CSCE 613: Fundamentals of VLSI Chip Design

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

2. MOSFET Theory p-type body: majority carriers are holes accumulation mode Vt depends on doping and tox channel is no longer at the same voltage as body (channel becomes decoupled from body)

3. Regions of Operation Gate to channel: Vds = Vgs - Vgd Vgs near source Vgd near drain • Switching delay is determined by: • time required to charge/discharge gate • time for current to travel across channel drain

4. Ideal I-V Characteristics Linear region (charge) (carrier velocity, m is mobility) (electric field)

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

6. Fabrication • Switching speed depends on Cg, Cs, and Cd • Shrink minimum feature size… • Given fixed W, L is reduced, therefore less gate area • However, tox is also reduced • Cgperm 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 • Cg, Cs, and Cd are reduced • Transistor switches faster

7. Nonideal I-V Effects • Velocity saturation and mobility degradation • Lower Ids than expected • At high lateral field strength (Vds/L), carrier velocity stops increasing linearly with field strength • At high vertical field strength (Vgs / tox) the carriers scatter more often • Channel length modulation • Saturation current increases with higher Vds • Subthreshold conduction • Current drops exponentially when Vgs drops below Vt (not zero) • Body effect • Vt 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

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)

9. Gate Capacitance • Gate’s capacitance • Relative to source terminal • Cgs=COXWL • Assuming minimum length… • Cgs=CpermW • Cperm = COXL = (eOX/tOX)L • Fab processes reduce length and oxide thickness simultaneously • Keeps Cperm relatively constant • 1.5 – 2 fF / um of width

10. Gate Capacitance Five components: Intrinsic: Cgb, Cgs, Cgd Overlap: Cgs(overlap), Cgd(overlap) C0 = WLCox Cgsol=Cgdol=0.2-0.4 fF / um of width

11. Parasitic Capacitance • Source and drain capacitance • From reverse-biased PN junction (diffusion to body) • Csb, Cdb • 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

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