This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Basic Physics of Semiconductors”.
1. At absolute zero, Si behaves as a ____________
a) Insulator
b) Conductor
c) Poor conductor
d) Poor insulator
View Answer
Explanation: Si displays the properties of a semiconductor. It will behave as an insulator at very low temperature while it will conduct current when exposed to a high temperature.
2. What is the relation between the intrinsic carrier concentration of Si (ni(si)) and Ge (ni(Ge)) at a fixed temperature?
a) (ni(si)) = (ni(Ge))
b) (ni(si)) > (ni(Ge))
c) (ni(si)) < (ni(Ge))
d) No relation
View Answer
Explanation: The intrinsic carrier concentration of Silicon is more than that of Germanium. This is proven from a comparison of the ni for both the materials.
3. The minimum energy required to separate an electron from it’s bond within a semiconductor material is _____________
a) Bandgap Energy
b) Threshold Voltage
c) Work function
d) Kinetic Energy
View Answer
Explanation: The term associated with the minimum energy required to dissociate an electron from it’s bond is Bandgap Energy. Threshold voltage is a term used to signify when a device would turn on or off. The work function is the minimum energy required to generate a photon.
4. In a doped semiconductor, the minority carrier concentration becomes equal to the ____________
a) Minority carrier concentration
b) Dopant concentration
c) Doesn’t get affected
d) Inversely proportional to the dopant concentration
View Answer
Explanation: The minority carrier concentration becomes \(\frac {n_i^2}{(dopant \, concentration)}\) in a doped semiconductor. This is because the concentration of majority carriers becomes approximately equal to the dopant concentration.
5. What is the relation between the drift velocity of electrons (ve) and that of holes (vp)?
a) ve > vp
b) ve = vp
c) ve < vp
d) No relation exists
View Answer
Explanation: The drift of velocity of flow of electrons is greater than holes since the mobility of electrons is seen to be more than holes. The mobility of electrons is 1350 cm2/V.s while the mobility of holes is 480 cm2/V.s.
6. In an n-type semiconductor, what is the conductivity?
a) neμn
b) peμp
c) neμn + peμp
d) \(\frac {1}{ne\mu_n}\)
View Answer
Explanation: The total conductivity due to the present of electrons and holes is given by (neμn + peμp). In an n-type semiconductor, n >> p and μn >> μn leads us to approximate the conductivity as neμn. The same approximation is valid for p-type semiconductors.
7. If the doping concentration increases by a factor of 100 in the n-side and by a factor of 1000 in the p-side, what is the factor by which the built-in potential changes?
a) 6
b) 5
c) 5.5
d) Doesn’t get affected
View Answer
Explanation: The built-in-potential is directly proportional to logarithm of the product of doping concentration. Thus, we find that it is multiplied by a factor of 6.
8. What is the key characteristic of a semi-conductor?
a) Barrier potential
b) Amplification
c) Stability
d) Filtering
View Answer
Explanation: The presence of a barrier potential is indicative of the fact that the semiconductor doesn’t always behave as a conductor and again as an insulator. It can switch from one characteristic to another.
9. When would the drift velocity saturate in a semiconductor?
a) High electric fields
b) Low electric fields
c) Medium electric fields
d) Independent of the electric field
View Answer
Explanation: The velocity of the carriers saturates at very high values of electric fields. This is because as the number of collisions increase, their acceleration decreases and the velocity don’t increase further.
10. Velocity saturation is more prone in short channel devices where a channel is the medium in which the information gets processed.
a) True
b) False
View Answer
Explanation: The velocity saturation is a cause of higher electric fields within the device. Since E=V/L, L is very less for short channel devices and the value of Electric field would increase significantly for small voltages and hence would saturate the velocity faster in comparison to devices with a longer L.
11. If two materials of different Fermi-levels are fused together, what would be the direction of flow of electrons?
a) Lower energy states to higher energy states
b) Higher energy states to lower energy states
c) State more than Fermi level of one material to states less than Fermi-level of another material
d) From the energy state of the material with a higher Fermi-level to the energy state of the material with lower Fermi-level
View Answer
Explanation: Electrons always occupy the lowest energy state. The Fermi Level reaches a constant throughout a material and if two materials are fused together, the Fermi-level gets adjusted to reach a constant Fermi-Level throughout the new solid. Hence, all the electrons will flow from the states of the material with a higher Fermi-level to the states of the material with a lower Fermi-level.
Sanfoundry Global Education & Learning Series – Microelectronics.
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