JNNCE, Shivamogga ECE
Experiment No. 2
Transfer and drain characteristics of a JFET and MOSFET.
Aim:
(a) Conduct experiment to study the transfer and drain characteristics of a
JFET and determine its parameters.
(b) Conduct experiment to study the transfer and drain characteristics of a
MOSFET and determine its parameters.
Components required:
SI No. Particulars Range Quantity
1 JFET (NPN) J2N3819 1
2 MOSFET (NMOS) MbreakN 1
3 Resistors 1K 1
100K 1
5 Variable power supply (0-30v) 2
6 Voltmeter (0-30v) 1
7 Ammeter (0-200mA) 1
(a) Conduct experiment to study the transfer and drain characteristics of a
JFET and determine its parameters.
JFET:
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 or p channel. In n-
channel JFET the drain is at the upper end, and the source is at the lower end.
Two p-type regions are diffused in the n-type material to form a channel, and
both p-type regions are connected to the gate lead. For simplicity, the gate lead
is shown connected to only one of the p regions. The inverse of n-channel JFET
is p-channel JFET.
(1) Transfer characteristics: A range of VGS values from zero to VGS(off)
controls the amount of drain current. For an n-channel JFET, VGS(off) is
negative, and for a p-channel JFET, VGS(off) is positive. Because VGS does
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JNNCE, Shivamogga ECE
control ID, the relationship between these two quantities is very important.
This curve is also known as a transconductance curve.
Drain saturation current IDSS: Maximum current flowing through JFET
when gate to source voltage is zero.
Pinch-off voltage VP: Gate to source voltage at which, drain current
becomes zero.
Transconductance gm: The forward transconductance (transfer
conductance), is the change in drain current ID for a given change in gate-
to-source voltage IGS with the drain-to-source voltage VDS constant.
(2) Drain characteristics: The curve between drain current ID and drain-source
voltage VDS of a JFET at constant gate-source voltage VGS is known as drain
characteristics of JFET.
Drain dynamic resistance rd: The drain dynamic resistance is defined as
the ratio of change in drain to source voltage V DS to the change in drain
current ID, when gate to source voltage remain constant.
Circuit Diagram:
Observations:
Transfer Characteristics
VDS= 15V VDS= 30V
VGS (V) ID(mA) VGS (V) ID(mA)
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JNNCE, Shivamogga ECE
Transfer characteristics:
Calculations:
ID
VGS
Calculations:
Drain saturation current IDSS =
Pinch-off voltage VP=
Procedure:
1. Connect the circuit as per given diagram properly.
2. Set the voltage VDS constant at 30 V.
3. Vary VGS by varying VGG in the step of 15 up to 30V and note down value
of drain current ID. Tabulate all the readings.
4. Plot the output characteristics VDS vs ID and transfer characteristics VGS
vs ID.
5. Calculate IDSS, vGS(off), gm from the graphs and verify.
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JNNCE, Shivamogga ECE
Observations:
Drain Characteristics
VGS= 0V VGS= -1V VGS= -2V
VDS (V) ID(mA) VDS (V) ID(mA) VDS (V) ID(mA)
ID(mA) Drain characteristics:
7
Ohmic
region Active region
6
5 VDS
ID
4
VDS(v)
-1 0 5 10 15 10
Cutoff region
Calculations:
Procedure:
1. Connect the circuit as per given diagram properly.
2. Keep VGS = 0V by varying VGG
3. Vary VDS in step of -1V up to -5 volts and measure the drain current ID.
Tabulate all the readings.
4. Repeat the above procedure for VGS as -1V, -2V, -3V, -4V, -5V etc.
5. Calculate rd from the graphs and verify.
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JNNCE, Shivamogga ECE
Result: Transconductance gm
Drain saturation current IDSS
Pinch-off voltage VP=
Drain dynamic resistance rd .
(b) Conduct experiment to study the transfer and drain characteristics of a
MOSFET and determine its parameters.
MOSFET:
The MOSFET (metal oxide semiconductor field-effect transistor) is another
category of field-effect transistor. The MOSFET, different from the JFET, has
no pn junction structure; instead, the gate of the MOSFET is insulated from the
channel by a silicon dioxide (SiO2) layer.
(1) Transfer characteristics: The transfer characteristics (drain-to-source
current IDS versus gate-to-source voltage VGS) of n-channel Enhancement-
type MOSFETs. From this, it is evident that the current through the device
will be zero until the VGS exceeds the value of threshold voltage V T.
Because VGS does control IDS, the relationship between these two quantities
is very important. This curve is also known as a transconductance curve.
Threshold voltage VT : Gate to source voltage at which, drain current starts
flowing.
Transconductance gm: The forward transconductance (transfer
conductance), is the change in drain current ID for a given change in gate-
to-source voltage VGS with the drain-to-source voltage VDS constant.
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JNNCE, Shivamogga ECE
(2) Drain characteristics: The curve between drain current IDS and drain-
source voltage VDS of a MOSFET at constant gate-source voltage VGS is
known as drain characteristics of MOSFET.
Drain dynamic resistance rd: The drain dynamic resistance is defined as
the ratio of change in drain to source voltage V DS to the change in drain
current ID, when gate to source voltage remain constant.
Circuit Diagram:
Observations:
Transfer Characteristics
VDS= 4V VDS= 5V
VGS (V) ID(mA) VGS (V) ID(mA)
Transfer characteristics:
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JNNCE, Shivamogga ECE
Calculations: Threshold voltage VT =
Procedure:
1. Connect the circuit as per given diagram properly.
2. Set the voltage VDS constant at 5V.
3. Vary VGS by varying VGG in the step of 1 up to 10V and note down value
of drain current ID. Tabulate all the readings.
4. Plot the output characteristics VDS vs ID and transfer characteristics VGS
vs ID.
5. Calculate IDSS, vGS(off), gm from the graphs and verify.
Observations:
Drain Characteristics
VGS= 5V VGS= 4V VGS= 3V
VDS (V) ID(mA) VDS (V) ID(mA) VDS (V) ID(mA)
ID(mA) Drain characteristics:
7
Ohmic
region Saturation region
6
5 VDS
ID
4
VDS(v)
-1 0 5 10 15 10
Cutoff region
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JNNCE, Shivamogga ECE
Calculations:
Procedure:
1. Connect the circuit as per given diagram properly.
2. Keep VGS = 5V by varying VGG
3. Vary VDS in step of 1V up to 10 volts and measure the drain current ID.
Tabulate all the readings.
4. Repeat the above procedure for VGS as 1V, 2V, 3V, 4V, 5V etc.
5. Calculate rd from the graphs and verify.
Result: Threshold voltage VT =
Transconductance gm .
Drain dynamic resistance rd
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