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MOSFET V-I Characteristics
Vijaylakshmi.B
Lecturer, Dept of Instrumentation Tech
Basaveswar Engg. College
Bagalkot, Karnataka
IUCEE-VLSI Design, Infosys, Mysore
Types of Transistors
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
n-channel
p-channel
Depletion type
n-channel
p-channel
Enhancement type
MOSFET
FET = Field-Effect Transistor
A four terminal device (gate, source, drain,
bulk)
Symbols of
MOSFET
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 switchedmode power supplies.
 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.
•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
n-Channel MOSFET
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
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
PMOS Structure
 PMOS transistor has a negative threshold voltage (Vtp) -0.3v~-1.2v
 A pMOS turns on when Vgs<Vtp
P-MOSFET Characteristics
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.
Switch models of MOSFETs
g=0
d
nMOS
d
OFF
g
s
d
pMOS
d
g=1
s
s
d
d
g
ON
OFF
ON
s
s
s
Thank You