Presentation on JFET

Advised by :
Engr. Md Asif Mahmood Chowdhury
Prepared by :
 Sayed Mohammad Jahed Hossain ET - 101003
 Ishtiaque Ahmed ET - 101014
 Md. Ashraf Uddin Chowdhury ET - 101015
 Mokammel Hossain ET - 101017

 Field Effect Transistor (FET)
 Junction Field Effect Transistor (JFET)
 Construction of JFET
 Theory of Operation
 I-V Characteristic Curve
 Pinch off Voltage (VP)
 Saturation Level
 Break Down Region
 Ohmic Region
 Cut off Voltage
 Advantages
 Disadvantages
 Application of JFET
OUTLINE

INTRODUCTION
The ordinary or bipolar transistor has two main disadvantage.
• It has a low input impedance
• It has considerable noise level
To overcome this problem Field effect transistor (FET) is introduced
because of its:
• High input impedance
• Low noise level than ordinary transistor
And Junction Field Effect Transistor (JFET) is a type of FET.

FET
 FET is a voltage controled device.
 It consists of three terminal .
• Gate
• Source
• Drain
 It is classified as four types.
JFET
MESFET MISFET
MOSFET
Field Effect Transistor (FET)

 Junction Field Effect Transistor is a three terminal semiconductor device in which
current conducted by one type of carrier i.e. by electron or hole.
Junction Field Effect Transistor (JFET)

 Source: The terminal through which the
majority carriers enter into the channel, is
called the source terminal S .
 Drain: The terminal, through which the
majority carriers leave from the channel, is
called the drain terminal D .
 Gate: There are two internally connected
heavily doped impurity regions to create two
P-N junctions. These impurity regions are
called the gate terminal G.
 Channel: The region between the source
and drain, sandwiched between the two gates
is called the channel .
Construction of JFET

 JFET has two types :
• n- Channel JFET
• p- Channel JFET
Types of JFET

 JFET is a voltage controlled device i.e. input voltage (VGS) control the
output current (ID).
 In JFETs, the width of a junction is used to control the effective cross-
sectional area of the channel through which current conducts.
 It is always operated with Gate-Source p-n junction in reverse bias.
 Because of reverse bias it has high input impedance.
 In JFET the gate current is zero i.e. IG=0.
Features of JFET

(i) When gate-source voltage(VGS) is applied and drain-source voltage is zero i.e. VDS= 0V
 When VGS = 0v , two depletion layers & channel are formed normally.
 When VGS increase negatively i.e. 0V > VGS > VGS(off) , depletion layers are also increased and
channel will be decrease.
 When VGS=VGS(off), depletion layer will touch each other and channel will totally removed. So no
current can flow through the channel.
Theory of Operation
Depletion layer

(ii) When drain-source voltage (VDS) is applied at constant gate-source voltage (VGS) :
 Now reverse bias at the drain end is larger than
source end and so the depletion layer is wider at
the drain end than source end.
 When VDS increases i.e. 0v < VDS < VP , depletion layer at
drain end is gradually increased and drain current also
increased.
 When VDS = VP the channel is effectively closed at drain
end and it does not allow further increase of drain
current. So the drain current will become constant.
Theory of Operation

It is the curve between drain current (ID)and drain-source voltage (VDS)for different
gate-source voltage (VGS). It can be characterized as:
 For VGS=0v the drain current is maximum. It’s denoted as IDSS and called shorted
gate drain current.
 Then if VGS increases Drain current ID decreases (ID < IDSS) even though VDS is
increased.
 When VGS reaches a certain value, the drain current will be decreased to zero.
 For different VGS, the ID will become constant after pinch off voltage (VP) though
VDS is increased.
I-V Characteristic Curve

Fig: Transfer Characteristic Curve
Transfer Characteristic Curve
 This curve shows the value of ID for a given value of VGS .

 It is the minimum drain source voltage at which the drain current essentially
become constant.
Pinch off Voltage (VP)
Pinch off Voltage

 After pinch off voltage the drain current become constant, this constant level is
known as saturation level .
Saturation Level
Saturation Level

 The region behind the pinch off voltage where the drain current increase rapidly is
known as Ohmic Region.
Ohmic Region

Break Down Region
 It is the region, when the drain-source voltage (VDS) is high enough to cause the
JFET’s resistive channel to breakdown and pass uncontrolled maximum current .

 The gate-source voltage, when the drain current become zero is called cut-
off voltage. Which is usually denoted as VGS(off).
Cut off Voltage
Here ID become Zero

 It is simpler to fabricate, smaller in size.
 It has longer life and higher efficiency.
 It has high input impedance.
 It has negative temperature coefficient of resistance .
 It has high power gain.
Advantages

 Greater susceptibility to damage in its handling.
 JFET has low voltage gain.
Disadvantages

 Voltage controlled resistor
 Analog switch or gate
 Act as an amplifier
 Low-noise amplifier
 Constant current source
Application of JFET

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