76 Questions
Where does the Fermi level for an intrinsic semiconductor lie?
Middle of valence and conduction band
Where does the Fermi level for an n-type semiconductor lie?
Near conduction band
Where does the Fermi level for a p-type semiconductor lie?
Near valence band
In an n-type semiconductor, as the donor impurities No increases, the Fermi level E
Moves towards conduction band
In an n-type semiconductor, as temperature T increases, the Fermi level
Moves towards middle of forbidden energy gap
If ND and NA are the donor and acceptor concentrations then if ND > NA the net impurity concentration is
ND - NA
In silicon, the concentration of thermally generated electrons at room temperature is of the order of
10^10 cm^-3
In a p-n junction at zero bias, the equilibrium current is
Zero because equal no. of charges cross the junction
Given the resistivities of the p-side and n-side of a step graded junction are 5Ωcm and 2.50-Ωcm respectively, what is the height of the potential barrier Vo?
593.4 mV
A Silicon p-n junction diode under reverse bias has a depletion region of width 10µm. What is the depletion capacitance of the diode per square meter?
10 µF
What is the ratio of conductance of a heavily doped n-type semiconductor to that of the intrinsic semiconductor of the same material and at the same temperature given the hole-electron mobility ratio is 0.4?
2,000
If the hole concentration p2 is twice that of p1 in a Hall effect experiment, what is the net Hall voltage VH2?
(1/2) VH1
What is the ratio of depletion-region penetration depths into p and n regions in a step-graded p-n junction diode if the ratio of acceptor to donor impurity atoms’ densities is 1:2?
1:2
An intrinsic semiconductor with an energy gap of 1 eV has a carrier concentration N at 200 K. What temperature will another intrinsic semiconductor with the same carrier concentration have if its energy gap is 3 eV?
653 K
In a heavily doped n-type semiconductor with hole-electron mobility ratio 0.4, what is the approximate doping concentration?
4.2 x 10^18 atoms/m3
If a p-n junction diode has a relative permittivity of 11.7 and the permittivity of free space is 8.85 x 10^-12 F/m, which capacitance closely matches the depletion capacitance if the depletion width is 10µm?
10 µF
What happens to the dynamic impedance of a zener diode as the current through it increases?
Decreases
Which principle does a zener diode operate on?
Tunneling of charge carriers across the junction
What type of temperature coefficient do avalanche breakdown diodes have?
Positive
Which device is commonly used as a voltage regulator?
Zener diode
In a regulated power supply using a zener diode, how should the unregulated input voltage compare to the regulated output voltage?
Larger with opposite polarity
Which characteristic is very close to that of an ideal voltage source?
Zener diode
The volt equivalent of temperature is given by-
KT
Which of the following devices is associated with voltage-controlled capacitance?
Varactor diode
In a p-n junction at zero bias, the equilibrium current is-
Zero because equal number of charges across the junction
What happens to the zener current in a zener regulator if the load resistance decreases?
Increases
For a forward bias pn-junction diode, diffusion capacitance varies-
Linearly with current
The difference between a hole and an electron is that a hole-
Always remains in the valence band
A photodiode is normally operated in which bias condition?
Reverse-biased
A Clipper circuit always-
Clips some part of the input signal
In a p-n-p transistor, current carriers in the base region are primarily ____.
Holes
The primary function of a clamper circuit is to-
Introduce a DC level into an AC signal
What happens to the reverse minority carrier current in a photodiode when light increases?
Increases
If the load resistance decreases in a zener regulator, the series current ____.
Increases
Depletion layer width.......under.......condition and...under.......condition.
Decreases, forward biased, increases, reverse biased
Leakage current of a Junction diode-
Decreases with decreasing temperature
Breakdown does not destroy a zener diode provided the zener current is less than the ____.
Maximum zener current rating
The conductivity of the emitter region in a BJT is kept ____ compared to the base region.
Much larger than that of base region
What is the penetration level of the depletion region when the p-side is lightly doped compared to the n-side?
Is more on the n-side
How can the Fermi level for an n-type material be expressed?
EF = Ec - kT In (Nc)
At very high temperatures, how does an n-type semiconductor behave?
Metal
What factor does NOT influence the built-in potential or barrier potential?
Current
What happens to the Fermi level of a p-type semiconductor when the temperature is increased significantly?
It moves downward
How is the drift velocity of electrons in silicon (Si) related to the electric field?
It is proportional to the electric field for all values of the electric field
Which components make up the space charge region in a pn-junction?
Ionized acceptors and Ionized donors
What causes the early effect in a bipolar junction transistor?
Large collector-base reverse bias
Determine the peak value of the current through the load resistor.
2.325 mA
Determine the average value of the current through the load resistor.
0 mA
What best describes the circuit?
Full-wave rectifier
Determine the peak value of the output waveform.
25 V
Determine the total discharge time for the capacitor in a clamper with C=0.01 µF and R=500 kΩ.
50 ms
Diffusion current in a pn-junction is influenced by which factors?
All of these
Under high electric fields, what happens to the mobility and velocity in a semiconductor?
Mobility decreases and velocity saturates
The ratio of diffusion constant to mobility of carriers in a semiconductor:
Depends on temperature
Which factor affects the gain of transistors?
Early effect
Which statement is true regarding the base width and beta of a transistor?
Both 1 and 2 are TRUE
What is the role of a capacitor in electronic circuits?
Coupling
What happens when all DC sources in a transistor circuit are reduced to zero?
AC equivalent circuit
To obtain an AC equivalent circuit, all of which component needs to be shorted?
Inductors
In the cascode amplifier design, why are the emitter stage (CE) and the common base stage (CB) connected?
The common base stage to amplify the output of the cascade amplifier
What happens to the gain of a bipolar transistor at high frequencies?
It drops
Which of the following is true about the $eta$ of a bipolar transistor?
It increases if the doping concentration in the base is increased
Which capacitor produces an AC ground in an amplifier circuit?
Bypass capacitor
What do the capacitors of a common-emitter (CE) amplifier appear to be to an AC signal?
Open to AC
What is one of the steps in deriving a DC equivalent circuit?
Reducing all DC sources to zero
How is the AC equivalent circuit derived from the original circuit?
By shorting all resistors
If $eta$ = 0.98, $I_{co}$ = 6 µA and $I_a$ = 100 μΑ for a transistor, what is the value of $I_c$?
3.1 mA
For an amplifier with a gain of 1000 and feedback of $eta = 0.1$, if the gain changes by 20% due to temperature, by how much does the gain of the feedback amplifier change?
0.2%
Which region in a transistor has the highest area?
Collector
Why are n-p-n transistors preferred over p-n-p transistors?
Mobility of electron is higher than the hole
Which is the most heavily doped region in a transistor?
Emitter
A BJT is said to be operated in the saturated region if:
Both junctions are forward biased
When is a transistor said to be in the quiescent state?
No signal is applied to the input
The beta (β) of a transistor may be determined directly from the plots of:
VCE versus ic for constant values of Ib
The cut-off frequency of a bipolar junction transistor increases with the:
Decrease in the base width
What does the arrow on the emitter of a transistor indicate?
The direction of conventional current flow
Study Notes
Electronics Devices
Objective Problems
Electronics Devices
- When p-side is lightly doped compared to the n-side, penetration of depletion region is more on the n-side.
- Fermi-Dirac function is given by f(E) = 1 / (1 + e(E-EF)/kT).
- For n-type material, Fermi level can be expressed as EF = Ec - kT In (Nc).
- For p-type material, Fermi level can be expressed as EF = Ev - kT In (Nv).
- At very high temperature, n-type semiconductor behaves as a metal.
- At very high temperature, p-type semiconductor behaves as an intrinsic semiconductor.
- Band gap for semiconductor is given by Eg = kT In (NcNv).
- Factors deciding the built-in potential or barrier potential include the intrinsic concentration (ni) before doping, the level of doping on p and n sides, and temperature.
- The drift velocity of electrons in Si is proportional to the electric field for all values of electric field.
Multiple Choice Questions
- Fermi level for an intrinsic semiconductor lies in the middle of the valence and conduction band.
- Fermi level for an n-type semiconductor lies near the conduction band.
- Fermi level for a p-type semiconductor lies near the valence band.
- In a n-type semiconductor, as the donor impurities No increases, the Fermi level E moves towards the conduction band.
- In a n-type semiconductor, as temperature T increases, the Fermi level moves towards the middle of the forbidden energy gap.
- If ND and NA are the donor and acceptor concentrations, the net impurity concentration is ND - NA.
- In a semiconductor, the Fermi level is proportional to √ND.
- In silicon, the concentration of thermally generated electrons at room temperature is of the order of 10^10 cm^-3.
- In a p-n junction at zero bias, the equilibrium current is due to diffusion of minority carriers.
- For a forward-biased p-n junction diode, diffusion capacitance varies inversely with current.
- The switching speed of a p-n junction depends primarily on the lifetime of minority carriers in the p-region.
- Dynamic resistance of a diode varies as I^-1.
Zener Diode
- Avalanche breakdown occurs in reverse bias conditions in a wider junction diode.
- Zener breakdown diodes have a negative temperature coefficient.
- Avalanche breakdown diodes have a positive temperature coefficient.
- Zener diode works on the principle of tunneling of charge carriers across the junction.
- The dynamic impedance of a zener diode decreases with an increase in current through it.
- Zener diode is commonly used as a voltage regulator.
- It has a DC power dissipation rating and a zener voltage rating.
- Zener resistance can be determined by the change of voltage with the change of current.
Bipolar Junction Transistor (BJT)
- The conductivity of the emitter region of a BJT is kept much larger than that of the base region.
- In a p-n-p transistor, in the base region, the main stream of current is due to the diffusion of holes.
- The number of depletion layers in a transistor is two.
- In a p-n-p transistor, current carriers are holes.
- At the base emitter junction of a transistor, one finds forward bias.
- Most of the majority carriers from the emitter pass through the base region to the collector.
- In a properly biased transistor, both depletion regions are forward biased.
- The phase difference between the input and output voltages in a common base arrangement is 0°.
Other Notes
- The current in a voltage multiplier circuit diodes and capacitors are suitable for high voltage low current devices.
- The current in the diode is 3mA when the voltage is 110V.### Transistor Parameters and Amplifiers
- AC equivalent circuit is derived by shorting all resistors.
- AC emitter resistance is calculated by dividing 25 mV by the DC emitter current.
- Transconductance of a cascode amplifier is the product of the transconductances of the common-emitter and common-base stages divided by 2.
- In a transistor, if Ic ≥ βdc * Ie, the transistor is in saturation.
- For a PNP transistor, Iceo = Ico / (1 - α).
- Collector current (Ic) of a transistor can be calculated using the given values of α, VBE, and Ie.
- IcEO = Ie - Ico.
- Gain of a feedback amplifier changes by a percentage of the original gain multiplied by the feedback factor (β) when the open-loop gain changes.
- Common Mode Rejection Ratio (CMRR) is the ratio of the differential gain to the common-mode gain, expressed in decibels.
- Collector current (Ic) of a transistor can be calculated using the given values of α, Ico, and Ie.
- Quiescent collector current of a common emitter amplifier can be calculated using the given circuit diagram and transistor parameters.
This quiz tests your knowledge of transistors, including their components, preferences, and operating modes. Learn about the emitter, base, and collector regions of a transistor and how they work together.
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