interview questions with
answer:
- Electronic Devices :: Semiconductors
Q1. What is Fermi level?
The maximum energy that an electron in a metal has at the absolute zero
temperature is called the Fermi level of energy.
Q2. What is the forbidden energy gap?
How does it occur? What is its magnitude for Ge and Si?
The energy gap between the valence band and conduction band is known as
forbidden energy gap. It is a region in which no electron can stay as there is
no allowed energy state. Magnitude of forbidden energy gap in germanium and
silicon is 0.72 eV and 1.12 eV respectively at 300 K and 0.785 eV and 1.21 eV
respectively at absolute zero temperature.
Q3. Is a hole a fundamental particle
in an atom?
Hole is not a fundamental particle in an atom. Holes may be thought of
as positive particles, and as such they move through an electric field in a
direction opposite to that of electrons.
Q4. What is hole current?
The movement of the hole (positively charged vacancy in the valence
band) from positive terminal of the supply to negative terminal through
semiconductor constitutes hole current.
Q5. What is the basis for classifying
a material as a conductor, semiconductor, or a dielectric? What is the conductivity
of perfect dielectric?
Conductors possess high conductivity
whereas the characteristic property of insulating materials (or dielectrics) is
poor conductivity. Semiconductors occupy an intermediate position between
conductors and insulators. Though there is no rigid line separating the
conductors from semiconductors and semiconductors from insulators, but still
according to resistivity the materials of resistivity of the order from 10-8 to 10-3 , 10-13 to 106 and 106 to 1018 ohm-meters
may be classified as conductors, semiconductors and dielectrics respectively.
Another classification is based on temperature coefficient of
resistivity. Metals have positive temperature coefficient of resistivity.
Semiconductors have small negative temperature coefficient of resistivity and
insulators have large negative temperature coefficient of resistivity.
To improve breakdown voltage.
Q7. Define a hole in a semiconductor.
When an energy is supplied to a semiconductor a valence electron is
lifted to a higher energy level. The departing electron leaves a vacancy in the
valence band. The vacancy is called a hole. Thus, a vacancy left in the valence
band because of lifting of an electron from the valence band to conduction band
is known as a hole.
Q8. What Is Doping?
The process of deliberately adding impurities to affect conductivity is
called Doping.
Q9. What is intrinsic semiconductor ?
An intrinsic semiconductor is one which is made of the semiconductor
material in the extremely pure form (impurity content not exceeding one part in
100 million parts of semiconductors).
Q10. Why silicon and germanium are
the two widely used semiconductor materials?
Because the energy required to release an electron from their valence
band (i.e. to break their covalent bonds ) is very small (1.12eV for Si and
0.72eV for Ge).
Conduction occurs in any given material when an applied electric field
causes electrons to move in a desired direction within the material. This may
be due to one or both of two processes, electron motion and hole transfer. In
case of former process, free electrons in the conduction band move under the
influence of the applied electric field. Hole transfer involves electrons
which are still attached to the atoms i.e. those in valence band.
Q12. Why does a pure semiconductor behave like an insulator at absolute zero temperature?
For a pure semiconductor at a
temperature of absolute zero (-273.15oC)the valence
band is usually full and there are may be no electron in the conduction band
and it is difficult to provide additional energy required for lifting electron
from valence band to conduction band by applying electric field. Hence the
conductivity of a pure semiconductor at absolute zero temperature is zero and
it behaves like an insulator.
Q13. What is the main factor for
controlling the thermal generation and recombination?
Temperature, because with the increase in the temperature,
concentrations of free electrons and holes increase and the rate of
recombination is proportional to the product of concentration of free electrons
and holes and also the rate of production of electron-hole pairs (thermal
generation) increases with the rise in temperature.
Q14. Define mean life of a carrier.
The amount of time between the creation and disappearance of a free
electron is called the life time. It varies from a few nanoseconds to several
microseconds depending how perfect the crystal is and other factors.
Q15. In which bands do the movement
of electrons and holes take place?
Free electrons move in valence band while holes in valence band.
Q16. Which of the two semiconductor
materials Si or Ge has larger conductivity at room temperature? Why?
Since energy required in transferring electrons from valence band to
conduction band is more in case of Si than that in case of germanium , the
conductivity of Ge will be more than that of Si at room temperature.
Q17. What do you mean by drift
velocity and mobility of a free electron?
The average velocity of an electron is known as drift velocity whereas
mobility of an electron is defined as the drift velocity per unit electric
field.
Q18. Define mobility of a carrier.
Show that the mobility constant of electron is larger than that of a hole.
Mobility is defined as the average particle drift velocity per unit
electric field.
The mobility of electrons is more than that of holes because the
probability of an electron having the energy required to move to an empty state
n the conduction band is much greater than the probability of an electron
having the energy required to move to the empty state in valence band. The
mobility of electron is about double that of a hole.
Q19. Define diffusion current in a
semiconductor.
The diffusion of charge carriers is a result of a gradient of carrier
concentration (i.e., the difference of carrier concentration from one region to
another). In this case concentrations of charge carriers (either electrons or
holes ) tend to distribute themselves uniformly throughout the
semiconductor crystal. This movement continues until all carriers are evenly distributed
throughout the material. This type of movement of charge carriers is called
diffusion current.
Q20. Define drift current in a
semiconductor.
The steady flow of electrons in one direction caused by applied electric
field constitutes an electric current, called the drift current.
Q21. What happens to the conductivity
of semiconductor with the rise in temperature? Compare with the conductivity of
metals.
With the increase in temperature, the concentration of charge carriers
increases resulting in increase in conductivity of semiconductors. The
conductivity of metal decreases with the increase in temperature.
Q22.
Why temperature coefficient of resistance of a semiconductor is negative?
With the increase
in temperature, the concentration of charge carriers (electrons and holes)
increases. As more charge carriers are made available, the conductivity of a
pure semiconductor increases i.e. resistivity of a pure semiconductor decreases
with the rise in temperature i.e. semiconductors have negative temperature
coefficient of resistance.
Q23.
What is meant by Fermi level in semiconductor? Where does the Fermi level lie
in an intrinsic semiconductor?
Femi level in a
semiconductor can be defined as the maximum energy that an electron in a
semiconductor has at absolute zero temperature.
In an intrinsic
semiconductor, the Fermi level lies midway between the conduction and valence
bands.
Q24.
Differentiate between intrinsic semiconductors and intrinsic semiconductors?
An intrinsic
semiconductor is one which is made of the semiconductor material in its
extremely pure form.
When a small amount
of impurity is added to a pure semiconductor crystal during the crystal growth
in order to increase its conductivity, the resulting crystal is called
extrinsic semiconductor.
Q25. For
the same order of doping, why does n-type semiconductor exhibit larger
conductivity than p-type semiconductor?
Since the mobility
of electrons is higher than that of holes, for same level of doping,
n-type semiconductor exhibits larger conductivity.
Q26.
Describe the difference between P-type and N-type semiconductor materials.
When a small amount
of trivalent impurity (such as boron, gallium, indium or aluminium) is added to
a pure semiconductor crystal during crystal growth, the resulting crystal is
called a P-type semiconductor.
When a small amount
of pentavalent impurity (such as arsenic, antimony, bismuth or phosphrous) is
added to a pure semiconductor crystal during crystal growth, the resulting
crystal is called the N-type semiconductor.
Q27.
What do you mean by donor and acceptor impurities?
Donor impurities
(such as arsenic, antimony, bismuth or phosphorous) when added to a pure
semiconductor lattice , form N-type extrinsic semiconductor. The pentavalent
impurities are called donor impurities as such impurities donate electrons to
the lattice.
Acceptor impurities
(such as boron, gallium, indium or aluminium) when added to a semiconductor
lattice form P-type extrinsic semiconductor. The trivalent impurities are
called acceptor impurities because such impurities accept electrons from the
lattice.
Q28.
Explain the term doping and its need.
The electrical
conductivity of intrinsic semiconductor, which has little current conducting
capability at room temperature and so is of little use, can be increased many
times by adding very small amount of impurity (of the order of one atom per
million atoms of pure semiconductor) to it in the process of
crystallization. This process is called doping.
Q30.
What is the effect of temperature on extrinsic semiconductor?
With the increase
in temperature of an extrinsic semiconductor, the number of thermally generated
carriers is increased resulting in increase in concentration of minority
carriers. At temperature exceeding critical temperature the extrinsic
semiconductor behaves like an intrinsic semiconductor but with higher
conductivity.
Q31.
What are the charge carriers in P-type and N-type semiconductors?
Fee electrons in
n-type semiconductors and holes in p-type semiconductors are the charge
carriers.
Q32. Why
doping is done in semiconductors?
Intrinsic (or pure
) semiconductor by itself is of little significance as it has little current
conduction capability at ordinary room temperature. However, if very small
amount of impurity (of the order of one atom per million atoms of pure
semiconductor) is added to it in the process of crystallization, the electrical
conductivity is increased many times.
Q33.
What is the ratio of majority and minority carriers in intrinsic and extrinsic
semiconductors?
For intrinsic
semiconductor the ratio of majority and minority carriers is Unity.
For extrinsic
semiconductor the ratio of majority and minority carriers is Very large.
Q34.
What is a p-n junction?
The contact surface
between the layers of p-type and n-type semiconductor pieces placed together so
as to form a p-n junction is called the p-n junction.
Q35.
How do the transition region width and contact potential across a p-n junction
vary with the applied bias voltage?
When the p-n
junction is forward biased , the transition region width is reduced and the
contact potential is also reduced with the increase in applied bias voltage.
When the p-n
junction is reverse biased, the transition is widened, and the contact
potential is increased and with the increase in applied bias voltage.
Q36.
Which type of charges present on the two opposite faces of the junction?
Positive charge on
n-side and negative charge on p-side of the junction.
Q37.
What types of carriers are present in space charge region?
No mobile carrier
is present in the space charge region.
Q38.
Why is space region called the depletion region?
The region around
the junction is completely ionized on formation of p-n junction. As a result,
there are no free electrons on the n-side nor the holes on the p-side. Since
the region around the junction is depleted of mobile charges, it is called the
depletion region.
Q39.
Why an electric field is produced in a depletion region of a p-n junction?
The separation of
positive and negative space charge densities in a p-n junction results in an
electric field.
Q40.
What is space charge width?
The space charge
region extends into the n and p-regions from the metallurgical junction. The
distance is known as the space charge width.
Q41.
The electric field in the space charge region decreases with forward bias and
increases with reverse bias. Why?
Because applied
electric field opposes built-in field.
Q42.
Define cut-in voltage of a p-n junction diode?
The forward
voltage, at which the current through the p-n junction starts increasing
rapidly, is called the cut-in voltage.
Q43. What
Does Led Stand For?
Light Emitting Diode
Q44.
What is the effect of temperature on the reverse current of a p-n junction?
Reverse current of
a p-n junction increases with the increase in junction temperature.
Q45.
Why is silicon preferred to germanium in the manufacturing of semiconductor
devices?
Silicon preferred
to germanium in the manufacturing of semiconductor devices because such devices
have higher peak inverse voltage and current ratings and wider temperature
range than germanium ones.
Q46. What
Does Mosfet Stand For?
Metal–oxide–semiconductor
field-effect transistor.
Q47.
Define breakdown voltage.
Breakdown voltage
is defined as the reverse voltage at which p-n junction breaks down with sudden
rise with reverse current.
Q48.
Define the limitations in the operation conditions of a p-n junction.
Every p-n junction
has limiting values of :
§ Maximum forward
current
§ Peak inverse
voltage (PIV)
§ Maximum power
rating
The p-n junction
provides satisfactory performance when operated within these limiting values.
The p-n junction diode may get destroyed due to excessive heat if any of these
values are exceeded.
Q 49. Which Semiconductor Has A Direct
Bandgap?
Gallium arsenide has a direct bandgap.
Q50. What Are The Four Elements In Cigs?
1.
Copper
2.
indium
3.
gallium
4. selenium
Q
51. What Is An
Exciton?
An electron-hole pair is called an exciton.
Q.52 Can Pure Silicon Alone Be Used
To Make A Semiconductor?
No. It needs impurities to become semi-conductive.
Q.53.What Does Bjt Stand For?
Bipolar junction transistor.
Q54. What do you understand by reverse saturation current of
a diode?
Reverse saturation
current of a diode is due to minority carriers and is caused when the diode is
reverse biased. Only a very small voltage is required to direct all minority
carriers across the junction, and when all minority carriers are flowing
across, further increase in bias voltage will not cause increase in current.
This current is referred to as reverse saturation current.
Q55. Define peak inverse voltage?
Peak inverse
voltage is the maximum voltage that can be applied to the p-n junction without
damaging the junction. If the reverse voltage across the junction exceeds its
peak inverse voltage(PIV), the junction may get destroyed owing to excessive
heat.
Q56. What is the importance of valence
shell and valence electrons?
The outermost shell of an atom is called valence shell and the
electrons in this shell are called valence electrons. Formation of energy bands
occur owing to overlapping of energy levels of these valence electrons in
valence shells. With the decrease in interatomic distance between the atoms in
a crystal, the energy levels of electrons in outermost shells of atoms overlap
to form energy bands.
Q57. Differentiate semiconductors, conductors
and insulators on the basis of band gap.
The distinction between conductors, insulators and semiconductors is largely
concerned with the relative width of the forbidden energy gaps in their energy
band structures. There is a wide forbidden gap (more than 5eV) for insulators,
narrow forbidden gap (about 1eV) in case of semiconductors and no forbidden gap
in case of conductors.
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