### GATE Questions on "PN Junction (Diode)" (1987 to Till Date)

1987

1.       The diffusion capacitance of a PN junction
a.       Decreases with increasing current and increasing temperature
b.      Decreases with decreasing current and increasing temperature
c.       Increases with increasing current and increasing temperature
d.      Does not depend on current and temperature

1988

1.       For a PN junction, Match the type of breakdown with phenomenon
i.         Avalanche breakdown
ii.       Zener breakdown
iii.      Punch through

a.       Collision of carriers with crystal ions
b.      Early effect
c.       Rupture of covalent bond due to strong electric field

2.       In the circuit shown below, the current voltage relationship when D1 and D2 are identical is given by  (assume Germanium diodes)

1989

1.       The switching speed of P+N junction depends primarily on
a.       the mobility of minority carriers in the P+ region
b.      the lifetime of minority carriers in the P+ region
c.       the mobility of majority carriers in the N region
d.      the lifetime of majority carriers in the N region

1990

1.  In a uniformly doped abrupt PN junction, the doping level of the N-side is four times the doping level of the P-side. Then the ratio of the depletion layer widths is......

Answer:     Wn / Wp = 1 / 4

2.  In a junction diode,
a. the depletion capacitance increases with increase in the reverse bias
b. the depletion capacitance increases with decrease in the reverse bias
c. the diffusion capacitance increases with increase in the forward bias
d. The diffusion capacitance is much higher than the depletion capacitance, when it  is  forward biased.

3. (a) Two ideal and identical junction diodes (with ideality factor Ƞ = 1) connected in series as shown below.

Show that exp(eV1/KT) + exp(-eV2/KT) = 2, where V1 and V2 are the voltage drop across the diodes D1 and D2.

(b) Assuming that the current flowing through the reverse biased diode is saturated at ICO, calculate the voltage drop across the forward biased diode. Assume KT = 26 meV.

1991

1.       The small signal capacitance of an abrupt P+N junction is 1 nF/cm2 at zero bias. If the built in voltage is 1 volt, the capacitance at a reverse bias voltage of 99 volts is equal to ……….

2.       Referring to the circuit shown, the switch is in position 1 initially and steady state conditions exist from time t=0 to t=t0. The switch is suddenly thrown into position 2. The current I flowing through the 10K resistor as function of time from t=0 is  ………. (Give the sketch showing the magnitudes of the current at t=0, t=t0 and t=)

3.       In the figure shown, the input Vi is a 100 Hz triangular wave having a peak to peak amplitude of 2 volts and an average value of zero volts. Given that the diode is ideal, the average value of the output voltage Vo is ……………..

4.       The current in a forward biased P+N junction shown in figure (a) is entirely due to diffusion of holes from x = 0 to x = L. The injected hole concentration distribution in the n-region is linear as shown in figure (b), with P(0) = 1022 per cm3 and L = 10-3 cm. Determine

a.       The current density in the diode, assuming that the diffusion coefficient of holes is 12 cm2/sec.
b.      The velocity of holes in the n-region at x = 0.

Jn = 19.2 x 106 A/cm2
υn = 12 x 103 cm/sec

1992

1.       A PN junction with a 100 resistor is forward biased so that a current of 100 mA flows. If the voltage across this combination is instantaneously reversed to 10 volts at t = 0, the reverse current that flows through the diode at t = 0 is approximately given by
a.       0 mA
b.      100 mA
c.       200 mA
d.      50 mA

1993

1.       In the figure, the ideal moving iron voltmeter M will read

2.       M in the figure shown is a rectifier type 200 volts full scale voltmeter having a sensitivity of 10 K/volt. What will be the reading in M if the source voltage VS is a symmetrical square wave of 800 volts peak to peak ?

Solution :

3.       The built in potential (Diffusion potential) in a PN junction………
a.       Is equal to the difference in the Fermi level of the two sides, expressed in volts
b.      Increases with the increase in the doping levels of the two sides
c.       Increases with the increase in temperature
d.      Is equal to the average of the Fermi levels of the two sides

4.       Consider the circuit shown in figure (a). if the diode used here has the V-I characteristics as shown in figure (b), then the output waveform Vo is
………

1994

1.       The forward dynamic resistance of a junction diode varies ……………… as the forward current.

1995

1.       The diffusion potential across a PN junction
a.       Decreases with increasing doping concentration
b.      Increases with decreasing band gap
c.       Does not depend on doping concentration
d.      Increases with increase in doping concentrations

2.       The depletion capacitance, CJ, of an abrupt PN junction with constant doping on either  side varies with reverse bias, VR as

3.   Two identical silicon junction diodes, D1 and D2 are connected back to back as shown. The reverse saturation current IS of each diode is 10-8 Amps and the breakdown voltage is 50 volts. Evaluate the voltage VD1 and VD2 across the diode D1 and D2 by assuming KT/q to be 25mV.

Answer:     -4.98 volts and 0.018 volts

1996

1.       The p-type substrate in a conventional PN junction isolated integrated circuit should be connected to
a.       Nowhere, i.e. left floating
b.      A dc ground potential
c.       The most positive potential available in the circuit
d.      The most negative potential available in the circuit

1997

1.       In the circuit shown, the current flowing through the ideal diode equal to

a.       0 Amp
b.      4 Amp
c.       1 Amp
d.      None of the above

1998

1.       The static characteristic of an adequately forward biased PN junction is a straight line, if the plot is of
a.       Log I vs. log V
b.      Log I vs. V
c.       I vs. log V
d.      I vs. V

1999

2000

1.       For the circuit in figure shown, the voltage Vo is

a.       2 volts
b.      1 volts
c.       -1 volts
d.      None of the above

2001

1.       For the circuit shown in figure, D1 and D2 are identical diodes with ideality factor of unity. The thermal voltage VT = 25 mV.
a.       If the reverse saturation current, IS, for the diode is 1 pA, then compute the current I through the circuit.
b.      Calculate Vf and Vr.

Answer:     (a) 1 pA (b) 0.018 mV and 0.032 mV

2002

1.       In the figure shown, a silicon diode is carrying a constant current of 1 mA. When the temperature of the diode is 20oC, diode voltage is found to be 700 mV. If the temperature rises to 40oC, diode voltage becomes approximately equal to

a.       740 mV
b.      660 mV
c.       680 mV
d.      700 mV

2003

1.       At 300oK, for a diode current of 1 mA, a certain Germanium diode requires a forward bias of 0.1435 volts, where as a certain Silicon diode requires a forward bias of 0.718 volts. Under the conditions stated above, the closest approximation of the ratio of reverse saturation current in Ge to that of Si diode is
a.       1
b.      5
c.       4 x 103
d.      8 x 103

2004

1.       In abrupt PN junction, the doping concentrations on the P side and N side are NA = 9 x 1016 cm-3 and ND = 1 x 1016 cm-3 respectively. The PN junction is reverse biased and the total depletion width is 3 µm. The depletion width on the P side is
a.       2.7 µm
b.      0.3 µm
c.       2.25 µm
d.      0.75 µm

2.       Consider an abrupt PN junction. Let Vbi be the built in potential of this junction and VR be the applied reverse bias. If the junction capacitance (CJ) is 1 pF for Vbi + VR = 1 volt, then for Vbi + VR = 4 volts, the value of CJ will be
a.       4 pF
b.      2 pF
c.       0.25 pF
d.      0.75 pF

2005

1.       A silicon PN junction at a temperature of 20oC has a reverse saturation current of 10 pA. The reverse saturation current at 40oC for the same bias is approximately
a.       30 pA
b.      40 pA
c.       50 pA
d.      60 pA

2.       A silicon PN junction diode under reverse bias has depletion region of width of 10 µm. The relative permittivity of silicon (Ԑr) is 11.7 and the permittivity of free space (Ԑo) is 8.85 x 10-12 F/m. The depletion capacitance of the diode per square meter is
a.       100 µF
b.      10 µF
c.       1 µF
d.      20 µF

2006

1.       In the circuit shown below, the switch was connected to position 1 at t < 0, and at t = 0, it is changed to position 2. Assume that the diode has zero voltage drop and a storage time tS. For 0<t<tS, the voltage across the 1 K resistor (VR) in volts is

2.   For the circuit shown below, assume the zener diode is ideal with a breakdown voltage of 6 volts. The waveform observed across R is

2007

1.       In a P+N junction diode under reverse bias, the magnitude of electric field is maximum at
a.       The edge of the depletion region on P side
b.      The edge of the depletion region on N side
c.       The P+N junction
d.      The centre of the depletion region on the N side

2.       The correct full wave rectifier circuit is :

3.       A P+N junction has a built in potential of 0.8 volts. The depletion layer width at a reverse bias of 1.2 volts is 2 µm. For a reverse bias of 7.2 volts, the depletion layer width will be
a.       4 µm
b.      4.9µm
c.       8 µm
d.      12 µm

2008

1.       Which of the following is NOT associated with a PN junction?
a.       Junction capacitance
b.      Charge storage capacitance
c.       Depletion capacitance
d.      Channel length modulation

2.       In the following limiter circuit, an input voltage Vi  = 10sin100πt applied. Assume that the diode drop is 0.7 volts when it is forward biased. The zener breakdown voltage is 6.8 volts.
The maximum and minimum values of the output voltage respectively are

a.       6.1 volts, -0.7 volts
b.      0.7 volts, -7.5 volts
c.       7.5 volts, -0.7 volts
d.      7.5 volts, -7.5 volts

2009

1.       In the circuit below, the diode is ideal. The voltage V is given by

a.       Min(Vi,1)
b.      Max(Vi,1)
c.       Min(-Vi,1)
d.      Max(-Vi,1)

2.       Common Data Question:
Consider a silicon PN junction at room temperature having the following parameters:
Doping on the N side = 1x1017 cm-3
Depletion width on N side  = 0.1 µm
Depletion width on the P side = 1 µm
Intrinsic carrier concentration = 1.4x1010 cm-3
Thermal voltage = 26 mV
Permittivity of free space = 8.85x10-14 F/cm
Dielectric constant of silicon = 12

i.        The built in potential of the junction is
a.       0.70 volts
b.      0.76 volts
c.       0.82 volts
d.      Cannot be estimated from given data

ii.        The peak electric field in the device is
a.       0.15 Mvolts/cm, directed from P region to N region
b.      0.15 Mvolts/cm, directed from N region to P region
c.       1.80 Mvolts/cm, directed from P region to N region
d.      1.80 Mvolts/cm, directed from N region to P region

2010

1.       Compared to a PN junction with ND=NA=1014 per cm3, which one of the following statements is TRUE for a PN junction with NA=ND=1020 per cm3?
a.       Reverse breakdown voltage is lower and depletion capacitance is lower
b.      Reverse breakdown voltage is higher and depletion capacitance is lower
c.       Reverse breakdown voltage is lower and depletion capacitance is higher
d.      Reverse breakdown voltage is higher and depletion capacitance is higher

2011

1.       A silicon PN junction is forward biased with a constant current at room temperature. When the temperature is increased by 10oC, the forward bias voltage across the PN junction
a.       Increases by 60 mV
b.      Decreases by 60 mV
c.       Increases by 25 mV
d.      Decreases by 25 mV

2012

1.       The I – V characteristics of the diode in the circuit is given below, then the current in the circuit is

a.       10 mA
b.      9.3 mA
c.       6.67 mA
d.      6.2 mA

2.       The diodes and capacitors shown in the circuit are ideal. The voltage V(t) across the diode D1 is

2013

1.       For a forward biased PN junction diode, the sequence of events that best describes the mechanism of current flow is
a.       Injection and subsequent diffusion and recombination of minority carriers
b.      Injection and subsequent drift and generation of minority carriers
c.       Extraction and subsequent diffusion and generation of minority carriers
d.      Extraction and subsequent drift and recombination of minority carriers

2014

Set – 1 (15th February 2014 (Forenoon))

1.       In the figure, assume that the forward voltage drops to the PN diode D1 and Schottky diode D2 are 0.7 volts and 0.3 volts respectively. If ON denotes conducting state of the diode and OFF denotes the non conducting state of the diode, then in the circuit,

a.       Both are ON
b.      D1 is ON and D2 is OFF
c.       Both are OFF
d.      D1 is OFF and D2 is ON

2.       The doping concentration on the P side and N side of a silicon diode are 1x1016 cm-3 and 1x1017 cm-3 respectively. A forward bias of 0.3 volts is applied to the diode. At T = 300oK, the intrinsic carrier concentration of silicon, ni = 1.5 x 1010 cm-3 and KT/q = 26 mV. The electron concentration at the edge of the depletion region on the P side is
a.       2.3 x 109 cm-3
b.      1 x 1016 cm-3
c.       1 x 1017 cm-3
d.      2.25 x 106 cm-3

SET – 2 (15th February 2014 (Afternoon))

1.       Consider an abrupt PN junction (at T = 300oK) shown in the figure. The depletion region width Xn on the N side of the junction is 0.2 µm and the permittivity of silicon (Ԑsi) is 1.044 x 10-12 F/cm. At the junction, the approximate value of the peak electric field (in kV/cm) is………..

2.       When a silicon diode having a doping concentration of NA = 9 x 1016 cm-3 on P side and ND = 1 x 1016 cm-3 on N side is reversed biased, the total depletion width is found to be 3 µm. Given that the permittivity of silicon is 1.04 x 10-12 F/cm, the depletion width on the P side and the maximum electric field in the depletion region, respectively, are
a.       2.7 µm and 2.3 x 105 V/cm
b.      0.3 µm and 4.15 x 105 V/cm
c.       0.3 µm and 0.42 x 105 V/cm
d.      2.1 µm and 0.42 x 105 V/cm

3.       The diode in the circuit shown, if Von = 0.7 volts but is ideal otherwise. If Vi = 5 sin(ωt) volts, the minimum and maximum values of Vo (in volts) are, respectively,

a.       -5 and 2.7
b.      2.7 and 5
c.       -5 and 3.85
d.      1.3 and 5

SET – 3  (16th February 2014 (Forenoon))

1.       The donor and acceptor impurities in an abrupt junction silicon diode are 1 x 1016 cm-3 and 5 x 1018 cm-3, respectively. Assume that the intrinsic carrier concentration in silicon ni = 1.5 x 1010 cm-3 at 300oK, KT/q = 26 mV and the permittivity of silicon Ԑsi = 1.04 x 10-12 F/cm. The built in potential and the depletion width of the diode under thermal equilibrium conditions, respectively are
a.       0.7 volts and 1 x 10-4 cm
b.      0.86 volts and 1 x 10-4 cm
c.       0.7 volts and 3.3 x 10-5 cm
d.      0.86 volts and 3.3 x 10-5 cm

SET  - 4  (16th February 2014 (Afternoon))

2015

1.       In the circuit shown, assume that the diodes D1 and D2 are ideal. The average value of voltage Vab (in volts), across terminals a and b is ________________

2.       The electric field profile in the depletion region of a PN junction in equilibrium is shown in the figure. Which one of the following is NOT TRUE?
a.       The left side of the junction is N-type and the right side is  P-type
b.      Both the N-type and P-type depletion regions are uniformly doped
c.       The potential difference across the depletion region is 700 mV
d.      If the P-type region has a doping concentration of 1015 cm-3, then the doping concentration in the N-type region will be 1016 cm-3

3.   A region of negative differential resistance is observed in the current voltage characteristics of a silicon PN junction if
a.       Both the P-region and N-region are heavily doped
b.      The N-region is heavily doped compared to P-region
c.       The P-region is heavily doped compared to N-region
d.      An intrinsic silicon region is inserted between the P-region and the N-region

Solution :     For the circuit shown, assume ideal diodes. The shape of the output (Vout) for the given sine wave input (Vin) will be ________________

5.   For a silicon diode DOPING with  P and N regions, the acceptor and donor impurity concentrations are 1x1017 cm-3 and 1x1015 cm-3 respectively. The lifetimes of electron in P-region and holes in N-region are both 100 µs. The electron and hole diffusion coefficients are 49 cm2/sec and 36 cm2/sec respectively. Assume thermal voltage is 26 mV, the intrinsic carrier concentration is 1x1010 cm-3 and q = 1.6x10-19 C. When a forward voltage of 208 mV is applied across the diode, the hole current density (in nA/cm2) injected from P-region to N-region is  ___________

6.   The built in potential of an abrupt PN junction is 0.75 volts. If its junction capacitance (CJ) at a reverse bias (VR) of 1.25 volts is 5 pF. The value of CJ (in pF) when VR = 7.25 volts is _____

7.   The diode in the circuit given below has VON = 0.7 volts but is ideal otherwise. The current (in mA) in the 4 kΩ resistor is __________________