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chap4 - MOS Capacitor Structure Substrate Conditions for...

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1 MOS Capacitor Structure First electrode- Gate: Consists of low-resistivity material such as polycrystalline silicon Second electrode- Substrate or Body: n - or p -type semiconductor Dielectric- Silicon dioxide: stable high-quality electrical insulator between gate and substrate. Substrate Conditions for Different Biases Accumulation V G <<V TN Depletion V G <V TN Inversion V G >V TN Low-frequency C-V Characteristics for MOS Capacitor on P-type Substrate MOS capacitance is non- linear function of voltage. Total capacitance in any region dictated by the separation between capacitor plates. Total capacitance modeled as series combination of fixed oxide capacitance and voltage-dependent depletion layer capacitance. NMOS Transistor: Structure 4 device terminals: Gate(G), Drain(D), Source(S) and Body(B). Source and drain regions form pn junctions with substrate. v SB , v DS and v GS always positive during normal operation. NMOS Transistor: Qualitative I-V Behavior V GS <<V TN : Only small leakage current flows. V GS <V TN : Depletion region formed under gate merges with source and drain depletion regions. No current flows between source and drain. V GS >V TN : Channel formed between source and drain. If v DS >0, significant i D flows from drain to source. i B =0 and i G =0. NMOS Transistor: Triode Region Characteristics where, K n = K n ’W/L K n ’=μ n C ox ’’ (A/V 2 ) C ox ’’= ε ox /T ox ε ox = oxide permittivity (F/cm) T ox= oxide thickness (cm) for 0 DS v TN V GS v i D = K n ( v GS V TN v DS /2) v DS i ( x ) = - i D (a constant) for all x
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2 NMOS Transistor: Triode Region Characteristics (contd.) Output characteristics appear to be linear. FET behaves like a gate-source voltage- controlled resistor between source and drain with = TN V GS V L W n K on R ' 1 NMOS Transistor: Saturation Region If v DS increases above triode region limit, channel region disappears, also said to be pinched-off. Current saturates at constant value, independent of v DS. Saturation region operation normally used for analog amplification. NMOS Transistor: Saturation Region (contd.) for TN V GS v DS v TN V GS v DSAT v = is also called saturation or pinch-off voltage i D = K n /2 ( v GS V TN ) 2 Transconductance of a MOS Device Transconductance relates the change in drain current to a change in gate-source voltage Taking derivative of the expression for the drain current in the saturation region, pt Q GS dv D di m g = TN V GS V D I TN V GS V L W n K m g = = 2 ) ( ' Channel-Length Modulation As v DS increases above v DSAT , length of depleted channel beyond pinch-off point, L, increases and effective L decreases .
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