CMOS Digital System Design

1. MOS Transistor DC Operation CMOS Digital System Design

2. Threshold Voltage Vt • Vgs < Vt: nMOS channel is cut off • Vt < Vgs: nMOS channel conducts • Vgs < Vt: pMOS channel conducts • Vt < Vgs: pMOS channel is cut off

3. Enhancement Mode Transistors Depletion Mode Transistors • Enhancement mode:channel is cut off when Vgs = 0 • Depletion mode:channel conducts when Vgs = 0 • Most CMOS ICs use enhancement-mode transistors.

4. n-MOS Channel Layers, Page 1 • When Vgs = 0,drain-to-substrate is reverse-biased pn junction. • When Vgs > 0,positive electric field in channel under gate: • repels holes • attracts electrons

5. n-MOS Channel Layers, Page 2 • When Vgs << Vt: • Mobile positive holes in p-type channel in substrate are evenly distributed. • Called accummulation layer • When Vt < Vgs: • Holes are repelled, causing a depletion region under the gate. • Called depletion layer

6. n-MOS Channel Layers, Page 3 • When Vt << Vgs: • Electrons are attracted, causing a conductive layer under the gate. • Called inversion layer

7. n-MOS Operating Regions, Page 1 • When Vds = 0: • Depletion and inversion layers uniform depth along length of channel • When Vds > 0: • Depletion and inversion layers same depth at source end of channel as for Vds = 0 • Inversion layer tapers off linearly toward drain end

8. n-MOS Operating Regions, Page 2 • When Vds < Vgs – Vt: • Inversion layer becomes deeper as Vgs increases • Ids depends on both Vgs and Vds. • Called linear region. • Also called resistive region. • Also called nonsaturated region. • Also called unsaturated region.

9. n-MOS Operating Regions, Page 3 • When Vds > Vgs – Vt: • Vgd < Vt. • Inversion layer pinched-off: no longer reaches drain from source end of channel • Electrons instead injected into depletion layer, then accelerated toward drain • Ids depends only on Vgs, independent of Vds. • Called saturated region

10. Body Effect • When have series-connected nMOS devices, only the bottom one has source connected to GND. • Others have Vsb = (Vsource – Vsubstrate) > 0 • For those, have greater gate-channel voltage difference • Increase in Vt.

11. Cutoff Region DC Equation • For Vgs <= Vt: • Ids = 0

12. Nonsaturation Region DC Equation • For 0 < Vds < Vgs – Vt: • Ids = Beta((Vgs – Vt)Vds - Vds**2 / 2) • Beta = MOS transistor gain factor

13. Saturation Region DC Equation • For 0 < Vgs - Vt < Vds: • Ids = Beta(Vgs – Vt)**2 / 2 • Beta = MOS transistor gain factor = ( (mu)(epsilon) / tox )( W / L ) • mu = channel carrier mobility • epsilon = gate insulator permittivity (SiO2) • tox = gate insulator thickness • W / L = channel dimensions

14. LOW Noise Margin • VIL = LOW input voltage • NML = LOW noise margin • Unity gain point, slope = -1 • VIL = 2.3 volts • NML = 2.3 volts

15. HIGH Noise Margin • VIH = HIGH input voltage • NMH = HIGH noise margin • Unity gain point, slope = -1 • VIH = 3.3 volts • NMH = 1.7 volts

16. Differential Amplifier, Page 1 • Pair of nMOS transistors,each with a pull-up resistor • Sources connectedthrough constant-current source to ground

17. Differential Amplifier, Page 2 • If Vin1 and Vin2 change equally from Vquiescent,Vout1 and Vout2 stay the same. • If only Vin1 changes: • current changes one way in resistor 1 and the other way in resistor 2 • So Vout1 changes one way and Vout2 changes the other.

18. Differential Amplifier, Page 3 • Common Mode Gain low • Differential Gain high • CMRR = Common Mode Rejection Ratio = Differential Gain/Common Mode Gain • Good for rejecting common mode noiseon input pins • Used in RAM sense amplifiers

19. Current Mirror • Pair of nMOS transistors with gates tied together • Tie drain of side device to its gateto put it in saturation • Feed constant current in side transistor • Identical current will flow in other transistor, since they are in saturation and Vgs1 = Vgs2.

20. Tri-State Driver • Inverter followed by a pass gate • For same size n- and p-devices, half the speed of inverter alone • Can omit connection between inverter devices • Used in bus drivers and latches • Can be drawn as one gate • (“Tri-State” is a registered trademark of National Semiconductor Corporation.)

21. Junctions and Diodes • At pn junction, junction diode formed • At metal-semiconductor junction,creates either: • Ohmic contact, or • Schottky diode (used extensively for high-frequency, low-noise mixer and switching circuits). • Only ohmic in most CMOS processes

22. Diode DC Equation • I = current in a diode = Ad Is(exp(qV/kmt) – 1), where: • Ad = area of the diode • Is = the saturation current/unit area • q = the charge of an electron • k = Boltzmann's constant • t = temperature • m = approx. 2.0 for pn-junction diodes, and = approx. 1.2 for Schottky diodes

23. BiCMOS Drivers • With extra processing steps added to a CMOS process, can build useful NPN transistors • NPN has high current gain • Can improve output drive of CMOS inverter