GATE EC 2016 Set 1 — Question 22
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Electronic Devices → MOS Capacitor & MOSFET → I-V & Transconductance
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Question
Consider the following statements for a metal oxide semiconductor field effect transistor (MOSFET):
P: As channel length reduces, OFF-state current increases.
Q: As channel length reduces, output resistance increases.
R: As channel length reduces, threshold voltage remains constant.
S: As channel length reduces, ON current increases.
Which of the above statements are INCORRECT?
P: As channel length reduces, OFF-state current increases.
Q: As channel length reduces, output resistance increases.
R: As channel length reduces, threshold voltage remains constant.
S: As channel length reduces, ON current increases.
Which of the above statements are INCORRECT?
Correct answer
(C) Q and R
Solution
Let's analyze each statement regarding the effects of reducing channel length () in a MOSFET (short-channel effects):P: As channel length reduces, OFF-state current increases.
(A) P and Q
(B) P and S
(C) Q and R
(D) R and SThe correct option is (C).The final answer is .
- Explanation: When the channel length is reduced, short-channel effects become prominent. These include Drain-Induced Barrier Lowering (DIBL) and subthreshold leakage. DIBL causes the potential barrier for electrons at the source end to be lowered by the drain voltage, leading to an increase in subthreshold current (OFF-state current) even when the gate-source voltage is below the threshold. Therefore, this statement is CORRECT.
- Explanation: The output resistance () of a MOSFET in saturation is given by , where is the channel length modulation parameter and is the drain current. As the channel length reduces, the channel length modulation effect becomes more significant, meaning increases. An increase in leads to a decrease in output resistance. Therefore, this statement is INCORRECT.
- Explanation: Due to short-channel effects like DIBL and velocity saturation, the threshold voltage () of a MOSFET typically decreases as the channel length reduces. The gate loses some control over the channel, and the drain voltage influences the channel more, effectively lowering the threshold voltage. Therefore, this statement is INCORRECT.
- Explanation: The ON current () in a MOSFET is generally proportional to (width-to-length ratio) and (in saturation) or (in linear region). For a given and , reducing directly increases the ratio, which tends to increase the ON current. Also, the decrease in (as per short-channel effects) further contributes to an increase in , thus increasing the ON current. Therefore, this statement is CORRECT.
(A) P and Q
(B) P and S
(C) Q and R
(D) R and SThe correct option is (C).The final answer is .
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