### GATE practice Problems on PN junction (Diode) Set - 2

1.       An ideal silicon PN junction has a reverse saturation current of 0.1 µA at a temperature of 125oC. Find the dynamic resistance at 105oC when the diode is forward biased with 0.8 volts.

2.       An ideal germanium diode at room temperature has a static resistance of 4.57 Ω at a point, where the current flowing is 43.8 mA. Find the dynamic resistance for a forward bias of 0.1 volt.

3.       For an alloy silicon PN junction with NA << ND, calculate depletion layer capacitance (CT), if the resistivity of P-material is 4 Ω-cm, the barrier height Vo = 0.3 volts, applied reverse voltage is 4 volts and the cross sectional area is circular of 50 mills in diameter.

4.       Find the resistivity of the P-type material in a silicon PN junction, where cross sectional area is circular and of 40 mils in diameter and the transition capacitance is 61 pF. The given barrier height is 0.35 volts and the applied reverse voltage is 5 volts.

5.       For a silicon P+N junction with ND = 1015 atoms per cm3 and the built in potential of 0.5 volts. Find the transition capacitance per square mil, if the applied reverse voltage is 10 volts.

6.       The transition capacitance of an abrupt PN junction is 10 pF at 4 volts. Find the decrease in capacitance for a 0.5 volts increase in bias.

7.       For a silicon PN junction with NA = ND = 1021 atoms per m3 and ni = 9.8 x 1015 atoms per m3. Calculate transition capacitance, if the area is 1 mm2 and the junction is reverse biased with 10 volts.

8.       Find static and dynamic resistances of a PN junction germanium diode for an applied forward bias of 0.2 volts, if the temperature is 300oK and reverse saturation current of 1 µA.

9.       Find the Diffusion capacitance of a silicon diode with NA >> ND, when carrying a current of 1 mA. Assume diffusion length of holes i 0.026 cm.

10.   The zero barrier height of an alloy silicon PN+ junction is 0.6 volts and acceptor concentration is 5 x 1016 atoms per cm3. Find space charge capacitance for an applied reverse voltage of 5.6 volts, if the cross sectional area is 1 mm2.

11.   For a silicon P+N junction, find the current flowing through the junction, if the diffusion length is 2.6 µm and diffusion capacitance is 1 nF.

12.   Calculate the barrier capacitance of a germanium PN junction, whose area is 0.5 mm X 0.5 mm and space charge thickness, is 3 x 10-4 cm.

13.   In the given figure, the V-I characteristics of the diode is given as I = 0.2(V – 1)1/2 for V ≥ 1 else zero. Find the current ‘I’ indicated.

14.   For the circuit shown, assume the drop across conducting diode is 0.7 volts. Find Vo if V1 = 10 volts and V2 = 5 volts.

15.   For the circuit shown, Find the voltage drop across diode D1, if V1 = 5 volts and V2 = 0 volts. Assume ideal diodes.

16.   For the circuit shown, assume that the silicon diode requires a minimum current of 1 mA to be above the knee of its I-V characteristic.
a.       What should be the value of R to establish 5 mA in the circuit?
b.      With the value of R calculated, what is the minimum value of voltage E, such that the diode current is above the knee point.

17.   For the circuit shown, assume that the silicon diode is biased above its knee and has a bulk resistance of 0.1 Ω. Find the total current in and total voltage across the diode. Sketch the current versus time.

18.   Determine which diodes are forward biased and which are reverse biased in each of the configurations shown in figure.

19.   Determine which diodes are forward biased and which are reverse biased in the circuits shown. Assume a 0.7 volts drop across each forward biased diode, determine the output voltage also.

20.   A diode conducts a current of 440 nA form cathode to anode, when the reverse biasing voltage across it is 8 volts. What is the diode resistance?

21.   For the circuit shown, the current I is 34.28 mA. What is the voltage drop across the diode and also find its DC resistance?

22.   For the circuit shown, assume that the voltage drop across a forward biased silicon diode is 0.7 volts and that across a germanium diode is 0.3 volts.
a.       If D1 and D2 are both silicon diodes, find the current I in the circuit.
b.      Find the current I in the circuit, if D1 is silicon and D2 is germanium.

23.   In the circuit shown below, assume the diode is germanium. Find the percent error caused by neglecting the voltage drop across the diode, when calculating the current I in the circuit. Assume voltage drop across forward biased germanium diode is 0.3 volts.

24.   In the circuit shown, the diode has 0.65 volts drop across it.
a.       Find the DC current in the diode
b.      Find ac resistance of the diode at room temperature
c.       Find total current in and total voltage across the diode
d.      What are the minimum and maximum values of current flowing through the diode?

25.   In the circuit shown, the voltage source is a square wave whose output alternates between + 2.5 volts and – 2.5 volts. Find the peak voltage across and current through the resistor, if the diode is germanium and R = 330 Ω.

26.   Determine which of the following diodes are forward biased and which are reverse biased.

27.   Determine which of the following diodes are forward biased and which are reverse biased.

28.   In the circuit shown, the inputs A and B can be either 0 volts or +10 volts. Each diode is silicon and has resistance 400 Ω when it is forward biased. Find Vo for all four possible combinations of A and B.

29.   In circuit shown, the inputs A, B and C can be either +10 volts or –5 volts. Each diode is silicon and has a resistance of 1200Ω when it is forward biased. Find Vo, when
a.       A = B = C = -5 volts
b.      A = B = C = +10 volts
c.       A = C = -5 volts and B = +10 volts
d.      A = B = +10 volts and C = -5 volts

30.   In the circuit shown, the inputs A and B can be either 0 volts or -5 volts. Assuming that the forward voltage of the diode is 0.7 volts, find Vo for all possible combinations of A and B.