MOSFET

Learn MOSFET structure, threshold voltage, gate capacitance and why CMOS uses little power through multiple-choice questions and explanations.

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MOSFET: key points

A MOSFET is a transistor that uses the gate voltage to create an inversion layer (channel) at the semiconductor surface and control the current between source and drain. Because the gate is separated by a thin insulator, almost no steady gate current flows.

Threshold voltage, the linear and saturation regions, short-channel effects, and CMOS circuits that combine nMOS and pMOS are key topics for understanding LSIs.

All MOSFET questions (50)

  1. What is the condition for an enhancement-mode n-channel MOSFET to turn on (conduct)? (VGS: gate-source voltage, Vth: threshold voltage)
  2. If only the thickness of the gate insulator is reduced, without changing its material or area, what happens to the gate capacitance per unit area?
  3. Why is the static (steady-state) power consumption of a CMOS inverter small?
  4. When a long-channel MOSFET operates in the saturation region, the drain current ID is roughly proportional to what? (VGS: gate-source voltage, Vth: threshold voltage)
  5. For nMOS and pMOS transistors of the same size, the nMOS generally has a larger drive current. What is the main reason?
  6. In an n-channel MOSFET, what happens to the threshold voltage when a negative voltage is applied to the substrate (body) relative to the source (body bias)?
  7. Which is a characteristic of a depletion-mode MOSFET?
  8. Which value is closest to the theoretical lower limit of a MOSFET's subthreshold swing at room temperature (the gate voltage needed to change the drain current by one decade below threshold)?
  9. How does a MOSFET behave in the linear region, where the drain-source voltage VDS is sufficiently small?
  10. What is the purpose of the LDD (lightly doped drain) structure in a MOSFET?
  11. In a MOSFET in saturation, what is the phenomenon in which raising the drain voltage shortens the effective channel length and slightly increases the drain current?
  12. What is the definition of the transconductance gm of a MOSFET?
  13. In a MOS structure on a p-type substrate, what is the state of the semiconductor surface when a negative voltage is applied to the gate?
  14. Which design makes the logic threshold of a CMOS inverter about half the supply voltage?
  15. What is the advantage of a transmission gate, which connects an nMOS and a pMOS in parallel?
  16. What failure can occur when a voltage far above the rating is applied between the gate and source of a MOSFET?
  17. What is the main reason the channel mobility of a MOSFET is lower than the mobility inside bulk silicon?
  18. In a short-channel MOSFET, what is the main reason the drain current falls below the (VGS − Vth) square law and approaches a linear dependence?
  19. Which expression roughly represents the delay of a CMOS logic gate? (C: load capacitance, V: supply voltage, I: drive current)
  20. What is the sign of the threshold voltage (relative to the source) of an enhancement-mode p-channel MOSFET?
  21. What is the effect in which the gate-drain capacitance appears (1 + gain) times larger when seen from the input of an amplifier?
  22. What problem results from depletion of the polysilicon on the gate electrode side of a polysilicon gate?
  23. What is the general effect of lowering a MOSFET's threshold voltage?
  24. How are the transistors connected in a CMOS two-input NAND gate?
  25. Which phenomenon increases off-state leakage because band-to-band tunneling current flows due to the strong field where the gate overlaps the drain?
  26. What circuit advantage results from almost no current flowing into the gate of a MOSFET?
  27. Which is a characteristic of a MOSFET source follower (common-drain) circuit?
  28. How does the absolute value of a MOSFET's threshold voltage generally change as temperature rises?
  29. How should the dimensions be changed to increase a MOSFET's drain current?
  30. Which combination lists the main components of leakage current in a MOSFET in the off state?
  31. What structure does the "MOS" in MOSFET stand for?
  32. Besides the gate, source and drain, what is the fourth terminal of a MOSFET?
  33. Which carriers carry the current in the channel of an n-channel MOSFET?
  34. What does the "C" (complementary) in CMOS mean?
  35. When a MOSFET is used as a switch, which statement correctly describes its on-resistance?
  36. For a typical MOSFET in an integrated circuit, which statement about the source and drain is correct?
  37. What is the condition for a long-channel nMOSFET to operate in the saturation region? (Assume VGS > Vth.)
  38. How does the threshold voltage change when the impurity concentration of the channel region (p-type substrate) of an nMOSFET is increased?
  39. If the gate insulator of an nMOSFET is made thicker with all else unchanged, how does the magnitude of the threshold voltage generally change?
  40. Which generally describes the relationship between a MOSFET's gate length (the length of the gate electrode on the mask or after patterning) and its effective channel length?
  41. Which correctly describes punch-through in a MOSFET?
  42. In a pass transistor (switch) made of a single nMOSFET with its gate at the supply voltage VDD, how high does the output rise when passing an input of VDD?
  43. In a CMOS inverter, what is the current that flows from the supply to ground while the input is switching called?
  44. Power MOSFETs are easy to use in parallel. Which statement about the temperature dependence of on-resistance explains why?
  45. Under Dennard (constant-field) scaling, which statement is correct when dimensions and voltages are each scaled by 1/k?
  46. Which correctly describes the flat-band voltage of a MOS structure?
  47. In a high-frequency C-V measurement of a MOS capacitor on a p-type substrate, what happens to the capacitance when a large positive gate voltage drives the surface into strong inversion?
  48. In a MOSFET with a very thin gate insulator, what is the effect of the electron distribution in the inversion layer peaking slightly away from the interface?
  49. Which expression appropriately gives the small-signal output resistance ro of a MOSFET in saturation, using the channel-length modulation coefficient λ and drain current ID?
  50. Which is the common condition for "strong inversion" used to define the threshold voltage of a MOSFET? (φF: Fermi potential)

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