FET and MOSFET
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Field Effect Transistor (FET)
A Field Effect Transistor (FET) is a three-terminal semiconductor device. Its operation is based
on a controlled input voltage(electric field).
Two types of FETs are available.
• Junction Field Effect Transistor (JFET)
• Metal Oxide Semiconductor FET (MOSFET) /Insulated Gate FET (IGFET)
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FET
The functioning of Junction Field Effect Transistor depends upon the flow of majority carriers
(electrons or holes) only. Basically, JFETs consist of an N type or P type silicon bar containing
PN junctions at the sides.
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Types of FET
Gate − By using diffusion or alloying technique, both sides of N type bar are heavily doped to
create PN junction. These doped regions are called gate (G).
Source − It is the entry point for majority carriers through which they enter into the
semiconductor bar.
Drain − It is the exit point for majority carriers through which they leave the semiconductor
bar.
Channel − It is the area of N type material through which majority carriers pass from the source
to drain.
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Types of FET
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FET Working
N-channel JFET Biasing
This is a transistor with N-type of channel and with P-type materials of the region. If the gate is diffused into
the N-type channel, then a reverse biased PN-junction is formed which results a depletion region around the
gate terminal when no external supply is applied to the transistor. Generally the JFETs are called as
depletion mode devices.
This depletion region produces a potential gradient with the variation of thickness around the PN-junction.
This PN-junction opposes the current flow through the channel by reducing the channel width and by
increasing the channel resistance.
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Pinch-off Effect
• If the gate voltage is zero, then the channel resistance is also zero and the conduction of the channel is
high. If the gate voltage (i.e. negative voltage) increases to above zero, then the resistance of the
channel also increases and the small amount of current will flows through the channel.
• If we apply a large amount of negative voltage at the gate terminal, then the channel totally blocks the
flow of current through it. In this condition, there is no current flow through the channel and now the
JFET acts as a perfect resistor.
• The state of JFET in which the channel closes is called “pinched-off” and the voltage applied at gate in
that situation is called “pinched-off voltage (VP)”.
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JFET V-I Characteristics
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Advantages of FET Disadvantages of FET
• High input impedance of several megaohms
• They are more costly than junction transistor
• FET has less effect by radiation than BJT
• Smaller gain bandwidth product compare to
• Temperature stable than BJT
BJT
• Less noise compare to BJT
• Can be fabricated with fewer processing • Transconductance is low hence voltage gain is
• Smaller in size low
• Longer life • It has lower switching time compare to BJT
• High efficiency
• It can be used low frequency application
• Uni-polar device
• Voltage control device
• They have better thermal stability
• They have voltage control device
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MOSFET
• The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a
semiconductor device that is widely used for switching purposes and for the amplification of
electronic signals in electronic devices.
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MOSFET working
• The charge carriers enter into the channel through the source terminal and exit via the drain.
• The width of the channel is controlled by the voltage on an electrode which is called the gate
and it is located in between the source and the drain. It is insulated from the channel near
an extremely thin layer of metal oxide.
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Working Principle of MOSFET
The working of the MOSFET depends on the MOS capacitor, which is the semiconductor surface
below the oxide layers between the source and drain terminal. It can be inverted from p-type to
n-type, simply by applying positive or negative gate voltage respectively.
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MOSFET Types
A MOSFET can function in two ways
• Depletion Mode
• Enhancement Mode
Depletion Mode
When there is no voltage across the gate terminal, the channel shows its maximum conductance. Whereas
when the voltage across the gate terminal is either positive or negative, then the channel conductivity
decreases.
Enhancement Mode
When there is no voltage across the gate terminal, then the device does not conduct. When there is the
maximum voltage across the gate terminal, then the device shows enhanced conductivity.
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MOSFET in Depletion Mode:
The depletion-mode MOSFETs are usually called the “Switched ON” devices as they are
generally in the closed state when there is no bias voltage at the gate terminal. When we
increase the applied voltage to the gate in positive the channel width will be increased in
depletion mode. This will increase the drain current ID through the channel. If the applied gate
voltage is highly negative, then the channel width will be less and the MOSFET might enter into
the cutoff region.
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MOSFET in Enhancement Mode:
The operation of MOSFET in Enhancement mode is similar to the operation of the open switch, it
will start to conduct only if the positive voltage(+VGS) is applied to the gate terminal and the
drain current starts to flow through the device. The channel width and drain current will
increase when the bias voltage increases. But if the applied bias voltage is zero or negative the
transistor will remain in the OFF state itself.
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Complimentary MOS (C-MOS)
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