GATE Practice Problems on JFET (Junction Field Effect Transistor)

1.       An N-channel JFET has V_P = -4 volts and I_DSS = 10 mA.

a.       Find the minimum V_DS for the device to operate in pinch-off, when V_GS = -2 volts.

b.      Find the drain current for V_GS = -2 volts, when V_DS is 3 volts and 5 volts.

2.       The JFET in the circuit has V_P = - 3 volts, I_DSS = 9 mA. Find the values of all resistors so that V_G = 5 volts, I_D = 4 mA and V_D = 11 volts. Design for 0.05 mA in the voltage divider.

3.       An N-channel silicon JFET has a donor concentration of 2 x 10²¹ /m³ and a channel width of 4 µm.

a.       Find the pinch off voltage, V_P

b.      Find the saturation voltage, V_DSsat, if V_GS is  – 2 volts.

4.       The Q-point of a JFET in a source self-bias arrangement is chosen at V_GS = - 1.5 volts and I_D = 5 mA. Find the value of the resistance R_S.

5.       An N-channel JFET has I_DSS = 12 mA and the Transconductance of 4mA/V when V_GS = 0 volts.

a.       Find the pinch off voltage, V_P

b.      Find the drain current when V_GS = - 2 volts.

6.       An N-channel JFET has pinch off voltage of – 4.5 volts and I_DSS = 9 mA.

a.       At what value of V_GS in the pinch off region will I_D equal to 3 mA?

b.      What is the value of V_DS(sat), when I_D = 3 mA?

7.       For the given JFET, I_DSS = 10 mA and V_P = - 4 volts. Find the quiescent values of I_D and V_DS, when V_GS = - 1.5 volts. Assume that JFET is biased in pinch off region.

 

8.       For the circuit shown, find the Q-point, when V_GS = - 3 volts.

 

9.       For the P-channel JFET shown, find the Q-point if I_DSS = 18 mA and V_P = 5 volts.

 

10.   An N-channel JFET has I_DSS = 16 mA and V_P = - 6 volts. Calculate the drain current and V_DS(sat) when V_GS = - 4 volts.

11.   A P-channel JFET has a pinch off voltage of 8 volts. At what value of V_GS does V_DS(sat) = - 3 volts?

12.   An N-channel JFET has pinch off voltage of – 5.8 volts and I_DSS = 15 mA. Find the value of I_D when V_GS is 0 volts, - 2 volts and – 6.5 volts.

13.   An N-channel JFET having a pinch off voltage of – 3.5 volts has a saturation current of 2.3 mA when V_GS = - 1 volt. What is its saturation current when V_GS = 0 volts and -2 volts.

14.   A P-channel JFET has a pinch off voltage of 6 volts and I_DSS = 18 mA. At what value of V_GS in the pinch off region will I_D equal 6 mA? What is the value of V_DS at pinch off point?

15.   The JFET shown has I_DSS = 14 mA and V_P = - 5 volts. Determine the Q-point for 

a.       V_GS = - 3.6 volts

b.      V_GS = - 3 volts

c.       V_GS = - 1.7 volts

16.   Find the Q-point of the JFET shown, if I_DSS = 6 mA and V_P = - 3 volts.

 

17.   Determine the values of I_D,V_GS and V_DS for the JFET shown, if I_DSS = 25 mA and V_P = 8 volts.

18.   Determine the values of I_D, V_GS and V_DS of the JFET circuit shown, if I_DSS = 2.42 mA and V_P = - 4 volts.

19.   Find the values of I_D and V_DS of the N-channel JFET shown, if I_DSS = 12 mA and V_P = - 5 volts.

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 125^oC. Find the dynamic resistance at 105^oC 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 N_A << N_D, calculate depletion layer capacitance (C_T), if the resistivity of P-material is 4 Ω-cm, the barrier height V_o = 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 N_D = 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 N_A = N_D = 10²¹ atoms per m³ and n_i = 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 300^oK and reverse saturation current of 1 µA.

9.       Find the Diffusion capacitance of a silicon diode with N_A >> N_D, 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 V_o 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 V_o 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 V_o, 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 V_o for all possible combinations of A and B.

 

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

1.       Calculate built in potential of a germanium PN junction at 300^oK, 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 300^oK 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 100^oC.

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 25^oC to – 55^oC.

12.   For a silicon P⁺N junction, N_D = 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 ε_r = 10.

14.   Find the magnitude of the electric field at the center of depletion region of silicon PN junction with N_A = N_D = 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 N_A = 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 N_D = 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 27^oC.

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

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 n_i = 1.5 x 10¹⁶ /m³ and is temperature independent.

a.       Find thermal voltage and barrier voltage at 40^oC.

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 27^oC.

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 27^oC. If I_o = 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 373^oK. What is the approximate value of Io at 273^oK?

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