ESE370: ircuit-level Modeling, Design, and Optimization for Digital Systems Today! PN Junction! MOS Transistor Topology! Threshold Lec 7: September 16, 2015 MOS Transistor Operating Regions Part 1! Operating Regions " Resistive " Saturation " Subreshold (net class) " Velocity Saturation (net class) 1 2 Last Time MOS model Refinement! Depletion region ecess carriers depleted 3 4 Bulk/Body ontact No Field! MOS actually has four contacts! Also effects fields! Usually common across transistors " Gnd for nmos, V dd for pmos D! V GS =0, V =0 G B S 5 6 1
Apply V GS >0 hannel Evolution -- Increasing Vgs! Accumulate negative charge " Repel holes (fill holes) + + + + + + + - - - - - - - - 7 8 Gate apacitance Gate apacitance hanges based on operating region. o dep! Depletion capacitance dependent on wid of depletion region and potential at oide-silicon border 9 10 hannel Evolution -- Increasing Vgs Inversion! Surface builds electrons " Inverts to n-type " Draws electrons from n + source terminal 11 12 2
Threshold Linear Region! Voltage where strong inversion occurs reshold voltage " V ~= 2ϕ F " Engineer by controlling doping (N A ) φ F = φ T ln N A ( n i 13 14 Linear Region Linear Region! V GS >V and V small OX = ε OX t OX 2! V GS >V and V small! V GS fied looks like resistor " urrent linear in V 2 µ n W OX ( L )( V V GS )V V 15 16 MOSFET IV haracteristics Dimensions V <V GS -V TH! hannel Leng (L)! hannel Wid (W)! Oide Thickness (T o ) V GS -V V V GS -V TH V 17 18 3
Preclass Preclass! I ds for identical transistors in parallel?! I ds for identical transistors in series? " (Vds small) 19 20 Transistor Streng (W/L) Transistor Streng (W/L) OX = ε OX t OX! Shape dependence match Resistance intuition " Wider = parallel resistors decrease R " Longer = series resistors increase R R = ρl A 2 2 21 22 L drawn vs. L effective hannel Voltage! Doping not perfectly straight! Spreads under gate! Effective L smaller an draw gate wid! Voltage varies along channel! Think of channel as resistor 23 24 4
Preclass Voltage in hannel! What is voltage in e middle of a resistive medium? " (halfway between terminals)! Think of channel as resistive medium " Leng = L " Area = Wid * Dep(inversion)! What is voltage in e middle of e channel? " L/2 from S and D? 25 26 hannel Voltage Voltage along hannel! Voltage varies along channel! If ink of channel as resistor " Serves as a voltage divider between V S and V D 27 28 Voltage along hannel Voltage along hannel Vd Vs 29 Vd Vs 30 5
Voltage along hannel hannel Field! When voltage gap V G -V drops below V, drops out of inversion " If V = V GS V V V GS = V Vd Vs 31 32 hannel Field Pinch Off! When voltage gap V G -V drops below V, drops out of inversion " What if V > V GS V V V GS > V? " Upper limit on current, channel is pinched off! When voltage along e channel drops below V, e channel drops out of inversion " Occurs when: V GS V < V T V > V GS V! onclusion: " current cannot increase wi V once V > V GS -V T 33 34 Saturation MOSFET IV haracteristics! In saturation, V -effective =V = V GS -V T " W ) OX ( V L GS V T )V V 2, +. * 2 -! Becomes: " W ) OX + V L GS V T * + = µ " n OX W V 2 L GS V T ( ( ) 2 V V GS T ) 2 2 [( ) 2 ],. -. 35 V <V GS -V TH V V GS -V TH V V GS -V 36 6
Approach Big Idea! Identify Region! 3 Regions of operation for MOSFET! Drives governing equations " Subreshold " See preclass reference " Linear! Use region and equations to understand operation " Saturation 37 38 Admin! Tet 3.3.2 highly recommend read!! " Second half on Friday! HW4 out " Get started over weekend " Long and time-consuming 39 7