OUTLINE

1
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, 18.3.4

2
Problem with the Square Law Theory

• Ignores variation in depletion width with
  distance y

3
Modified (Bulk-Charge) Model

• linear region

• saturation region

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
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
6
MOSFET VT Measurement

• VT can be determined by plotting IDS vs. VGS,
  using a low value of VDS

IDS
VGS
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.

8
P-Channel MOSFET

• The PMOSFET turns on when VGS lt 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 lt 0
• IDS lt 0
• IDS increases with
• VGS - VTp
• VDS (linear region)

VS
VD
GATE
IDS
P
P
N
VB
9
PMOSFET I-V

• Linear region
• Saturation region

m 1 (3Toxe/WT) is the bulk-charge factor
10
Small Signal Model

• Conductance parameters

11
Inclusion of Additional Parasitics
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