.pdf · sε 0 qn a v bi ⎡ ⎣ ⎢ ⎤ ⎦...
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ECE 305 Spring 2015
ECE-‐305 Spring 2015 1
ECE 305 Homework SOLUTIONS: Week 8
Mark Lundstrom Purdue University
1) The sketch below shows the carrier concentrations in a PN junction at room
temperature. Answer the following questions.
1a) Is the diode forward or reverse biased? Explain your answer.
Solution: Forward biased because there are excess electrons on the P-‐side and excess holes on the N-‐side.
1b) What is the acceptor concentration on the P-‐side?
Solution: NA = 1016 cm-3
1c) What is the donor concentration on the N-‐side?
Solution: ND = 1014 cm-3
1d) What is the intrinsic carrier concentration?
Solution:
n0 p0 = ni2
On the P-‐side: n0 p0 = 1016 ×107 = 1023 ni = 10
23 = 3.16×1011 cm-3
On the N-‐side: n0 p0 = 1014 ×109 = 1023 ni = 10
23 = 3.16×1011 cm-3
ni = 3.16×10
11 cm-3
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HW8 solutions (continued): 1e) Do low level injection conditions apply?
Solution: YES. On the P-‐side:
Δn −xp( ) = 1010
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HW8 solutions (continued):
2a) Is the diode forward or reverse biased?
Solution: Forward biased because
Fn > Fp . 2b) What is the value of the applied bias?
Solution:
qVA = Fn − Fp
VA = +0.5 V
2c) What is the bandgap of the semiconductor?
Solution: Reading from the graph:
EC − EV = 1.25 eV
2d) What is the built-‐in potential of the junction.
Solution: From the plot:
Vj =Vbi −VA = 0.25 V
Since: VA = +0.5 V
Vbi =Vj +VA = 0.75 V
Vbi = 0.75 V
3) A silicon diode is asymmetrically doped at N D = 10
19 cm-‐3 and N A = 1016 cm-‐3. (Note
that at N D = 1019 the semiconductor is on the edge of degeneracy, but we can assume
that non-‐degenerate carrier statistics are close enough for this problem.) Answer the following questions assuming room temperature. Assume that the minority electron and hole lifetimes are
τ n = τ p = 10
−6 s. The lengths of the N and P regions are
L = 500 µm and L >> xp ,xn .
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ECE-‐305 Spring 2015 4
HW8 Solutions (continued): 3a) Find the zero-‐bias depletion region capacitance per cm2 of diode area.
Solution: The junction capacitance per unit area is:
CJ =
Ksε0W VA( )
The depletion region width for a one-‐sided junction is:
W VA = 0( ) = 2KSε0qNAVbi
⎡
⎣⎢
⎤
⎦⎥
1/2
The built-‐in potential is
Vbi =kBTqln NDNA
ni2
⎛⎝⎜
⎞⎠⎟
Putting in numbers, we find:
Vbi =kBTqln NDNA
ni2
⎛⎝⎜
⎞⎠⎟= 0.026 ln 10
191016
1020⎛⎝⎜
⎞⎠⎟= 0.90 V
W VA = 0( ) = 2KSε0qNAVbi
⎡
⎣⎢
⎤
⎦⎥
1/2
= 2 ×11.8 × 8.854 ×10−14
1.6 ×10−19 ×1016× 0.90⎡
⎣⎢
⎤
⎦⎥
1/2
= 3.43×10−5 cm
(Note that we used ε0 in F/cm and N A in cm
-‐3 so that the result would come out in cm not in meters.)
CJ 0 =
Ksε0W VA = 0( )
= 11.8×8.845×10−14
2.8×10−5= 3.05×10−8 F/cm2
CJ 0 = 3.05×10
−8 F/cm2
3b) Find the depletion capacitance at VA = −5 V (reverse biased).
Solution:
CJ =
Ksε0W VA( )
=Ksε0
W VA = 0( )×
W VA = 0( )W VA( )
= CJ 0Vbi
Vbi −VA=
CJ 01−VA Vbi
CJ =
3.05×10−8
1+5 0.9= 1.19×10−8 F/cm2 < CJ 0
CJ VA = −5 V( ) = 1.19×10−8 F/cm2 Reverse bias decreases the junction capacitance.
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ECE-‐305 Spring 2015 5
HW8 Solutions (continued): 3c) Find the depletion capacitance at VA = +0.5V (forward biased).
Solution:
CJ =
CJ 01−VA Vbi
= 3.73×10−8
1− 0.5 0.9= 5.6×10−8 F/cm2 > CJ 0
CJ VA = −5 V( ) = 4.58×10−8 F/cm2 Forward bias increases the junction capacitance.
4) A silicon diode is asymmetrically doped at N D = 10
19 cm-‐3 and N A = 1016 cm-‐3. (Note
that at N D = 1019 the semiconductor is on the edge of degeneracy, but we can assume
that non-‐degenerate carrier statistics are close enough for this problem.) Assume that the minority electron and hole lifetimes are
τ n = τ p = 10
−6 s. The lengths of the N and
P regions are L = 500 µm and L >> xp ,xn . 4a) Estimate the applied forward bias at which the P-‐region enters high-‐level
injection. Solution:
High injection will occur first on the lightly doped side, the P-‐side. The maximum excess electron concentration occurs at the beginning of the P-‐side. From the law of the junction:
Δn 0( ) = ni
2
N AeqVA kBT −1( )
Low level injection means: Δn 0( )
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ECE-‐305 Spring 2015 6
HW8 Solutions (continued):
4b) Compute the current density at the onset of high-‐injection.
Solution: The current density is given by:
J = J0 e
qVA kBT −1( ) From HW8, problem 1): J0 = 9.1×10
−12 A/cm2
J = J0 e
qVA kBT −1( ) = 9.1×10−12 e0.718/0.026 −1( ) = 9 A/cm2 J = 9 A/cm2
5) A silicon diode is asymmetrically doped at N D = 10
19 cm-‐3 and N A = 1016 cm-‐3. (Note
that at N D = 1019 the semiconductor is on the edge of degeneracy, but we can assume
that non-‐degenerate carrier statistics are close enough for this problem.) Answer the following questions assuming room temperature. Assume that the minority electron and hole lifetimes are
τ n = τ p = 10
−6 s. The lengths of the N and P regions are
L = 500 µm and L >> xp ,xn . What is the reverse breakdown voltage of this diode?
Assume a critical field for breakdown of E cr = 3×105 V/cm.
Solution:
E 0( ) = 2VbiW
=2 Vbi +VR( )
2KSε0qNA
Vbi +VR( )⎡
⎣⎢
⎤
⎦⎥
1/2 =2qNA Vbi +VR( )
KSε0
⎡
⎣⎢
⎤
⎦⎥
1/2
E cr =E 0( ) =
2qNA Vbi +VR( )KSε0
⎡
⎣⎢
⎤
⎦⎥
1/2
VBR =
KSε0E cr2
2qNA−Vbi
Vbi =kBTqln NDNA
ni2
⎛⎝⎜
⎞⎠⎟= 0.026 ln 10
191016
1020⎛⎝⎜
⎞⎠⎟= 0.90
VBR =
KSε0E cr2
2qNA− 0.90 =
11.8 × 8.854 ×10−14 × 3×105( )22 ×1.6 ×10−19 ×1016
− 0.90 = 29.4 − 0.90 = 28.5 V
VBR = 28.5 V This assumes a planar junction. If there is junction curvature at the edge of the diode, the breakdown voltage will be lower.