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Tutorial - 3: Questions on PN junction Resistances and Capacitances (EDC)

Electronic Devices & Circuits

Unit – 1 : Tutorial – 3 (11th August 2015)
Topic: V-I characteristics 
           Static & Dynamic resistances
           Diffusion & Transition Capacitance
           Energy band Diagram
Submission Date : 20th August 2015 (before 12:30 pm)

Theory Questions:
1. Define Cut-in voltage and draw V-I characteristics of Silicon and Germanium PN junction separately.
2. Why reverse saturation current of Germanium is more than silicon at any temperature?
3. Explain importance of modeling and in how many ways a diode can be modeled.
4. Derive mathematical expression for dynamic resistance.
5. Why Diffusion capacitance dominates in forward bias and Transition capacitance dominates in reverse bias?
6. Derive the expression for transition capacitance of abrupt P+N junction.
7. Find the expression for built in potential barrier energy (Eo) with the help of energy band diagram under open circuit PN junction.
8. Draw Energy Band diagram of PN junction under forward and reverse bias.
9. Explain how Diffusion capacitance is formed in forward bias and give the expression for it.
10. Define Mean lifetime (τ) and Diffusion length (L) of minority carrier in a PN junction.

Numerical Questions:
1. Find the voltage at “C” in both circuits, when A and B are binary signals, which can hold either 0 volts (LOW) or VCC (HIGH). Write comments on the results obtained.

2. Find V0, I, I1 and I2 when (a) the diodes are ideal (b) when D1 has cut-in voltage of 0.2 volts and D2 has cut-in voltage of 0.7 voltage.

3.Find VD1, VD2 and IO when (a) the diodes are ideal (b) when D1 has cut-in voltage of 0.2 volts and D2 has cut-in voltage of 0.7 volts.

4. Find the dynamic resistance of a silicon PN junction for a forward bias of 0.6 volts. Assume reverse saturation current is 50 nA at room temperature.

5. A silicon P+N junction has Diffusion capacitance of 1 µF, when carrying a current of 1 mA. Calculate Mean life time of holes.

6. A silicon PN junction under reverse bias has depletion region width of 10 µm. Find the depletion capacitance of the diode per square meter.

7. consider an Abrupt PN junction with a built in potential of 0.8 volts. If the transition capacitance is 1 pF when the reverse bias voltage is 1.2 volts. Find the transition capacitance when the reverse voltage is 7.2 votls.

8. Calculate the barrier capacitance of a Germanium PN junction, whose area is 0.5 x 0.5 mm2 and space charge thickness is 3 µm.

9. The transition capacitance of an abrupt PN junction is 10 pF at barrier voltage (VB) of 4 volts. Calculate the change in capacitance for a 1 volt increase in barrier voltage.

10. (a) calculate the factor by which the reverse saturation current is multiplied, when the temperature is increased from 27oC to 100oC.
     (b) A silicon PN junction has a reverse saturation current of 1 µA at 27oC. Find the dynamic resistance at 27oC and at 100oC.

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