4.11. DESIGN PROBLEMS 213
Calculate the diode current at a forward bias of 0.5 V and 0.6 V at 300 K. What is the
ideality factor of the diode in this range?
Problem 4.7Consider a narrow diode with the same parameters as given above. Calculate
the total electron- and hole-injected charge in then-andp- sides at a forward bias of 0.4 V.
The widths of then-andp-sides are both 1.0μm.
Problem 4.8Discuss how ap-ndiode can be used as a temperature sensor. Assuming an
ideal Sip-ndiode, calculate the value ofxandywhere
x=
1
Io
dIo
dT
,y=
1
I
dI
dT
In real diodes the value ofxandyis smaller than what is expected for an ideal diode.
Discuss the reason for this.
Problem 4.9Assume that a Si diode suffers Zener breakdown at a field of 2× 105 V/cm if
bothn-andp-sides are doped above 1018 cm−^3. Design a diode that suffers Zener
breakdown at a reverse bias of 5 V. Draw the I-V characteristics for this diode assuming
reasonable material parameters.
Problem 4.10Consider a 20 μm diameterp-ndiode fabricated in silicon. The donor
density is 1016 cm−^3 and the acceptor density is 1018 cm−^3. Calculate the following in
this diode at 300 K: i) The depletion widths and the electric field profile under reverse
biases of 0, 2, 5, and 10 V, and under a forward bias of 0.5 V. ii) What are the charges in
the depletion region for these biases?
Problem 4.11Consider the diode discussed in design problem 4.10. Calculate the average
field in the depletion region at the four reverse-bias values considered. Calculate the
velocity of the electrons at these average fields using the velocity-field results given in
chapter 3 What can be said about the change in the drift components of the diode current
with the change in bias?
Problem 4.12Consider an ideal diode model for a siliconp-ndiode with
Nd=10^16 cm−^3 andNa=10^18 cm−^3. The diode area is 10 −^3 cm^2.
The transport properties of the diode are given by the following values at 300 K:
n−side
{
μp= 300 cm^2 V−^1 s−^1 ; μn= 1300 cm^2 V−^1 s−^1
Dp=7.8cm^2 s−^1 ; Dn=33cm^2 s−^1
p−side
{
μp= 100 cm^2 V−^1 s−^1 ; μn= 280 cm^2 V−^1 s−^1
Dp=2.6cm^2 s−^1 ; Dn=7.3cm^2 s−^1
(Note that the mobility is a lot lower in the heavily dopedp-side because of the increased
ionized impurity scattering.) Assume thatτn=τp=10−^6 s. Calculate the diode current.
