This set of Microelectronics Multiple Choice Questions & Answers (MCQs) focuses on “Behaviour of a MOSFET – Set 2”.
1. Which term to the flat band voltage if oxide thickness increases for a PMOS?
a) It increases
b) It decreases
c) It remains the same
d) It increases proportionately
View Answer
Explanation: An increase in the oxide thickness in a PMOS leads to a decrease in the flat band voltage. This means that to achieve the flat band condition, we’ve to reduce the gate voltage below the previous flat band voltage.
2. Which gas can help in the reduction of fixed oxide charges at the Si-oxide interface?
a) Nitrogen
b) Hydrogen
c) Oxygen
d) Krypton
View Answer
Explanation: The nitrogen gas can prevent the formation of charges at the Si-Oxide interface by preventing the breaking away of Silicon during oxide formation. This leads to a reduction in the charge content and the flat-band voltage becomes almost equal to the metal-semiconductor work function.
3. What happens to the maximum induced space charge width if the doping concentration of the substrate increases?
a) It decreases
b) It increases
c) It remains the same
d) It decreases proportionately
View Answer
Explanation: The doping concentration decreases the length of the maximum space charge width. This can be understood from the fact that it would be difficult to establish a wider depletion region if the substrate is heavily doped. Moreover, the maximum space charge width is given by \(\sqrt { (\frac {4€_s \Phi_{fp}}{qN_A} )}\) and ΦFP=VTLN(NA/ni). Note that ΦFP is having less significance on the doping concentration of the substrate and the denominator NA is more dominant.
4. What are the regions faced by the MOSFET when the surface potential faces a decrease of 2ΦFP? Assume that originally it was at the flat band condition.
a) Depletion and Weak inversion
b) Accumulation and Depletion
c) Weak inversion and Accumulation
d) It gets destroyed
View Answer
Explanation: From the flat band condition, the MOSFET undergoes a phase termed as depletion until the surface potential rises to ΦFP. As it crosses this mark, the MOSFET undergoes the weak inversion phase up to a potential of 2ΦFP.
5. The transconductance of a FET becomes constant when the velocity of electrons gets saturated.
a) True
b) False
View Answer
Explanation: The transconductance is really the power of the MOSFET which suggests how much current it generates due to a change in the voltage. However, as the drift velocity gets saturated due to high electric field-the transconductance becomes independent of the gate or drain voltage.
6. What happens to the speed of electrons due to ballistic transport?
a) It decreases heavily
b) It increases heavily
c) It increases slowly
d) It decreases slowly
View Answer
Explanation: As the mean time between collisions becomes comparable to the channel length, the electrons generated in the channel can travel between the symmetrically doped terminals without any scattering. This leads to heavy increase in their speed.
7. What happens to the transconductance of the MOSFET due to channel-length modulation for a constant drain current and aspect ratio?
a) It doesn’t get affected
b) It increases
c) It decreases
d) It decreases logarithmically
View Answer
Explanation: The transconductance of the MOSFET decreases due to the presence of Channel Length modulation. This effect is more prominent in long-channel MOSFETs and we further note that the original transconductance does get divided by a factor of √(1+λVDS) for a constant drain current and aspect ratio.
8. Due to faulty manufacturing, a MOSFET is affected by a high body effect. What would happen to the threshold voltage if we reduce body effect?
a) It will decrease
b) It will increase
c) It will increase logarithmically
d) It will decrease logarithmically
View Answer
Explanation: If the body effect decreases, the threshold voltage of a MOSFET decreases. This is because body effect increases the quantity of carriers near the Si-oxide interface which leads to an increase in the voltage required to create an inversion layer, which is the threshold voltage.
9. Subthreshold conduction is most prominent in the weak-inversion region.
a) True
b) False
View Answer
Explanation: The subthreshold region is regarded as that part of the operation of a MOSFET when the gate voltage is almost near to the threshold voltage. A very small current flows through the device which has an exponential relationship with the gate voltage.
10. How does constant field scaling affect the drain current?
a) It decreases the current
b) It increases the current
c) It has no impact on the drain current
d) It decreases exponentially
View Answer
Explanation: If all the parameters of the MOSFET are scaled by a factor of p, the drain current will also reduce by factor of p. This is a direct consequence of the constant field scaling method.
Sanfoundry Global Education & Learning Series – Microelectronics.
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