CSCE 613: Fundamentals of VLSI Chip Design Instructor: Jason D. Bakos

Fund. of VLSI Chip Design 2 MOSFET Theory p-type body: majority carriers are holes accumulation mode V t depends on doping and t ox channel is no longer at the same voltage as body (channel becomes decoupled from body)

Fund. of VLSI Chip Design 3 Regions of Operation Gate to channel: V ds = V gs - V gd V gs near source V gd near drain Switching delay is determined by: time required to charge/discharge gate time for current to travel across channel drain

Fund. of VLSI Chip Design 4 Ideal I-V Characteristics Linear region(charge) (carrier velocity,  is mobility) (electric field)

Fund. of VLSI Chip Design 5 Ideal I-V Characteristics Saturation region: into equation… cutoff linear saturation nmos pmos Holes have less mobility than electrons, so pmos’s provide less current (and are slower) than nmos’s of the same size Which parameters do we change to make MOSFETs faster?

Fund. of VLSI Chip Design 6 Fabrication Switching speed depends on C g, C s, and C d Shrink minimum feature size… –Given fixed W, L is reduced, therefore less gate area –However, t ox is also reduced –C gper stays constant –However, smaller channel length decreases carrier time Yields more current for per unit of W –Therefore, W may also be reduced for given current –C g, C s, and C d are reduced –Transistor switches faster

Fund. of VLSI Chip Design 7 Nonideal I-V Effects Velocity saturation and mobility degradation –Lower I ds than expected At high lateral field strength (V ds /L), carrier velocity stops increasing linearly with field strength At high vertical field strength (V gs / t ox ) the carriers scatter more often Channel length modulation –Saturation current increases with higher V ds Subthreshold conduction –Current drops exponentially when V gs drops below V t (not zero) Body effect –V t affected by source voltage relative to body voltage Junction leakage –S/D leaks current into substrate/well Tunneling –Gate current due to thin gate oxides Temperature dependence –Mobility and threshold voltage decrease with rising temperature

Fund. of VLSI Chip Design 8 C-V Characteristics Capacitors are bad –Slow down circuit (need to use more power), creates crosstalk (noise) Gate is a good capacitor –Gate is one plate, channel is the other –Needed for operation: attracts charge to invert channel Source/drain are also capacitors to body (p-n junction) –Parasitic capacitance –“Diffusion capacitance” –Depends on diffusion area, perimeter, depth, doping levels, and voltage Make as small as possible (also reduces resistance)

Fund. of VLSI Chip Design 9 Gate Capacitance Gate’s capacitance –Relative to source terminal –C gs =C OX WL –Assuming minimum length… –C gs =C per W –C per = C OX L = ( OX /t OX )L –Fab processes reduce length and oxide thickness simultaneously Keeps C per relatively constant 1.5 – 2 fF / um of width

Fund. of VLSI Chip Design 10 Gate Capacitance Five components: Intrinsic: C gb, C gs, C gd Overlap: C gs(overlap), C gd(overlap) C 0 = WLC ox ParameterCutoffLinearSaturation C gb C0C0 00 C gs 0C 0 /22/3 C 0 C gd 0C 0 /20 SumC0C0 C0C0 2/3 C 0 C gsol =C gdol = fF / um of width

Fund. of VLSI Chip Design 11 Parasitic Capacitance Source and drain capacitance –From reverse-biased PN junction (diffusion to body) –C sb, C db –Depends of area and perimeter of diffusion, depth, doping level, voltage –Diffusion has high capacitance and resistance Made small as possible in layout –Approximately same as gate capacitance (1.5 – 2 fF / um of gate width) Isolated, shared, and merged diffusion regions for transistors in series

Fund. of VLSI Chip Design 12 Switch-Level RC Delay Models Delay can be estimated as R * 6C FET passing weak value has twice the resistance