Basic construction and operation FET and MOSFET PPT UNIT 2.pdf

Asst. Prof. Manoj Kr. Sharma
ECE/EN deptt.
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UNIT 2 PART B
JFET
(JUNCTION FIELD EFFECT TRANSISTOR)
The term field-effect relates to the depletion region formed in the channel
of a FET as a result of a voltage applied on one of its terminals (gate).
(OPERATION)

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DIFFERENCES B/W BJT & JFET
• FETs are unipolar devices because, unlike BJTs that use both
electron and hole current, they operate only with one type of charge
carrier.
• Recall that a BJT is a current-controlled device; that is, the base
current controls the amount of collector current.
• A FET is different. It is a voltage-controlled device, where thevoltage
between two of the terminals (gate and source) controls the current
through the device.
• A major advantage of FETs is their very high input resistance.
• However, FETs are the preferred device in low-voltage switching
applications because they are generally faster than BJTs when
turned on and off.

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DIFFERENCES B/W BJT &
JFET

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JFET
The relationship between ID and VGS is defined by Shockley’s
equation:

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JFET

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COMPARISION B/W BJT & FET

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CLASSIFICATION OF FET(FIELD EFFECT TRANSISTOR)
p

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TYPES OF FET
• The JFET (junction field-effect transistor)
is a type of FET that operates with a
reverse-biased pn junction to control
current in a channel.
• Depending on their structure:
• JFETs fall into either of two categories,
n channel JFET or p channel JFET.

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JFET -Construction
(Junction Field-Effect Transistor)

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n Channel JFET (OPERATION)

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PINCH-OFF VOLTAGE
WHEN DRAIN CURRENT ID = 0 mAmp at VGS = │VP│ is called PINCH-OFF VOLTAGE

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V-I CHARACTERSTICS

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Voltage-Controlled Resistor
In this region the JFET can actually be employed as a variable resistor
(possibly for an automatic gain control system) whose
resistance is controlled by the applied gate-to-source voltage.
Where ro is the resistance with VGS =0 V and rd the resistance at a particular level
of VGS.
For an n-channel JFET with ro equal to 10 kΩ (VGS =0 V, VP = - 6 V), above equation
will result in 40 kΩ at VGS = - 3 V.

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V-I CHARACTERSTICS

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P-CHANNEL JFET

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n channel JFET & p channel JFET
SYMBOLS

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TYPES OF MOSFET
• The metal oxide semiconductor field-effect transistor (MOSFET).
• DEPLETION (MOSFET)
• n channel D-MOSFET or p channel D-MOSFET
• ENHANCEMENT (MOSFET)
• n channel E-MOSFET or p channel E-MOSFET

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DEPLETION (MOSFET)
Depletion-type MOSFETs, Shockley’s equation is applied to relate the input and
output quantities:

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DEPLETION (MOSFET)
n channel D-MOSFET p channel D-MOSFET

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DEPLETION (MOSFET)
p channel E-MOSFET SYMBOLS

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DEPLETION (MOSFET)
OPERATION

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OPERATION

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V-I CHARACTERSTICS

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V-I CHARACTERSTICS

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ENHANCEMENT (MOSFET)
For enhancement-type MOSFETs, the following equation is applicable:

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n channel E-MOSFET

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SYMBOLS

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n channel E-MOSFET(OPERATION)

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n channel E-MOSFET OPERATION

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V-I CHARACTERSTICS

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V-I CHARACTERSTICS

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FET Biasing
• FIXED-BIAS CONFIGURATION
• SELF-BIAS CONFIGURATION
• VOLTAGE-DIVIDER BIASING

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FET Biasing summary

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FET Biasing summary

Basic construction and operation FET and MOSFET PPT UNIT 2.pdf

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