# VE311 Electronic Circuits Homework 03 solved

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• Diodes and Rectifiers
1. A diode is doped with NA = 1019 /cm3 on the p-type side and ND = 1018/cm3
on the n-type side. (a) What is the depletion-layer width wdo ? (b) What are
the values of xp and xn? (c) What is the value of the built-in potential of the
junction? (d) What is the value of EMAX? Use Eq. (3.3) and Fig. 3.5.
2. A diode has wdo = 0.4 µm and φj = 0.85 V. (a) What reverse bias is required
to triple the depletion-layer width? (b) What is the depletion region width if a
reverse bias of 7 V is applied to the diode?
3. Suppose a drift current density of 5000 A/cm2
exists in the neutral region on the
p-type side of a diode that has a resistivity of 2.5 Ω·cm. What is the electric field
needed to support this drift current density?
4. Suppose that NA(x) = No exp(−x/L) in a region of silicon extending from x =
0 to x = 12 µm, where No is a constant. Assume that p(x) = NA(x). Assuming
that jp must be zero in thermal equilibrium, show that a built-in electric field
must exist and find its value for L = 1 µm and No = 1018/cm3
5. A diode has n=1.05 at T =320 K. What is the value of n·VT ? What temperature
would give the same value of n·VT if n=1.00?
6. A diode has IS = 10−17A and n = 1.07. (a) What is the diode voltage if the diode
current is 70 µA? (b) What is the diode voltage if the diode current is 5 µA? (c)
What is the diode current for vD = 0 V? (d) What is the diode current for vD =
-0.075 V? (e) What is the diode current for vD = -5 V?
7. The saturation current for diodes with the same part number may vary widely.
Suppose it is known that 10−14 A ≤ IS ≤ 10−12 A. What is the range of forward
voltages that may be exhibited by the diode if it is biased with iD=1mA?
8. A diode with IS = 2.5×10−16A at 30◦C is biased at a current of 1 mA. (a) What
is the diode voltage? (b) If the diode voltage temperature coefficient is -2 mV/K,
what will be the diode voltage at 50◦C?
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9. A diode has a doping of ND = 1020/cm3 on the n-type side and NA = 1018/cm3
on the p-type side. What are the values of wdo and φj? What is the value of wd
at a reverse bias of 5 V? At 25 V?
10. (a) Plot the load line and find the Q-point for the diode circuit in Fig.P3.57 if V
=10 V and R = 5 kΩ . Use the i − v characteristic in Fig. P3.42. (b) Repeat for
V = -10 V and R = 5 kΩ . (c) Repeat for V = -2 V and R = 2 kΩ
11. Use the i − v characteristic in Fig.1a. (a) Plot the load line and find the Q-point
for the diode circuit in Fig. 1 if V = 6V and R = 4kΩ. (b) For V = -6 V and R
= 3 kΩ.(c) For V = -3 V and R = 3 kΩ. (d) For V = 12 V and R = 8 kΩ. (e)
For V = -25 V and R = 10kΩ
Figure 1: i − v diode characteristics and schematic diagram for the diode…
12. Simulate the behavior of the half-wave rectifier in Fig.2 for vI = 10 sin 120πt,
R = 0.025 Ω and C = 0.5 F. (Use IS = 10−10 A, RS=0 and RELTOL = 10−6
.)
Compare the simulated values of dc output voltage, ripple voltage, and peak diode
current to hand calculations. Repeat simulation with RS =0.02Ω.
Figure 2: Half-wave rectifier
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13. What are the dc output voltages V1 and V2 for the rectifier circuit in Fig. 3 if
vI = 40 sin 377t and C = 20 000 µF?
Figure 3: Full-wave rectifier
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