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
V2 causes movement
of negative charges,
thus current.
V1 can control the
resistivity of the channel.
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.
P-Type Silicon is
slightly conductive.
The gate
draws no current!

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

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.
VTH=300mV to 500 mV
(OFF)
(ON)
Positive charges repel the holes
creating a depletion region, a region free of holes.
Free electrons appear at VG=VTH.

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-VOXIDE-Silicon
VOXIDE-Silicon can change along the channel! Low VOXIDE-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 Dependence on VDS)
No channel

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<(VGS-VTH) and VGS>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 gm/ID when W and ID 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
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