Questions & Answers of Electronic Devices

A small percentage of impurity is added to an intrinsic semiconductor at 300 K. Which one of the following statements is true for the energy band diagram shown in the following figure?


Consider the following statements for a metal oxide semiconductor field effect transistor (MOSFET):
P: As channel length reduces, OFF-state current increases.
Q: As channel length reduces, output resistance increases.
R: As channel length reduces, threshold voltage remains constant.
S: As channel length reduces, ON current increases.

Which of the above statements are INCORRECT?

What is the voltage Vout in the following circuit?


Consider a silicon p-n junction with a uniform acceptor doping concentration of 1017 cm−3 on the pside and a uniform donor doping concentration of 1016 cm−3 on the n-side. No external voltage is applied to the diode. Given: $kT/q=26\;\mathrm{mV},\;{\mathrm n}_\mathrm i=1.5\times10^{10}\;\mathrm{cm}^{-3},{\mathrm\varepsilon}_\mathrm{Si}=12{\mathrm\varepsilon}_0,\;{\mathrm\varepsilon}_0=8.85\times10^{-14}\mathrm F/\mathrm m$, and q = 1.6 ×10−19 C.
The charge per unit junction area (nC cm−2) in the depletion region on the p-side is ___________

Consider an n-channel metal oxide semiconductor field effect transistor (MOSFET) with a gate-tosource voltage of 1.8 V. Assume that WL=4,μNCox=70×10-6AV-2 the threshold voltage is 0.3V, and the channel length modulation parameter is 0.09 V−1. In the saturation region, the drain conductance (in micro seimens) is ________

The figure below shows the doping distribution in a p-type semiconductor in log scale

The magnitude of the electric field (in kV/cm) in the semiconductor due to non uniform doping is _________

Consider a silicon sample at T = 300 K, with a uniform donor density Nd = 5×1016 cm−3, illuminated uniformly such that the optical generation rate is Gopt= 1.5×1020 cm−3 s-1 throughout the sample. The incident radiation is turned off at t = 0. Assume low-level injection to be valid and ignore surface effects. The carrier lifetimes are τpo=0.1= 0.1 μs andτn0=0.5 = 0.5 μs.

The hole concentration at t=0 and the hole concentration at t=0.3 μs, respectively, are

The Ebers-Moll model of a BJT is valid

A long-channel NMOS transistor is biased in the linear region with VDS = 50 mV and is used as a resistance. Which one of the following statements is NOT correct?

Assume that the diode in the figure has Von = 0.7 V, but is otherwise ideal.

The magnitude of the current i2 (in mA) is equal to ________

A voltage VG is applied across a MOS capacitor with metal gate and p-type silicon substrate at T=300 K. The inversion carrier density (in number of carriers per unit area) for VG = 0.8 V is 2×1011 cm−2. For VG = 1.3 V, the inversion carrier density is 4×1011 cm−2. What is the value of the inversion carrier density for VG = 1.8 V?

Consider avalanche breakdown in a silicon p+n junction. The n-region is uniformly doped with a donor density ND. Assume that breakdown occurs when the magnitude of the electric field at any point in the device becomes equal to the critical field Ecrit. Assume Ecrit to be independent of ND. If the built-in voltage of the p+n junction is much smaller than the breakdown voltage, VBR, the relationship between VBR and ND is given by

Consider a region of silicon devoid of electrons and holes, with an ionized donor density of Nd+=1017 cm−3. The electric field at x=0 is 0 V/cm and the electric field at x=L is 50 kV/cm in the positive x direction. Assume that the electric field is zero in the y and z directions
Given q=1.6×10−19 coulomb, 0=8.85×10-14 F/cm, r=11.7 for silicon, the value of L in nm is ________

The I-V characteristics of three types of diodes at the room temperature, made of semiconductors X, Y and Z, are shown in the figure. Assume that the diodes are uniformly doped and identical in all respects except their materials. If EgX, EgY and EgZ are the band gaps of X, Y and Z, respectively, then


The figure shows the I-V characteristics of a solar cell illuminated uniformly with solar light of power 100 mW/cm2. The solar cell has an area of 3 cm2 and a fill factor of 0.7. The maximum efficiency (in %) of the device is __________


The injected excess electron concentration profile in the base region of an npn BJT, biased in the active region, is linear, as shown in the figure. If the area of the emitter-base junction is 0.001 cm2, μn = 800 cm2/(V-s) in the base region and depletion layer widths are negligible, then the collector current IC (in mA) at room temperature is __________
(Given: thermal voltage VT = 26 mV at room temperature, electronic charge q = 1.6 × 10−19 C)


Figures I and II show two MOS capacitors of unit area. The capacitor in Figure I has insulator materials X (of thickness t1 = 1 nm and dielectric constant ε1=4) and Y (of thickness t2 = 3 nm and dielectric constant ε2=20). The capacitor in Figure II has only insulator material X of thickness tEq. If the capacitors are of equal capacitance, then the value of tEq (in nm) is __________


The I-V characteristics of the zener diodes D1 and D2 are shown in Figure I. These diodes are used in the circuit given in Figure II. If the supply voltage is varied from 0 to 100 V, then breakdown occurs in


A region of negative differential resistance is observed in the current voltage characteristics of a silicon PN junction if

A silicon sample is uniformly doped with donor type impurities with a concentration of 1016/cm3. The electron and hole mobilities in the sample are 1200 cm2/V-s and 400 cm2/V-s respectively. Assume complete ionization of impurities. The charge of an electron is 1.6 X 10-19 C. The resistivity of the sample (in Ω-cm) is _______.

In the circuit shown below, the Zener diode is ideal and the Zener voltage is 6V. The output voltage Vo (in volts) is _______.

The built-in potential of an abrupt p-n junction is 0.75 V. If its junction capacitance (CJ) at a reverse bias (VR) of 1.25 V is 5 pF, the value of CJ (in pF) when VR = 7.25 V is_________.

A MOSFET in saturation has a drain current of 1 mA for VDS =0.5V. If the channel length modulation coefficient is 0.05 V-1, the output resistance (in kΩ) of the MOSFET is_________

For a silicon diode with long P and N regions, the accepter and donor impurity concentrations are 1 X 1017 cm-3 and 1X1015 cm-3, respectively. The lifetimes of electrons in P region and holes in N region are both 100 μs. The electron and hole diffusion coefficients are 49 cm2/s and 36 cm2/s, respectively. Assume kT/q = 26m V , the intrinsic carrier concentration is 1 X 1010 cm-3, and q = 1.6 X 10-19 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 _________.

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 of VSS is represented by

An n-type silicon sample is uniformly illuminated with light which generates 1020 electron hole pairs per cm3 per second. The minority carrier lifetime in the sample is 1 μs. In the steady state, the hole concentration in the sample is approximately 10x, where x is integer. The value of x is __________

A piece of silicon is doped uniformly with phosphorous with a doping concentration of 1016/cm3. The expected value of mobility versus doping concentration for silicon assuming full dopant ionization is shown below. The charge of an electron is 1.6X10-19 C. The conductivity (in S cm-1) of the silicon sample at 300 K is ______.

A dc voltage of 10 V is applied across an n-type silicon bar having a rectangular cross-section and a length of 1 cm as shown in figure. The donor doping concentration ND and the mobility of electrons μn are 1016 cm-3 and 1000 cm2 V-1s-1 , respectively . The average time (in μs) taken by the electrons to move from one end of the bar to other end is________.

In a MOS capacitor with an oxide layer thickness of 10 nm, the maximum depletion layer thickness is 100 nm. The permittivities of the semiconductor and the oxide layer are εs and εos respectively. Assuming εsos= 3, the ratio of the maximum capacitance to the minimum capacitance of this MOS capacitor is _____________.

The energy band diagram and the electron density profile n(x) in a semiconductor are shown in figures. Assume that nx=1015eqaxkTcm-3 , with α = 0.1 V/cm and x expressed in cm. Given kTq=0.026V, Dn = 36cm2s–1 , and Dμ=kTq . The electron current density (in A/cm2) at x = 0 is

Which one of the following process is preferred to form the gate dielectric (SiO2) of MOSFETs ?

If the base width in a bipolar junction transistor is doubled, which one of the following statements will be TRUE?

The electric field profile in the depletion region of a p-n junction in equilibrium is shown in the figure. Which one of the following statements is NOT TRUE?

The current in an enhancement mode NMOS transistor biased in saturation mode was measured to be 1 mA at a drain-source voltage of 5 V. When the drain-source voltage was increased to 6 V while keeping gate-source voltage same, the drain current increased to 1.02 mA. Assume that drain to source saturation voltage is much smaller than the applied drain-source voltage. The channel length modulation parameter λ (in V-1) is ______.

An npn BJT having reverse saturation current Is = 10-15 A is biased in the forward active region with VBE = 700 mV. The thermal voltage (VT) is 25 mV and the current gain (β) may vary from 50 to 150 due to manufacturing variations. The maximum emitter current (in μA) is _____.

When the optical power incident on a photodiode is 10µW and the responsivity is 0.8 A/W, the photocurrent generated (in µA) is ________.

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

 If fixed positive charges are present in the gate oxide of an n-channel enhancement type MOSFET, it will lead to

A BJT is biased in forward active mode. Assume VBE = 0.7 V, kT/q = 25 mV and reverse saturation IS=10-13 A. The transconductance of the BJT (in mA/V) is________.

The doping concentrations on the p-side and n-side of a silicon diode are 1×1016CM-3 m and 1×1017CM-3 m , respectively. A forward bias of 0.3 V is applied to the diode. At T = 300 K, the intrinsic carrier concentration of silicon ni=1.5×1010CM-3 m and kTq=26 mV. The electron concentration at the edge of the depletion region on the p-side is ___________.

A depletion type N-channel MOSFET is biased in its linear region for use as a voltage  controlled resistor. Assume threshold voltage VTH= -0.5 V, VGS= 2.0 V, VDS= 5 V, W/L = 100, COX= 10-8 F/ Cm2 and μn= 800 Cm2 /V-s. The value of the resistance of the voltage Controlle resistor (in Ω) is_____.

A silicon bar is doped with donor impurities ND = 2.25 x 1015 atoms / cm3. Given the intrinsic carrier concentration of silicon at T = 300 K is ni = 1.5 x 1010 cm-3. Assuming complete impurity ionization, the equilibrium electron and hole concentrations are

An increase in the base recombination of a BJT will increase

In CMOS technology, shallow P-well or N-well regions can be formed using

Assume electronic charge q = 1.6×10-19 C, kT/q = 25 mV and electron mobility μn = 1000 cm2/V-s. If the concentration gradient of electrons injected into a P-type silicon sample is 1×1021/cm4, the magnitude of electron diffusion current density (in A/cm2) is _________.

Consider an abrupt PN junction (at T = 300 K) 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 (εsi) is 1.044×10-12 F/cm. At the junction, the approximate value of the peak electric field (in kV/cm) is _________.

When a silicon diode having a doping concentration of NA = 9 × 1016 cm-3 on p-side and ND = 1 × 1016 cm-3 on n-side is reverse biased, the total depletion width is found to be 3 μm. Given that the permittivity of silicon is 1.04 × 10–12 F/cm, the depletion width on the p-side and the maximum electric field in the depletion region, respectively, are

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

For the n-channel MOS transistor shown in the figure, the threshold voltage VTh is 0.8 V. Neglect channel length modulation effects. When the drain voltage VD=1.6 V, the drain current ID was found to be 0.5 mA. If VD is adjusted to be 2 V by changing the values of R and VDD, the new value of ID (in mA) is

For the MOSFETs shown in the figure, the threshold voltage |Vt| = 2 V and

K=12μCoxWL=0.1 mA/V2 The value of ID (in mA) is _______ .

In MOSFET fabrication, the channel length is defined during the process of

A thin P-type silicon sample is uniformly illuminated with light which generates excess carriers. The recombination rate is directly proportional to

At T = 300 K, the hole mobility of a semiconductor μp=500 cm2/V-s and kTq=26mV The hole diffusion constant Dp in cm2/s is ________

The donor and accepter 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 300 K, kTq = 26 mV and the permittivity of silicon si =1.04×10−12F/cm. The built-in potential and the depletion width of the diode under thermal equilibrium conditions, respectively, are

The slope of the ID vs.VGS curve of an n-channel MOSFET in linear regime is 10−3Ω−1 at VDS=0.1 V. For the same device, neglecting channel length modulation, the slope of the ID vs. VGS curve (in AV) under saturation regime is approximately _________.

An ideal MOS capacitor has boron doping-concentration of 1015 cm-3 in the substrate. When a gate voltage is applied, a depletion region of width 0.5 μm is formed with a surface (channel) potential of 0.2 V. Given that ε0 = 8.854 × 10-14 F/cm and the relative permittivities of silicon and silicon dioxide are 12 and 4, respectively, the peak electric field (in V/μm) in the oxide region is _____________.

For the MOSFET M1 shown in the figure, assume W/L = 2, VDD = 2.0 V,μnCox=100  μA/V2 and VTH = 0.5 V. The transistor M1 switches from saturation region to linear region when Vin (in Volts) is__________.

At T = 300 K, the band gap and the intrinsic carrier concentration of GaAs are 1.42 eV and 106 cm-3, respectively. In order to generate electron hole pairs in GaAs, which one of the wavelength (λC) ranges of incident radiation, is most suitable? (Given that: Plank’s constant is 6.62 × 10-34 J-s, velocity of light is 3 × 1010 cm/s and charge of electron is 1.6 × 10-19 C)

In the figure, ln (ρi) is plotted as a function of 1/T, where ρi is the intrinsic resistivity of silicon, T is the temperature, and the plot is almost linear.

The slope of the line can be used to estimate

The cut-off wavelength (in μm) of light that can be used for intrinsic excitation of a semiconductor material of bandgap Eg= 1.1 eV is ________

Consider a silicon sample doped with ND = 1×1015/cm3 donor atoms. Assume that the intrinsic carrier concentration ni = 1.5×1010/cm3. If the sample is additionally doped with NA = 1×1018/cm3 acceptor atoms, the approximate number of electrons/cm3 in the sample, at T=300 K, will be ______.

Consider two BJTs biased at the same collector current with area A1 = 0.2 μm × 0.2 μm and A2 = 300 μm × 300 μm. Assuming that all other device parameters are identical, kT/q = 26 mV, the intrinsic carrier concentration is 1 × 1010 cm-3, and q = 1.6 × 10-19 C, the difference between the base-emitter voltages (in mV) of the two BJTs (i.e., VBE1 – VBE2) is _____.

An N-type semiconductor having uniform doping is biased as shown in the figure.

If EC is the lowest energy level of the conduction band, EV is the highest energy level of the valance band and EF is the Fermi level, which one of the following represents the energy band diagram for the biased N-type semiconductor?

A BJT in a common-base configuration is used to amplify a signal received by a 50 Ω antenna. Assume kT/q = 25 mV. The value of the collector bias current (in mA) required to match the input impedance of the amplifier to the impedance of the antenna is________.

In a forward biased pn junction diode, the sequence of events that best describes the mechanism of current flow is

In IC technology, dry oxidation (using dry oxygen) as compared to wet oxidation (using steam or water vapor) produces

The return loss of a device is found to be 20 dB. The voltage standing wave ratio (VSWR) and magnitude of reflection coefficient are respectively

The i-v characteristics of the diode in the circuit given below are

i=v-0.7500A,v0.7v0 A,v<0.7v

The current in the circuit is

The source of a silicon (ni = 1010 per cm3) n-channel MOS transistor has an area of 1 sq μm and a depth of 1 μm. If the dopant density in the source is 1019/cm3, the number of holes in the source region with the above volume is approximately

In the three dimensional view of a silicon n-channel MOS transistor shown below, δ= 20 nm. The transistor is of width 1 μm. The depletion width formed at every p-n junction is 10 nm. The relative permittivities of Si and SiO2, respectively, are 11.7 and 3.9, and ε0 = 8.9 × 10-12 F/m.

The gate-source overlap capacitance is approximately

In the three dimensional view of a silicon n-channel MOS transistor shown below, δ= 20 nm. The transistor is of width 1 μm. The depletion width formed at every p-n junction is 10 nm. The relative permittivities of Si and SiO2, respectively, are 11.7 and 3.9, and ε0 = 8.9 × 10-12 F/m.

The source-body junction capacitance is approximately

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 Zener diode, when used in voltage stabilization circuits, is biased in

For a BJT the common base current gain α = 0.98 and the collector base junction reverse bias saturation current ICO  = 0.6μA. This BJT is connected in the common emitter mode and operated in the active region with a base drive current IB=20μA. The collector current IC for this mode of operation is

At room temperature, a possible value for the mobility of electrons in the inversion layer of a silicon n-channel MOSFET is

In a uniformly doped BJT, assume that NE, NB and NC are the emitter, base and collector dopings in atoms/cm3, respectively. If the emitter injection efficiency of the BJT is close unity, which one of the following conditions is TRUE?

Compared to a p-n junction with NA=ND=1014/cm3, which one of the following statements is TRUE for a p-n junction with NA=ND=1020/cm3?

Consider the following two statements about the internal conditions in an n-channel MOSFET operating in the active region
             S1: The inversion charge decreases from source to drain
             S2: The channel potential increases from source to drain
Which of the following is correct?

Which of the following is NOT associated with a p-n junction?

The drain current of a MOSFET in saturation is given by  ID=KVGS-VT2 where K is a constant. The magnitude of the transconductance gm is

The measured transconductance gm of an NMOS transistor operating in the linear region is plotted against the gate voltage VG at constant drain voltage VD. Which of the following figures represents the expected dependence of gm on VG ?

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?

In a p+n junction diode under reverse bias, the magnitude of electric field is maximum at

A p+n junction has a built-in potential of 0.8 V. The depletion layer width at a reverse bias of 1.2V is 2µm. For a reverse bias of 7.2 V, the depletion layer width will be

Group I lists four types of p-n junction diodes. Match each device in Group I with one of the option in Group II to indicate the bias condition of that device in its normal mode of operation.

 Group I Group II
P. Zener Diode 1. Forward bias
Q. Solar cell 2. Reverse bias
R. LASER diode  
S. Avalanche Photodiode  

Group I lists four different semiconductor devices. Match each device in Group I with its characteristic property in Group II.

 Group I Group II
P. BJT 1. Population inversion
Q. MOS capacitor 2. Pinch-off voltage
R. LASER diode 3. Early effect
S. JFET 4. Flat-band voltage

Subjects of Electronics and Communication Engineering