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Mosfet
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METAL OXIDE SEMICONDUCTOR FIELD
EFFECT TRANSISTOR
Depletion MOSFET N- channel & P- channel Enhancement MOSFET N- channel & P- channel
Types
N-Channel Depletion MOSFET Structure
Working ( VGS = 0V & VDS > 0V)
Cont.
Drain & Transfer Characteristics of N-channel D-MOSFET
Working – Enhancement mode
JFET & D-MOSFET have the same transconductance equation
Cont.
D-MOSFET Schematic Symbols
P-channel D-MOSFET Structure
P-channel D-MOSFET Characteristics
N- channel Enhancement Type MOSFET
VGS = 0V; VDS = +V
No Current ID flows
Working (VGS > 0V & VDS > 0V)
Cont.
It is the minimum gate to source voltage (VGS) that turns the E-MOSFET ON or that produces the induced channel which result in the flow of drain current (ID) in E-MOSFET
Threshold Voltage (VT or VGS(Th))
Working of E-MOSFET
Condition
Drain Current Induced Channel
VGS < VT VDS > 0V No current flows No channel
VGS = VT
VDS > 0V
No Current flows since the channel is smaller at the drain end than at the source end
Induced Channel is only just created
VGS > VT
VDS > 0VSignificant amount of
current flows
The density of free carriers in the
Induced Channel will increase
Drain Characteristics (n-channel E-MOSFET)
Drain Characteristics (n-channel E-MOSFET)
The saturation level for VDS is related to the level of applied VGS by
VDS (sat) = VGS – VT
Therefore for a fixed value of VT , the higher the level of VGS , the greater is the saturation level for VDS
Cont.
Regions Condition
Triode (Ohmic)VDS< VGS – VT
&VGS > VT
SaturationVDS > VGS – VT
&VGS > VT
Cut-off VGS < VT
Different Regions in Drain Characteristics (n-channel E-MOSFET)
Transfer & Drain Characteristics(n-channel E-MOSFET)
The equation for the E-MOSFET transconductance curve (for VGS > VT):
Cont.
P-channel E-MOSFET
P-channel E-MOSFET Drain & Transfer Characteristics
E-MOSFET Schematic Symbols
MOSFET as a Resistor
Used in Digital CMOS logic Used in almost all electronics appliances including
personal computers Used as Power Amplifers
Application of MOSFETs
JFET MOSFETTwo types – p channel & n channel JFET
Two types – Depletion & Enhancement type
Do not have the insulated gate Have the insulated gate structure
Input impedance is lower than MOSFET
Input Impedance is higher due to the insulated gate structure
Drain resistance is lower than MOSFET Drain resistance is higher than JFET
The transverse electric field across the reversed biased pn junction controls the channel conductivity
The transverse electric field induced across the insulating layer deposited on the semiconductor material controls the channel conductivity
JFET operated only in depletion mode D-MOSFET operated in both Depletion & Enhancement mode whereas E-MOSFET only in E-mode
Difficult to fabricate compared to MOSFETs
Easier to Fabricate
Not Susceptible to overload voltage Very Susceptible to overload voltage & needs special handling during installation. (easily damaged if not handled properly)
MOS Capacitor
Type Basic Relationships Symbol
Junction Field Effect Transistor (n-
channel)
Depletion type MOSFET (n-
channel)
Enhancement type MOSFET (n-
channel)
Summary of Field Effect Transistors
• Electronic Devices & Circuit Theory by Robert Boylestad and Louis Nashelsky
Reference