4vfor180nmprocess s2vtlnnaniissurfacepotential

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Unformatted text preview: age Current Leakage current equals Ids when Vgs= 0 Leakage current, Ids = I0 exp(­Vth/nVT) At cutoff, Vgs = Vth , and Ids = I0 Lowering leakage to 10­bI0 Vth = bnVT ln 10 = 1.5b × 26 ln 10 = 90b mV Example: To lower leakage to I0/1,000 Vth = 270 mV Copyright Agrawal & Srivaths, 2007 Low-Power Design and Test, Lecture 5 25 Threshold Voltage Vth = Vt0 + γ[(Φs+Vsb)½­ Φs½] Vt0 is threshold voltage when source is at body potential (0.4 V for 180nm process) Φs = 2VT ln(NA /ni ) is surface potential γ = (2qεsi NA)½tox /εox is body effect coefficient (0.4 to 1.0) NA is doping level = 8×1017 cm­3 ni = 1.45×1010 cm­3 Copyright Agrawal & Srivaths, 2007 Low-Power Design and Test, Lecture 5 26 Threshold Voltage, Vsb = 1.1V Thermal voltage, VT = kT/q = 26 mV Φs = 0.93 V εox = 3.9×8.85×10­14 F/cm εsi = 11.7×8.85×10­14 F/cm tox = 40 Ao γ = 0.6 V½ Vth = Vt0 + γ[(Φs+Vsb)½­ Φs½] = 0.68 V Copyright Agrawal & Srivaths, 2007 Low-Power Design and Test, Lecture 5 27 A Sample Calculation VDD = 1.2V, 100nm CMOS process Transistor width, W = 0.5μm OFF device (Vgs = Vth) leakage I0 = 20nA/μm, for low threshold transistor I0 = 3nA/μm, for high threshold transistor 100M transistor chip Power = (100×106/2)(0.5×20×10­9A)(1.2V) = 600mW for all low­threshold transistors Power = (100×106/2)(0.5×3×10­9A)(1.2V) = 90mW for all high­threshold transistors Copyright Agrawal & Srivaths, 2007 Low-Power Design and Test, Lecture 5 28 Dual­Threshold Chip Low­threshold only for 20% transistors on critical path. Leakage power = 600×0.2 + 90×0.8 = 120 + 72 = 192 mW Copyright Agrawal & Srivaths, 2007 Low-Power Design and Test, Lecture 5 29 Dual­Threshold CMOS Circuit Copyright Agrawal & Srivaths, 2007 Low-Power Design and Test, Lecture 5 30 Dual­Threshold Design To maintain performance, all gates on the critical path are assigned low Vth . Most of the other gates are assigned high Vth . But, Some gates on non­critical paths may also be assigned low Vth to prevent those paths from becoming critical....
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