3. MOSFET (Types)
Four types:
n-channel enhancement mode
• Most common since it is cheapest to manufacture
p-channel enhancement mode
n-channel depletion mode
p-channel depletion mode
Depletion type
n-channel p-channel
Enhancement type
n-channel p-channel
4. MOSFET
FET = Field-Effect Transistor
A four terminal device (gate, source, drain,
bulk)
Symbols of
MOSFET
5. MOSFET characteristics
Basically low voltage device. High voltage
device are available up to 600V but with limited
current. Can be paralleled quite easily for higher
current capability.
Internal (dynamic) resistance between drain and
source during on state, RDS(ON), , limits the
power handling capability of MOSFET. High
losses especially for high voltage device due to
RDS(ON) .
Dominant in high frequency application
(>100kHz). Biggest application is in switched-
mode power supplies.
6. The transistor consists of three regions, labeled the
``source'', the ``gate'' and the ``drain''.
The area labeled as the gate region is actually a
``sandwich'' consisting of the underlying substrate
material, which is a single crystal of semiconductor
material (usually silicon); a thin insulating layer
(usually silicon dioxide); and an upper metal
layer.
Electrical charge, or current, can flow from the
source to the drain depending on the charge applied
to the gate region.
The semiconductor material in the source and drain
region are ``doped'' with a different type of material
than in the region under the gate, so an NPN or PNP
type structure exists between the source and drain
region of a MOSFET.
7. •Most important device in digital design
•Very good as a switch
•Relatively few parasitics
•Rather low power consumption
•High integration density
•Simple manufacturing
•Economical for large complex circuits
9. NMOS Structure
MOS (Metal-Oxide-Semiconductor) Nowadays gate is made of poly-silicon
Channel length L and width W
In most digital design, L is set at the minimum feature size
W is selectable by the designer
Bulk is connected to the Gnd in NMOS to prevent forward-biased PN junction
On state Off state
10. n-MOSFET Characteristics
Plots V-I characteristics
of the device for various
Gate voltages (VGS)
At a constant value of VDS , we can
also see that IDS is a function of the
Gate voltage, VGS
The transistor begins to conduct
when the Gate voltage, VGS , reaches
the Threshold voltage: VT
11. PMOS Structure
PMOS transistor has a negative threshold voltage (Vtp) -0.3v~-1.2v
A pMOS turns on when Vgs<Vtp
12. The terminal characteristics of the device are given by
drain-to-source current Ids against drain-to-source
voltage Vds for different values of gate-to-source
voltage Vgs. All voltages are referenced with respect
to the source voltage, which is assumed to be at
ground potential.
P-MOSFET Characteristics
13. Switch models of MOSFETs
g
s
d
g = 0
s
d
g = 1
s
d
g
s
d
s
d
s
d
nMOS
pMOS
OFF
ON
ON
OFF
