GATE EC 2019 Set 1 — Question 20
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Electronic Devices → MOS Capacitor & MOSFET → C-V Characteristics
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Question
The figure shows the high-frequency C-V curve of a MOS capacitor (at T = 300 K) with V and no oxide charges. The flat-band, inversion, and accumulation conditions are represented, respectively, by the points
Correct answer
(B) Q, R, P
Solution
The C-V curve of a MOS capacitor shows the capacitance variation with the applied gate voltage ().
1.Accumulation: This occurs at large gate voltages that attract majority carriers to the semiconductor-oxide interface. For a p-type substrate, this is a large negative . For an n-type substrate, this is a large positive . In this region, the capacitance is the highest and is equal to the oxide capacitance (). In the given figure, point P corresponds to the accumulation region, as the capacitance is maximum.
2.Inversion: This occurs at gate voltages with a polarity opposite to that required for accumulation. This voltage repels majority carriers and attracts minority carriers, forming an inversion layer at the interface. In strong inversion, the high-frequency capacitance saturates at its minimum value, . In the figure, point R corresponds to the strong inversion region, as the capacitance is minimum.
3.Flat-band: This is the condition where there is no band bending in the semiconductor. The voltage required to achieve this is the flat-band voltage, . The problem states that the metal-semiconductor work function difference V and there are no oxide charges. Therefore, the flat-band voltage is V. In the figure, point Q is at V, which corresponds to the flat-band condition.
The question asks for the points representing flat-band, inversion, and accumulation conditions, respectively. This corresponds to the sequence: Q, R, P.Therefore, option (B) is the correct answer.Continue learning with Success Tracker
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