1. Reading: Finish Chapter 17, 18.3.4
Lecture #38
OUTLINE
The MOSFET:
Bulk-charge theory
Body effect parameter
Channel length modulation parameter
PMOSFET I-V
Small-signal model
Reading: Finish Chapter 17,
EE130 Lecture 38, Slide 1

2. Problem with the “Square Law Theory”
Ignores variation in depletion width with distance y
EE130 Lecture 38, Slide 2

3. Modified (Bulk-Charge) Model
linear region:
saturation region:
EE130 Lecture 38, Slide 3

4. MOSFET Threshold Voltage, VT
The expression that was previously derived for VT is the gate voltage referenced to the body voltage that is required reach the threshold condition:
Usually, the terminal voltages for a MOSFET are all referenced to the source voltage. In this case,
and the equations for IDS are
EE130 Lecture 38, Slide 4

5. The Body Effect Note that VT is a function of VSB:
where g is the body effect parameter
When the source-body pn junction is reverse-biased, |VT| is increased. Usually, we want to minimize g so that IDsat will be the same for all transistors in a circuit
EE130 Lecture 38, Slide 5

6. MOSFET VT Measurement
VT can be determined by plotting IDS vs. VGS, using a low value of VDS
IDS
VGS
EE130 Lecture 38, Slide 6

7. Channel Length Modulation Parameter, l
Recall that as VDS is increased above VDsat, the width DL of the depletion region between the pinch-off point and the drain increases, i.e. the inversion layer length decreases.
EE130 Lecture 38, Slide 7

8. P-Channel MOSFET The PMOSFET turns on when VGS < VTp
Holes flow from SOURCE to DRAIN
 DRAIN is biased at a lower potential than the SOURCE
In CMOS technology, the threshold voltages are usually symmetric: VTp = -VTn VG
VDS < 0
IDS < 0
|IDS| increases with
|VGS - VTp|
|VDS| (linear region) VS VD
GATE
IDS P+ P+ N VB
EE130 Lecture 38, Slide 8

9. m = 1 + (3Toxe/WT) is the bulk-charge factor
PMOSFET I-V
Linear region:
Saturation region:
m = 1 + (3Toxe/WT) is the bulk-charge factor
EE130 Lecture 38, Slide 9

10. Small Signal Model
Conductance parameters:
EE130 Lecture 38, Slide 10

11. Inclusion of Additional Parasitics
EE130 Lecture 38, Slide 11

12. Cutoff Frequency
fmax is the frequency where the MOSFET is no longer amplifying the input signal
Obtained by considering the small-signal model with the output terminals short-circuited, and finding the frequency where |iout / iin| = 1
Increased MOSFET operating frequencies are achieved by decreasing the channel length
EE130 Lecture 38, Slide 12