454Lec05_MOSFET2-1

454Lec05_MOSFET2-1 - ECEN 454 Lecture 5 1 ECEN 454 Digital...

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Unformatted text preview: ECEN 454 Lecture 5 1 ECEN 454 Digital Integrated Circuit Design Lecture 5 MOS Transistor II ECEN 454 Lecture 5 2 MOS Transistors N-channel MOS transistor Enhancement mode vs. depletion mode transistors ¡ Enhancement: no conducting channel exists at zero gate bias ¡ Depletion: a conducting channel exists at zero gate bias Symbols for enhancement-mode MOSFETs ECEN 454 Lecture 5 3 Switch Model of NMOS ECEN 454 Lecture 5 4 Threshold Voltage (1) Band diagram when the inversion layer is formed Four factors that determine the threshold voltage ¡ Work function difference between the gate and the channel ¡ Change the surface potential at the silicon surface ¡ Offset the depletion region charge ¡ Offset the nonideal charge at the interface between the gate oxide and silicon ECEN 454 Lecture 5 5 Threshold Voltage -- NMOS (1) Work function difference (built-in potential) or Change the surface potential by Offset the depletion region charge ¡ Depletion charge density: no body effect (V SB =0) ¡ Depletion charge density: w/ body effect ) ) ( metal substrate M F GC ( φ φ − = Φ ) )( ( ) ( poly gate substrate F F GC φ φ − = Φ F φ 2 − SB F Si A B V N q Q + − ⋅ ⋅ − = φ ε 2 2 F Si A B N q Q φ ε 2 2 − ⋅ ⋅ − = ECEN 454 Lecture 5 6 Threshold Voltage -- nMOS (2) ox ox ox B F GC T C Q C Q V − − − Φ = 2 φ ( ) F SB F ox Si A ox B B V C N q C Q Q φ φ ε 2 2 2 − + − ⋅ ⋅ − = − ox B B T ox ox ox B F GC T C Q Q V C Q C Q V 2 − − = − − − Φ = φ ( ) F SB F T T V V V φ φ γ 2 2 − + − + = ox Si A C N q ε γ ⋅ ⋅ = 2 Adding things up Body effect coefficient: ECEN 454 Lecture 5 7 nMOS and pMOS φ F < 0 in nMOS, φ F > 0 in pMOS Q B0 and Q B : negative in nMOS, positive in pMOS γ > 0 in nMOS, γ < 0 in pMOS V SB > 0 in nMOS, V SB < 0 in pMOS How to change threshold voltage?...
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This note was uploaded on 04/17/2008 for the course ELEN 454 taught by Professor Jianghu during the Spring '08 term at Texas A&M.

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454Lec05_MOSFET2-1 - ECEN 454 Lecture 5 1 ECEN 454 Digital...

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