Electronics and Communication Engineering - Electronic Devices and Circuits
Exam Duration: 45 Mins Total Questions : 30
The drift velocity of electrons, in silicon
- (a)
is proportional to the electric field for all values of electric field
- (b)
is independent of the electric field
- (c)
increases at low values of electric field and decreases as high values of electric field exhibiting negative differential resistance.
- (d)
increases linearly with electric field at low values of electric field and gradually saturates at higher values of electric field.
If an intrinsic semiconductor is doped with a very small amount of aluminium, then in the extrinsic semiconductor so formed, the number of electrons and holes will
- (a)
decrease
- (b)
increase and decrease respectively
- (c)
increase
- (d)
decrease and increase respectively
A GaAs device is doped with a donor concentration of 3 x 1015 cm-3 . For the device to operate properly, the intrinsic carrier concentration must remain less than 5% of the total concentration. The maximum temperature on which the device may operate is
- (a)
763 K
- (b)
769 K
- (c)
486 K
- (d)
243 K
A p-n junction diode's dynamic conductance is directly proportional to
- (a)
the applied voltage
- (b)
the temperature
- (c)
its current
- (d)
the thermal voltage
The diffusion capacitance of a p-n junction diode
- (a)
increases exponentially with forward-bias voltage
- (b)
decreases exponentially with forward-bias voltage
- (c)
decreases linearly with forward-bias voltage
- (d)
increases linearly with forward-bias voltage
The static characteristic of an adequately forward-biased p-n junction is a straight line, if the plot is of
- (a)
\(\log { |vs\log { V } } \)
- (b)
\(\log { |vsV } \)
- (c)
\(|vs\log { V } \)
- (d)
\(|vsV\)
Avalanche breakdown diodes have breakdown voltage
- (a)
having positive temperature coefficient
- (b)
having negative temperature coefficient
- (c)
independent of temperature
- (d)
None of the above
A p+ -n silicon diode is forward biased at a current of 1 mA. The hole life time in the n-region is \(0.1\mu s\). Neglecting depletion capacitance, the diode impedance at 1 MHz is
- (a)
\(38.7+j12.1\Omega \)
- (b)
\(23.5+j7.5\Omega \)
- (c)
\(38.7-j12.1\Omega \)
- (d)
\(23.5-j7.5\Omega \)
Match List I (biasing of the junctions) with List II (functions) and select the correct answer using the codes given below the lists.
List I | List II |
A. E-B junction forward-bias and C-B junction reverse-bias | Very low gain amplifier |
B. Both E-B and condition forward-bias | Saturation C-B junctions |
C. E-B junction reverse bias and C-B junction forward-bias | High gain and amplifier |
D. Both E-B and C-B junction reverse-bias | Cut-off condition |
- (a)
A B C D 2 3 1 4 - (b)
A B C D 3 2 1 4 - (c)
A B C D 3 2 4 1 - (d)
A B C D 2 3 4 1
An n-channel JFET has a pinch-off voltage of VD = - 5 V, VDS (max) = 20 V and gm = 2 mA/V. Minimum on resistance is achieved in the JFET for
- (a)
VGS = - 7 V and VDS = 0
- (b)
VGS = 0 and VDS = 0
- (c)
VGS = 0 and VDS = 20 V
- (d)
VGS = - 7 V and VDS = 20 V
In a GaAs sample the electrons are moving under an electric field of 5 kV/cm and the carrier concentration is uniform at 1016 cm-3. The electron velocity is the saturated velocity of 107 cm/s. The drift current density is
- (a)
1.6 x 104 A/cm2
- (b)
2.4 x 104 A/cm2
- (c)
1.6 x 108 A/cm2
- (d)
2.4 x 108 A/cm2
In an RC coupled common-emitter amplifier
- (a)
coupling capacitance affects the hf response and bypass capacitance affects the If response
- (b)
both coupling and bypass capacitance affect the If response only
- (c)
coupling capacitance affects the If response and the bypass capacitance affects the hf response
- (d)
both coupling and bypass capacitances affects the hf response only
The bandgap of silicon at room temperature is
- (a)
1.3 eV
- (b)
0.7 eV
- (c)
1.1 eV
- (d)
1.4 eV
The primary reason for the widespread use of silicon in semiconductor device technology is
- (a)
abundance of silicon on the surface of the earth
- (b)
larger bandgap of silicon in comparison to germanium
- (c)
favourable properties of silicon-dioxide (SiO2)
- (d)
lower melting point
The current gain of a bipolar transistor drops at high frequencies because of
- (a)
transistor capacitance
- (b)
high current effects in the base
- (c)
parasitic inductive elements
- (d)
the early effect
Two p+ -n junction is reverse-biased at VR = 7 V. The impurity doping concentration in junction A and NA = 1016 cm-3 and ND = 1016 cm-3, and those in junction B are Na = 1016 cm-3 and ND = 1018 cm-3 . The ratio of the space charge width is
- (a)
9.8
- (b)
19
- (c)
4.3
- (d)
1.39
The hole concentration in p-type GaAs is given by
\(p={ 10 }^{ 16 }\left( 1-\frac { X }{ L } \right) { cm }^{ -3 }\) for \(0\le X\le L\)
where, \(L=10\mu m\). The hole diffusion coefficient is 10 cm2/s. The hole diffusion current density at \(X=5\mu m\) is
- (a)
20 A/cm2
- (b)
16 A/cm2
- (c)
24 A/cm2
- (d)
30 A/cm2
In a silicon semiconductor material the doping concentration are NA = 1016 cm-3 and NA = 0. The equilibrium recombination rate is Rp0 = 1011 cm-3 s-1, A uniform generation rate produces an excess-carrier concentration of \(\delta n=\delta p={ 10 }^{ 14 }{ cm }^{ -3 }\) The factor, by which the total recombination rate increase is
- (a)
2.3 x 1013
- (b)
4.4 x 1013
- (c)
2.3 x 109
- (d)
4.4 x 109
Silicon is doped with boron to a concentration of 4 x 1017 atom/cm3 . Assuming the intrinsic carrier concentration of silicon to be 1.5 x 10 10 /cm3 and the value of \(\frac { kT }{ q } \) to be 25 mV at 300 K. Compared to undoped silicon, the Fermi level of doped silicon
- (a)
goes down by 0.13 eV
- (b)
goes up by 0.13 eV
- (c)
goes down by 0.427 eV
- (d)
goes up by 0.427 eV
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 are both true and S2 is a reason for S1
- (d)
Both S1 and S2 are false
A uniformly doped silicon p+ -n junction is to be designed such that at a reverse-bias voltage of VR = 10 V. The maximum electric field is limited to Emax = 106 V/cm. The maximum doping concentration in the n-region is
- (a)
3.2 x 1019 cm-3
- (b)
3.2 x 1017 cm-3
- (c)
6.4 x 1017 cm-3
- (d)
6.4 x 1019 cm-3
The majority carriers in an n-type semiconductor have an average drift velocity v in a direction perpendicular to a uniform magnetic field B. The electric field E induced due to Hall effect acts in the direction
- (a)
v x B
- (b)
B x v
- (c)
along v
- (d)
opposite to v
A heavily doped n-type semiconductor has the following data :
Hole-electron mobility ratio : 0.4
Doping concentration : 4.2 x 102 atom/m3
Intrinsic concentration : 1.5 x 104 atom/m3
The ratio of conductance of the n-type semiconductor to that of the intrinsic semiconductor of same material and at the same temperature is given by
- (a)
0.00005
- (b)
2000
- (c)
10000
- (d)
20000
A silicon sample A is doped with 1010 atom/cm3 of boron. Another sample B of identical dimensions is doped with 1010 atom/cm3 of phosphorous. The ratio of electron to hole mobility is 3. The ratio of conductivities of the sample A to B is
- (a)
3
- (b)
1/3
- (c)
2/3
- (d)
3/2
The neutral base width of a bipolar transistor, biased in the active region, is 0.5 \(\mu m\). The maximum electron concentration and the diffusion constant in the base are 1014 / cm3 and Dn = 25 cm2 /s respectively. Assuming negligible recombination 1 the vase, the collector current density is (the electron charge is 1.6 x 10-19 C)
- (a)
800 A/cm2
- (b)
8 A/cm2
- (c)
200 A/cm2
- (d)
2 A/cm2
For a particular NMOS device the parameters are \({ V }_{ TN }=1V,L=2.4\mu m,{ \mu }_{ n }=600{ cm }^{ 2 }/V-s\) and \({ t }_{ ox }=400\dot { A } \). When device is biased in the saturation region at VGS = 5 V, the drain current is ID = 1.2 mA. The channel width of device is
- (a)
7.21 \(\mu m\)
- (b)
10.46 \(\mu m\)
- (c)
5.23 \(\mu m\)
- (d)
20.92 \(\mu m\)
Consider the ideal long silicon p-n junction diode shown below. The n-region is doped with 1016 organic atom per cm3 and the p-region is doped with 5 x 1016 boron atom per cm 3 . The minority carrier lifetimes are Dn = 23 cm2 /s and Dp = 8 cm2/s. The forward bias voltage is Va = 0.61 V.
The hole diffusion current density at X = 3 \(\mu m\) is
- (a)
0.6 A/cm2
- (b)
0.6 x 10-3 A/cm2
- (c)
0.4 A/cm2
- (d)
0.4 x 10-3 A/cm2
Consider a silicon p-n junction at room temperature having the following parameters :
Doping on the n-side = 1 x 1017 cm-3
Depletion width of the n-side = 0.1 \(\mu m\)
Depletion width of the p-side = 1.0 \(\mu m\)
Intrinsic carrier concentration = 1.4 x 1010 F/cm
Thermal voltage = 26 mV
Permittivity of free space = 8.85 x 10-14 F/cm
Dielectric constant of silicon = 12
The built-in potential of the junction is
- (a)
0.70 V
- (b)
0.76 V
- (c)
0.82
- (d)
Cannot be estimated from the data given
For the transistor in circuit shown below. The parameters are \({ \beta }_{ R }=1,{ \beta }_{ F }=100,{ I }_{ S }=1fA\)
The current IE is
- (a)
1 fA
- (b)
- 1 fA
- (c)
2 fA
- (d)
- 2 fA
Determine the region of operation for the transistor shown in circuit in questions.
In the following circuit the transistor is in
- (a)
forward-active
- (b)
reverse-active
- (c)
saturation
- (d)
cut-off