VLSI Multiple Choice Questions on “MOS Transistor Threshold Voltage”.
1. The electrical equivalent component for MOS structure is:
A. Resistor
B. Capacitor
C. Inductor
D. Switch
Answer: B
Clarification: The MOS structure acts as a capacitor with a metal gate and semiconductor acting as parallel plate conductors and oxide as dielectric between them.
2. The Fermi potential is the function of:
A. Temperature
B. Doping concentration
C. Difference between Fermi level and intrinsic Fermi level
D. All of the mentioned
Answer: D
Clarification: The Fermi potential, which is a function of temperature and doping, denotes the difference between the intrinsic Fermi level and the Fermi level.
3. The direction of electric field when the gate voltage is zero:
A. Metal to semiconductor
B. Semiconductor to metal
C. No electric field exists
D. None of the mentioned
Answer: A
Clarification: Metal being more positive compared to semiconductor. The electric field exists from metal to semiconductor.
4. Consider a MOS structure with equilibrium Fermi potential of the doped silicon substrate is given as 0.3eV. Electron affinity of Si is 4.15eV and metal is 4.1eV. Find the built in potential of the MOS system.
A. -0.8eV
B. 0.8eV
C. 0.9eV
D. -0.9eV
Answer: D
Clarification: Surface potential: qΦs = 4.15eV + 0.55eV + 0.3eV = 5.0eV
qΦm-qΦs = 4.1eV – 5.0eV = -0.9eV.
5. When gate voltage is negative for enhancement mode n-MOS, the direction of electric field will be:
A. Metal to semiconductor
B. Semiconductor to metal
C. No field exists
D. None of the mentioned
Answer: B
Clarification: When gate voltage is negative, holes in substrate are attracted towards surface creating electric field from semiconductor to metal.
6. At threshold Voltage, the surface potential is:
A. – Fermi potential
B. Fermi potential
C. 2 Fermi potential
D. -2 Fermi potential
Answer: A
Clarification: When surface potential reaches –fermi potential, the surface inversion occurs. The gate voltage which brings these changes is known as threshold voltage.
7. Surface inversion occurs when gate voltage is:
A. Less than zero
B. Less than threshold voltage
C. Equal to threshold voltage
D. Greater than threshold voltage
Answer: C
Clarification: Surface inversion occurs when gate voltage is equal to threshold voltage.
8. The energy band diagram of the MOS system when gate voltage is zero is:
A. vlsi-questions-answers-mos-threshold-voltage-q8a”>
B. vlsi-questions-answers-mos-threshold-voltage-q8b”>
C. vlsi-questions-answers-mos-threshold-voltage-q8c”>
D. vlsi-questions-answers-mos-threshold-voltage-q8d”>
Answer: A
Clarification: The energy band diagram of enhancement mode nMOSFET when gate voltage is zero is :vlsi-questions-answers-mos-threshold-voltage-q8a”>