March, 007 MOSFET Physcal Operaton Some fgures of ths presentaton were taken from the nstructonal resources of the followng textbooks: B. Razav, Desgn of Analog CMOS Integrated Crcuts. New York, NY: McGraw Hll, 00. A. S. Sedra and K. C. Smth, Mcroelectronc Crcuts. New York, NY: Oxford Unversty Press, 003. A. R. Hambley, Electroncs: A Top-Down Approach to Computer-Aded Crcut Desgn. Englewood Clffs, NJ: Prentce Hall, 000. Outlne Introducton MOSFET physcal structure MOSFET symbols MOSFET physcal operaton I-V characterstcs Regons of operaton Channel length modulaton effect Body effect Strong nverson VS weak nverson
March, 007 Introducton There are many types of FET technologes: MOS, CMOS, JFET, E-MOS, D-MOS, V-MOS, etc. MOSFETs are easer to fabrcate and can be made smaller than BJTs Resstors and capactors can be easly mplemented n ICs usng MOSFET technology Most of the logc and memory crcuts can be mplemented usng MOSFET technology Most of the VSI crcuts are currently mplemented n MOSFET technology 3 Termnology FET: Feld Effect Transstor MOSFET: Metal Oxde Semconductor FET JFET: Juncton FET E-MOSFET: Enhancement MOSFET D-MOSFET: Depleton MOSFET V-MOSFET: MOSFET type V 4
March, 007 E-MOSFET Physcal Structure Typcal dmensons: 0.µ m 0µ m µ m 500µ m 0.005µ m H (SO 0.µ m 5 E-MOSFET Physcal Structure (cont Symbol: Polyslcon ( poly s used for the gate: eff = drawn D = eff 6 3
March, 007 Substrate Connectons n NMOS and PMOS NMOS transstor: B must be connected to the most negatve voltage PMOS transstor: B must be connected to the most postve voltage 7 n-ell CMOS Technology Process NMOS transstor PMOS transstor (wells can also be used to fabrcate resstors 8 4
March, 007 Symbols for E-MOS Transstors 9 E-MOSFET Operaton (v > V, v = 0 0 5
March, 007 E-MOSFET Operaton (v > V, v small E-MOSFET Operaton (v > V, v large 6
March, 007 The Inverson ayer (Channel Formaton If V G = 0V If 0 < V G < V If V G > V 3 Increasng V Channel thckness depends on v V(x where V(x = 0 0 and V(x = V D 4 7
March, 007 Increasng V (cont Channel densty at x depends on v V(x where V(x = 0 0 and V(x = V D Mnmum channel densty occurs when v V(x = V 5 Dervaton of I-V Characterstcs I = Q v d Q Q d d = C ( v [ v ( x ] = C = Q v = C [ v ( x ]v d dv ( x Snce v = µ E and E( x = dx dv ( x = C [ v ( x ] µ n dx 6 8
March, 007 Dervaton of I-V Characterstcs (cont 0 = C µ dx 0 v = = µ C n n [ v ( x ] C µ v n dv ( x dx [ v ( x ] v v ( dv Snce s constant along the channel length, 7 I-V Characterstcs Trode Regon = µ C n v ( v v The peak of each parabola occurs at v = v V, peak = µ nc ( v / aspect rato If v V and v v V, trode regon 8 9
March, 007 I-V Characterstcs Saturaton Regon If v V and v > v V, saturaton regon or actve regon, s approxmately constant µ nc ( v ' Condton for saturaton: <V + 9 Saturaton Regon, Channel ength Modulaton If v V and v > v V, saturaton regon or actve regon = µ nc ( v ' = µ nc ( v ( + λv channel length modulaton factor λ / 0 0
March, 007 Regons of Operaton Saturaton Regon, Channel ength Modulaton = µ nc ( v ( + λv channel length modulaton
March, 007 Saturaton Regon, Channel ength Modulaton = µ nc ( v ( + λv Snce λ /: channel length modulaton arger MOSFETs are less senstve to the channel length modulaton effect 3 Transconductance Curve Saturaton Regon (t neglects the channel length modulaton effect v > v V = µ nc ( v = K( v K = µ n C 4
March, 007 Small Sgnal Transconductance, g m v > v V = K( v g m v K = v µ n C = V, v = V g g m m = K( V I = V gm = 4KI m n g = µ C ( / I 5 How does g m changes wth V? 6 3
March, 007 Ohmc Regon = µ C n v ( v v If v V and v s small (v << v V, ohmc regon or deep trode regon = µ nc ( v v r v = µ C n ( v = K( v para v pequeño 7 Ohmc Regon (cont If v V and v s small (v << v V, ohmc regon or deep trode regon v r = = µ ( K( v V nc v for v small 8 4
March, 007 Ohmc Regon (cont 9 Body Effect Usually, V B = V S If V B < V S then V ncreases 30 5
March, 007 Subthreshold Conducton (eak Inverson Above threshold conducton, v v for v V = K( v strong nverson Subthreshold conducton, for v V ξ > V T KT = q = I v ξvt 0e 5mV (@ room temp weak nverson 3 Strong Inverson eak Inverson 3 6
March, 007 Strong Inverson eak Inverson (cont weak nverson strong nverson 33 Strong Inverson eak Inverson (cont 34 7
March, 007 Strong Inverson eak Inverson (cont 35 8