Lecture Notes_23-26_MOSFET_2014

For depletion mode normally on mosfet we need a

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Unformatted text preview: eshold voltage. (A) is incorrect because increasing the semiconductor doping means the metal- semiconductor work function difference becomes larger, which decreases the threshold voltage. (B) is incorrect because an increase in φFp increases the threshold voltage. For p- channel MOSFET, enhancement- mode means normally off, so we need a large, negative voltage to achieve inversion. For depletion- mode, normally on MOSFET, we need a positive gate voltage to reduce the number of inversion holes. We are assuming “long- channel” approximation, which, in practice, means about 2 micrometer gate- length L. Therefore, the vertical electric field εox through the oxide and the horizontal electric field due to the drain bias, εD, can be treated independently. Note that at Drain, it is reverse- biased pn junction between the Drain and the semiconductor body (or bulk). The depletion (space charge) is thus larger than that between Source and the body. For small Drain- Source bias (voltage) VDS, the inversion channel behaves like a resistor, that is, the current increases linearly with applied voltage. 11 12 13 14 15 16 17 18 19 20 21 At higher VDS (positive), the voltage difference between the gate (positive) and the drain voltage become...
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