1. Qualitative Discussion of MOS Transistors

2. Big Picture ES230 – Diodes – BJT – Op-Amps ES330 – Applications of Op-Amps – CMOS Analog applications Digital applications

3. A Crude Metal Oxide Semiconductor (MOS) Device P-Type Silicon is slightly conductive. Positive charge attract negative charges to interface between insulator and silicon. A conductive path is created If the density of electrons is sufficiently high. Q=CV. V2 causes movement of negative charges, thus current. V1 can control the resistivity of the channel. The gate draws no current!

4. An Improved MOS Transistor n+ diffusion allows electrons move through silicon. (provide electrons)(drain electrons)

5. Typical Dimensions of MOSFETs These diode must be reversed biased. tox is made really thin to increase C, therefore, create a strong control of Q by V.

6. A Closer Look at the Channel Formulation Need to tie substrate to GND to avoid current through PN diode. Positive charges repel the holes creating a depletion region, a region free of holes. Free electrons appear at VG=VTH. VTH=300mV to 500 mV (OFF)(ON)

7. MOSFET as a Variable Resistor As VG increases, the density of electrons increases, the value of channel resistance changes with gate voltage. You can build an attenuator circuit. (i.e. a voltage divider)

8. Change Drain Voltage Resistance determined by VG.

9. Change Gate Voltage Higher VG leads to a lower channel resistance, therefore larger slope.

10. Length Dependence The resistance of a conductor is proportional to the length.

11. Dependence on Oxide Thickness Q=CV C is inversely proportional to 1/tox. Lower Q implies higher channel resitsance.

12. Width Dependence The resistance of a conductor is inversely proportional to the cross section area. A larger device also has a larger capacitance!

13. Channel Pinch Off Q=CV – V=VG-V OXIDE-Silicon V OXIDE-Silicon can change along the channel! Low V OXIDE-Silicon implies less Q.

14. VG-VD is sufficiently large to produce a channel VG-VD is NOT sufficiently large to produce a channel No channel Electrons are swept by E to drain. Drain can no longer affect the drain current!

15. Regions No channel (No Dependence on VDS)

16. Determination of Region How do you know whether a transistor is in the linear region or saturation region? – If VDS>(VGS-VTH) and VGS>VTH, then the device is in the saturation region. – If VDS VTH, then the device is in the linear region.

17. Graphical Illustration

18. Limited VDS Dependence During Saturation As VDS increase, effective L decreases, therefore, ID increases.

19. Pronounced Channel Length Modulation in small L

20. Transconductance As a voltage-controlled current source, a MOS transistor can be characterized by its transconductance: It is important to know that

21. What Happens to g m /I D when W and I D are doubled ?

22. Body Effect The threshold voltage will change when VSB=0!

23. Experimental Data of Body Effect The threshold voltage will increase when VSB increases.

24. What if we drive the base with a small signal ?

25. Input and Output Vout, m=46 mV Vin, m=1 mV

26. Replace the transistor by its small signal equivalent circuit (EQ 5.157) Comparision: ADS Simulation: 46 EQ 5.157: 49.33

27. Small Signal Model for NMOS Transistor

28. PMOS Transistor

29. IV Characteristics of a PMOS

30. Small Signal Model of PMOS

31. Small Signal Model of NMOS