Lect4

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Unformatted text preview: y • A good metric for a transistor is the transconductance normalized to the DC current. Since the power dissipation is determined by and large by the DC current, we’d like to get the most “bang” for the “buck”. • From this perspective, the weak and moderate inversion region is the optimal place to operate. EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 14 Efficiency gm/ID • High efficiency is good for low power • Higher gm/ID at low VGS • Approaches BJT for VGS < VTH gm/IC = 1/Vt ~ 40 V-1 • NMOS / PMOS about same EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 15 Efficiency gm/ID • Let’s define V* = e.g. V* = 200mV 2ID gm ⇔ gm 2 = ID V * gm/ID = 10 V-1 • Square-law devices: V* = VGS-VTH = Vdsat Square law : 2I D 2I D gm = = VGS − VTH V * EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 16 SPICE Charge Model • Charge conservation • MOSFET: – – – – – 4 terminals: S, G, D, B 4 charges: QS + QG + QD + QB = 0 (3 free variables) 3 independent voltages: VGS, VDS, VSB 9 derivatives: Cij = dQi / dVj, e.g. CG,GS ~ CGS Cij != Cji Ref: HSPICE manual, “Introduction to Transcapacitance”, pp. 15:42, Metasoft, 1996. EECS 240 Lecture 2: CMOS - passive devices © 2006 A. M. Niknejad and B. Boser 17 Small Signal Capacitances Weak inversion Strong inversion linear Strong inversion saturation CGS Col CGC/2 + Col 2/3 CGC + Col CGD Col CGC/2 + Col Col CGB CGC // CCB 0 0 CSB CjSB CjsB + CCB/2 CjsB + 2/3 CCB CDB CjDB CjDB + CCB/2 CjDB Cox = 5.3 fF/µm 2 CGC = CoxWL CCB = ε Si xd WL 0.35u Process EECS 240 Lecture 2: CMOS - passive devices ColN = 0.24 fF/µm ColP = 0.48 fF/µm © 2006 A. M. Niknejad and B. Boser 18 MOS Capacitance Example subthreshold : C gs = Col = 24fF W 100 = L 0 .5 Cox = ε o C gd = Col = 24fF ε SiO 2 tox = 5 .3 fF µm 2 triode : C gs = 1 C gc + Col = 157fF 2 C gc = CoxWL = 265fF C gd = 1 C gc + Col = 157fF 2 Col = ColNW = 24f...
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This note was uploaded on 03/18/2014 for the course EECS 240 taught by Professor Boser during the Spring '04 term at University of California, Berkeley.

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