Name:
Student No.
1. (a) Consider an ideal pn junction diode at T = 300 K operating in the forward-bias region.
Calculate the change in diode voltage that will cause a factor of 10 increase in current. (b)
Repeat part (a) for a factor of 100 increase in current.
2. A silicon pn junction has impurity doping concentrations of Nd = 2*1015 cm-3 and Na = 8*1015
cm-3. Determine the minority carrier concentrations at the edges of the space charge region
for (a) Va = 0.45 V, (b) Va = 0.55 V, and (c) Va = - 0.55 V.
3. Consider a GaAs pn junction with doping concentrations Na = 5*1016 cm-3 and Nd = 1016 cm-3.
The junction cross-sectional area is A = 10-3 cm2 and the applied forward-bias voltage is Va =
1.10 V. Calculate the (a) minority electron diffusion current at the edge of the space charge
region, (b) minority hole diffusion current at the edge of the space charge region, and (c) total
current in the pn junction diode.
4. Fill in the missing data in the following table.
Case
Va (V)
I (mA)
Is (mA)
1
0.65
0.50
2
0.70
2*1012
3
0.80
4
0.72
1.20
Js (mA/cm2)
1*10-7
2*10-8
A(cm2)
2*10-4
1*10-3
1*10-4
5. Consider an ideal silicon pn junction diode. (a) What must be the ratio of Nd /Na so that 90
percent of the current in the depletion region is due to the flow of electrons? (b) Repeat part
(a) if 80 percent of the current in the depletion region is due to the flow of holes.
6. Consider a silicon pn junction diode with an applied reverse-biased voltage of VR = 5V. The
doping concentrations are Na = Nd = 4*1016 cm-3 and the cross-sectional area is A = 10-4 cm2.
Assume minority carrier lifetimes of τ0 =τno =τpo = 10-7 s. Calculate the (a) ideal reversesaturation current, (b) reverse-biased generation current, and (c) the ratio of the generation
current to ideal saturation current.
7. A one-sided pn silicon diode has doping concentrations of Na = 4*1017 cm-3 and Nd = 8*1015
cm-3. The diode cross-sectional area is A = 5*10-4 cm2. (a) The maximum diffusion capacitance
is to be limited to 1 nF. Determine (i) the maximum current through the diode, (ii) the
maximum forward-bias voltage, and (iii) the diffusion resistance. (b) Repeat part (a) if the
maximum diffusion capacitance is limited to 0.25 nF.
8. (a) Explain physically why the diffusion capacitance is not important in a reversebiased pn
junction. (b) Consider a silicon, a germanium, and gallium arsenide pn junction. If the total
current density is the same in each diode under forward bias, discuss the expected relative
values of electron and hole current densities.