# Topic: Electronic Devices (Test 4)

Topic: Electronic Devices
Q.1
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 the P-region
C. The P-region is heavily doped compared to the N-region
D. An intrinsic silicon region is inserted between the P-region and the N-region
Explaination / Solution:

In case of Tunnel diode formed by PN junction, tunnel diode gives negative resistance and works in forward bias, and in tunnel diode both N and P regions are heavily doped.

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Q.2
For the NMOSFET in the circuit shown, the threshold voltage is Vth, where Vth > 0. The source voltage VSS is varied from 0 to VDD. Neglecting the channel length modulation, the drain current ID as a function VSS is represented by

A.
B.
C.
D.
Explaination / Solution:

We have the following conditions

Since it is in saturation, current ID is given by

Thus, ID - VSS graph shows Parabolic relation for Vss < VDD - Vth and zero for Vss > VDD - Vth. Only graph shown in option (A) satisfies this result.

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Q.3
Which of the following is true?
A. A silicon wafer heavily doped with boron is a p+ substrate
B. A silicon wafer lightly doped with boron is a p+ substrate
C. A silicon wafer heavily doped with arsenic is a p+ substrate
D. A silicon wafer lightly doped with arsenic is a p+ substrate
Explaination / Solution:

Trivalent impurities are used for making p - type semiconductors. So, Silicon wafer heavily doped with boron is a p+ substrate.

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Q.4
The drain current of MOSFET in saturation is given by  where K is a constant.
The magnitude of the transconductance gm is
A.
B.
C.
D.
Explaination / Solution:

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Q.5
The OPAMP circuit shown above represents a

A. high pass filter
B. low pass filter
C. band pass filter
D. band reject filter
Explaination / Solution:

and from this equation it may be easily seen that this is the standard form of T.F. of low pass filter
and form this equation it may be easily seen that this is the standard form of T.F. of low pass filter

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Q.6
Consider the following assertions. S1 : For Zener effect to occur, a very abrupt junction is required. S2 : For quantum tunneling to occur, a very narrow energy barrier is required. Which of the following is correct ?
A. Only S2 is true
B. S1 and S2 are both true but S2 is not a reason for S1
C. S1 and S2 and are both true but S2 is not a reason for S1
D. Both S1 and S2 are false
Explaination / Solution:
No Explaination.

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Q.7
A fully charged mobile phone with a 12 V battery is good for a 10 minute talktime. Assume that, during the talk-time the battery delivers a constant current of 2 A and its voltage drops linearly from 12 V to 10 V as shown in the figure. How much energy does the battery deliver during this talk-time?

A. 220 J
B. 12 kJ
C. 13.2 kJ
D. 14.4 J
Explaination / Solution:

The energy delivered in 10 minutes is

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Q.8
A Zener diode, when used in voltage stabilization circuits, is biased in
A. reverse bias region below the breakdown voltage
B. reverse breakdown region
C. forward bias region
D. forward bias constant current mode
Explaination / Solution:

Zener diode operates in reverse breakdown region.

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Q.9
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
Explaination / Solution:

For every 1º C increase in temperature, forward bias voltage across diode decreases by 2.5 mV. Thus for 10º C increase, there us 25 mV decreases

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Q.10
In a uniformly doped BJT, assume that NENB and NC are the emitter, base and collector doping in atoms/cm3, respectively. If the emitter injection efficiency of the BJT is close unity, which one of the following condition is TRUE
A. NE  = NB = NC
B. NE  >> NB and NB > NC
C. N = NB and NB < NC
D. N < NB < NC