GATE practice Problems on PN junction (Diode) Set - 1



1.       Calculate built in potential of a germanium PN junction at 300oK, if P-side is doped with 5 x 1016 acceptors/cm3 and N-side with 5 x 1014 donors/cm3.


2.       The resistivites of the two sides of an abrupt silicon PN junction are 9.6 Ω-cm on P-side and 100 Ω-cm on N-side. Find contact potential at room temperature.


3.       For what voltage, will the reverse current in a PN junction silicon diode reach 95% of its saturation value at room temperature?


4.       What is the ratio of currents for a forward bias of 0.1 volt to the current for the same magnitude of reverse bias of silicon PN junction at room temperature?


5.       If the reverse saturation current in a silicon PN junction is 1 nA. Find the applied voltage for a forward current of 0.5 µA?


6.       A silicon diode at room temperature conducts 5 mA at 0.7 volts. If the voltage increases to 0.8 volts, find the diode current.


7.       What is the change in voltage at 300oK for a tenfold increase in current for a silicon diode operating in the conducting region?


8.       For a silicon diode operating under conducting region at room temperature, if the voltage is increased by 0.24 volts, then the current flowing through it increases by _______ number of times.


9.       Calculate the factor by which the reverse saturation current of a germanium diode is multiplied, when the temperature is increased from 20 to 100oC.


10.   What increase in temperature would result in a reverse saturation current, which is 50 times its value at room temperature?


11.   A silicon diode operates at a forward voltage of 0.7 volts. Calculate the factor by which the current will be multiplied, when the temperature is decreased from 25oC to – 55oC.


12.   For a silicon P+N junction, ND = 1015 atoms/cm3 and the built in potential of 0.5 volts. If the applied reverse voltage is 10 volts, find the value of electric field at the junction.


13.   Calculate the width of depletion region of a PN junction under open circuit, when doped equally on both sides with 1021 atoms/m3. Assume built in potential of 0.2 volts and εr = 10.


14.   Find the magnitude of the electric field at the center of depletion region of silicon PN junction with NA = ND = 1021 atoms per m3 and built in potential of 0.6 volts.


15.   If the reverse saturation current of diode is 30 nA, then voltage across the diode is ___________________





16.   The zero voltage barrier height of an alloy silicon PN+ junction is 0.6 volts, given NA = 5 x 1016 atoms/cm3. Calculate the width of depletion layer for an applied reverse voltage of 5.6 volts.


17.   The zero voltage barrier height of an alloy silicon P+N junction is 0.6 volts, given ND = 5 x 1016 per cm3. Calculate the width of depletion layer for an applied forward voltage of 0.5 volts.


18.   A silicon PN junction is formed from P material doped with 1022 acceptors/m3 and N material doped with 1.2 x 1021 donors/m3. Find the thermal voltage and barrier voltage at 27oC.


19.   A silicon diode has a saturation current of 0.1 pA at 20oC. Find its current when it is forward biased by 0.55 volts. Find the current in the same diode when the temperature rises to 100oC.


20.   A silicon PN junction is formed from N material doped with  2.5 x 1021 donors/m3 and P material doped to have the same impurity density. Assume ni = 1.5 x 1016 /m3 and is temperature independent.
a.       Find thermal voltage and barrier voltage at 40oC.
b.      To what temperature would the junction have to be raised (in oC) in order that the thermal voltage be 30 mV? What would be the barrier voltage at that temperature?


21.   A silicon PN junction has a saturation current of 1.8 x 10-14 A. Assuming that η=1 and the temperature is 27oC.
a.       Find the current in the junction when the forward biasing voltage is 0.6 volts .
b.      Repeat (a) when forward biasing voltage is 0.65 volts.


22.   The forward current in a PN junction is 1.5 mA at 27oC. If Io = 2.4 x 10-14 A and η = 1. What is the forward biasing voltage across the junction?


23.   The forward current in a PN junction is 22 mA, when the forward biasing voltage is 0.64 volts. If the thermal voltage is 26 mV and η = 1, find the reverse saturation current?


24.   A bar of silicon is doped so that one side (side A) has 1.85 x 1022  electrons/m3 and the other side (side B) has 2.66 x 1010 electrons/m3. If the bar is to be used as a junction diode, which side should be the anode and which side is cathode?


25.   The reverse breakdown voltage of a certain diode is 150 volts and its reverse saturation current is 0.1 pA. What is the current in the diode, when the reverse bias voltage is 149.95 volts? Assume constant n = 2.


26.   For a certain diode, a reverse current of 9.3 nA was measured when the reverse voltage across the diode was 349.99 volts. If the breakdown voltage is 350 volts and reverse saturation current is 0.1 pA, find the value of the constant ‘n’?


27.   A diode has its maximum power dissipation rating as 0.1 watts and its reverse breakdown voltage as 200 volts. What maximum reverse current does it sustain at breakdown without damage?


28.   A diode has a reverse breakdown voltage of 100 volts and a saturation current of 0.05 pA. How much power does it dissipate when the reverse voltage is 99.99 volts? Assume constant n = 2.5.


29.   A diode has a reverse saturation current of 45 pA at a temperature of 373oK. What is the approximate value of Io at 273oK?


30.   When the voltage across a forward biased diode at T = 10oC is 0.621 volts and the current is 4.3 mA. If the current is held constant, what is the voltage at T = 40oC and T= - 30oC.

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