Lect_04 - ECE442 ElectronicCircuits 4.MOSTransistors Jose...

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1 Jose E. Schutt Aine ECE 442 ECE 442 Electronic Circuits 4. MOS Transistors Jose E. Schutt-Aine Electrical & Computer Engineering University of Illinois jschutt@emlab.uiuc.edu
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2 Jose E. Schutt Aine ECE 442 NMOS Transistor – N-Channel MOSFET – Built on p-type substrate – MOS devices are smaller than BJTs – MOS devices consume less power than BJTs NMOS Transistor
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3 Jose E. Schutt Aine ECE 442 NMOS Transistor - Layout Top View Cross Section
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4 Jose E. Schutt Aine ECE 442 GT T VV > GS T > GS T > Resistive Nonlinear Saturation MOS Regions of Operation < ( ) DS GS T V DS GS T V ≥− DS Vs m a l l Triode Active
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5 Jose E. Schutt Aine ECE 442 As V G increases from zero – Holes in the p substrate are repelled from the gate area leaving negative ions behind – A depletion region is created – No current flows since no carriers are available MOS Transistor Operation As V G increases – The width of the depletion region and the potential at the oxide-silicon interface also increase – When the interface potential reaches a sufficiently positive value, electrons flow in the “ channel ”. The transistor is turned on As V G rises further – The charge in the depletion region remains relatively constant – The channel current continues to increase
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6 Jose E. Schutt Aine ECE 442 () μ ⎡⎤ =− ⎣⎦ D ox GS T DS W IC V V V L ( ) ± DS GS T VV V C ox : gate oxide capacitance : electron mobility L : channel length W : channel width V T : threshold voltage MOS – Triode Region - 1 3.9 ε == ox o ox ox ox C tt
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7 Jose E. Schutt Aine ECE 442 FET is like a linear resistor with () 1 μ = ds no x G S T r W CV V L MOS – Triode Region
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8 Jose E. Schutt Aine ECE 442 () 2 1 2 μ =− D n ox GS T DS DS W IC V V V V L ( ) <− D SG S T VV V > GS T – Charge distribution is nonuniform across channel – Less charge induced in proximity of drain MOS – Triode Region - 2
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9 Jose E. Schutt Aine ECE 442 MOS – Active Region Saturation occurs at pinch off when ( ) = −= DS GS T DSP VV V V () 2 2 μ =− Dn o x G S T W IC V V L DS GS T V >− > GS T (saturation)
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10 Jose E. Schutt Aine ECE 442
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Lect_04 - ECE442 ElectronicCircuits 4.MOSTransistors Jose...

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