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Lecture_13_first_posting - Lecture 13-14 MOS Transistor...

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MOS Transistor Reading Assignment Sedra and Smith Chap 4 Howe Chap 4 Handout on MOS (see blackboard) Lecture 13-14
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N-MOSFET Source and drain are n + (providing contacts and source of electrons). Substrate often p-type Positive gate bias can induce an electron sheet in the channel or capacitively induced a forward bias in the source diode! If the drain bias is reverse biased, V DS will not affect I D much Only drain and source can have large current! In ideal cases, the only W or L dependence will be the prefactor. ( 29 BS DS GS D V V V f L W I , , = Fig. 4.1
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P-MOSFET Outlook Source and drain are p + (providing contacts and source of holes ). Substrate often n-type Negative gate bias can induce a hole sheet in the channel or capacitively induced a forward bias in the source diode! If the drain bias is reverse biased, V DS will not affect I D much Only drain and source can have large current! ( 29 BS DS GS D V V V f L W I , , = In ideal cases, the only W or L dependence will be the prefactor. p + p + n
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MOS Transistor The MOS transistor is essentially a MOS- capacitor with two PN junctions placed immediately adjacent to the region controlled by the MOS gate. The device is symmetrical Typically the source is placed at ground potential and the drain is held at a positive potential for a n- channel device or negative potential for a p- channel device If no voltage is applied to the gate the circuit looks like two PN junctions back to back in series. Therefore the voltage will drop these diodes and the current flowing will be essentially zero
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The Back-to-Back Diode and Gate Capacitance Control W d Back-to-back diodes from S to substrate (channel) to D will result in most V DS dropped at the reverse-bias D junction. Source Gate Drain Substrate (or bulk) Try to induce a forward bias at S capacitively!! V GS d si si W C 0 ε ε = in F/cm 2 ox ox ox t C 0 ε ε = ψ s Surface potential ψ s Fig. 4.2
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Ideal I D (V GS , V DS ) in nMOSFET ( 29 BS DS GS D V V V f L W I , , = Fig. 4.11 Three regions of operation: Linear, triode and saturation
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Critical FET specifications In digital circuits, the strength of the transistor (or
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