Jfet and Mosfet Slides

FET AND MOSFET
From : A.A.Chaudhary
Department of Physics
Institute of Science ,Nagpur
Introduction
Classification
CONSTRUCTION OF THE JFET
 The n-channel JFET consists of a uniformly doped n-type silicon semiconductor bar with
ohmic contacts at both ends and semiconductor junctions made on either sides of the bar.
 The top portion of the n-type channel is connected through the ohmic contact to a terminal
called the drain (D) while the lower end is connected to the terminal referred to as the
source (S ).
 The two p-type materials, fabricated on the two sides, are connected together and then to
the third terminal called gate (G).
 The source terminal gets its name from the fact that the carriers contributing to the current
flow move out from the external circuit into the semiconductor at this electrode.
 The carriers travel through the bulk of the semiconductor and are subsequently collected
at the drain electrode.
 The gate is called so because it controls the flow of charges though the bulk.
JFET CONSTRUCTION
Symbol of N CHANNEL FET
 There are three terminals: Drain (D) and Source (S) are
connected to n-channel
Gate (G) is connected to the p-type material
N-Channel JFET Operation &
Working
Transfer Characteristics
 The input-output transfer characteristic of the

JFET is not as straight forward as it is for the BJT
 In a BJT,  (hFE) defined the relationship
between IB (input current) and IC (output current).
 In a JFET, the relationship (Shockley’s Equation)
between VGS (input voltage) and ID (output
current) is used to define the transfer
characteristics, and a little more complicated (and
not linear):
Transfer and Drain characteristics
JFET Parameters
• 1) Drain resistance (rd) :-- (VDS/ ID) VGS
• 2) Transconductance (gm):--(ID / VGS) VDS
• 3) Amplification factor (µ):--(VDS / VGS) ID

JFET as an amplifier
Applications of JFET
• JFET are used as an Amplifier.
• JFET are used for mixer operation of FM and
TV recievers.
• It can be used as voltage variable resistor.
• It can be used for computer memories
because of small size.
Advantages of JFET over BJT
• 1) It has higher input impedance than that of the
BJT.
• 2) It has negative temperature coefficient of
resistance and hence better thermal stability.
• 3)It has small size,longer life and high efficiency.
• 4)It has low noise level.
• 5)It has high power gain
• 6) It has square law characteristics
MOSFET
CONSTRUCTION
Internal Diagram
TYPES
Types of MOSFETS
n-channel p-channel
Enhancement Mode Enhancement Mode
(nMOSFET) (pMOSFET)
n-channel p-channel
Depletion Mode Depletion Mode
(nMOSFET) (pMOSFET)
MOSFET
(Metal Oxide Semiconductor
FET)
There are two types of MOSFET’s:
• Depletion mode MOSFET (D-MOSFET)
• Operates in Depletion mode the same way
as a JFET when VGS  0
• Operates in Enhancement mode like E-
MOSFET when VGS > 0
• Enhancement Mode MOSFET (E-MOSFET)

• Operates in Enhancement mode
• IDSS = 0 until VGS > VT (threshold
voltage)
22
Depletion MOSFET N channelConstruction
The Drain (D) and Source (S) leads connect to the to n-

doped regions
These N-doped regions are connected via an n-channel
This n-channel is connected to the Gate (G) via a thin
insulating layer of SiO2
23
The n-doped material lies on a p-doped substrate that
D-MOSFET Symbols
24
Working of DMOSFET(Depletion mode)
Working of DMOSFET(Enhancement
mode)
• VGS : Apply positive voltage to gate terminal
• N channel: Enriched with more electrons.
• Drain Current: increases
Drain Characteristics (D MOSFET)
Transfer Characteristics (D MOSFET)
Enhancement MOSFET(N channel)
Construction
The Drain (D) and Source (S) connect to the to n-

doped regions
These n-doped regions are not connected via an n-
channel without an external voltage. The Gate (G)
connects to the p-doped substrate via a thin insulating
29
layer of SiO2.The n-doped material lies on a p-doped
E-MOSFET Symbols
30
Working E MOSFET(N channel)
Drain characteristics E MOSFET
(N channel)
Transfer characteristics E MOSFET(N
channel)
Working of EMOSFET(P channel)
Drain & Transfer characteristics E MOSFET
(P channel)
Merits of MOSFET
1. It has very high input impedance

Approx.= 10 11 to 10 15 ‎Ω
2. Requires very small space for fabrication.
3.lower power consumption and high noise

immunity.
4.MOSFET Transconductance increases with

drain current ,it gives less distortion.
Difference between E MOSFET & D
MOSFET
1. E MOSFET Consists induced channel where as

D MOSFET Consists inbuilt diffused channel.
2. E MOSFET operates only in Enhancement

mode where as D MOSFET operates in both
Enhancement & Depletion mode.
3.In E MOSFET no current ID flows when VGS =

0 while in D MOSFET significant current flows
De merits of MOSFET
1. MOSFET requires very careful handling.
2.MOSFET is highly susceptible to overload

voltages pick from any stray or static
charges and may destroy it.
Applications of MOSFET
1. MOSFET are used in Switch mode power

supply(SMPS) & are widely used in battery
charging applications.
2. They can be used as Hi-Fi Amplifiers when

configured in complementary pairs.
3.MOSFET provide large output current with a

small input.This Characteristics in transducer
drivers for high power devices like motor and
CFL bulbs
Comparision of MOSFET with JFET
1.Principle of operation:-- In MOSFET the

conductivity of channel is controlled by the
transverse electric field across capacitor where
as in JFET conductivity of channel is controlled
by the transverse electric field across reverse
biased PN junction.
2. Input Resistance:-- Input Resistance of JFET is

of the order of 10 8 to 10 9 ohms where as Input
Resistance of MOSFET is very high of the order
of 10 10 to 10 15 ohm.
Comparision of MOSFET with JFET
3. Output Resistance:-- Output Resistance of
MOSFET is of the order of 1 to 50K ohm10 8 to
10 9 ohms where as output Resistance of JFET is
very high of the order of 0.1 to 1 Megaohm.
4.Mode of operation :-- DMOSFET operates in

both Enhancement & Depletion mode while
JFET operates only in depletion mode.
5.MOSFETs are easier to fabricate than JFET.

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FET AND MOSFET

 The input-output transfer characteristic of the

• 2) Transconductance (gm):--(ID / VGS) VDS

• 3) Amplification factor (µ):--(VDS / VGS) ID

• Enhancement Mode MOSFET (E-MOSFET)

The Drain (D) and Source (S) leads connect to the to n-

The Drain (D) and Source (S) connect to the to n-

1. It has very high input impedance

2. Requires very small space for fabrication.

3.lower power consumption and high noise

4.MOSFET Transconductance increases with

1. E MOSFET Consists induced channel where as

2. E MOSFET operates only in Enhancement

3.In E MOSFET no current ID flows when VGS =

1. MOSFET requires very careful handling.

2.MOSFET is highly susceptible to overload

1. MOSFET are used in Switch mode power

2. They can be used as Hi-Fi Amplifiers when

3.MOSFET provide large output current with a

1.Principle of operation:-- In MOSFET the

2. Input Resistance:-- Input Resistance of JFET is

4.Mode of operation :-- DMOSFET operates in

5.MOSFETs are easier to fabricate than JFET.

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