GATE EC 2014 Set 1 — Question 20
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Electronic Devices → MOS Capacitor & MOSFET → Threshold Voltage
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Question
If fixed positive charges are present in the gate oxide of an n-channel enhancement type MOSFET, it will lead to
Correct answer
(A) a decrease in the threshold voltage
Solution
The threshold voltage () of an n-channel MOSFET is given by the equation:where:
The term related to the fixed oxide charge is . Since and , this term is negative.A negative term is added to the threshold voltage expression, which causes the overall value of to decrease.Physically, the positive charges in the oxide help in attracting electrons to the surface of the silicon substrate. This means less positive voltage is required on the gate to form the inversion layer (the channel). Therefore, the threshold voltage decreases.Hence, the correct option is (A).
- is the work function difference between the metal gate and the semiconductor.
- is the fixed charge per unit area in the oxide.
- is the oxide capacitance per unit area.
- is the charge per unit area in the depletion region.
- is the surface potential at strong inversion.
The term related to the fixed oxide charge is . Since and , this term is negative.A negative term is added to the threshold voltage expression, which causes the overall value of to decrease.Physically, the positive charges in the oxide help in attracting electrons to the surface of the silicon substrate. This means less positive voltage is required on the gate to form the inversion layer (the channel). Therefore, the threshold voltage decreases.Hence, the correct option is (A).
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