MOSFET

3. R.M.K COLLEGE OF ENGINEERING AND
TECHNOLOGY
DEPARTMENT OF ECE
EC8252-ELECTRONIC DEVICES
SECOND SEMESTER-I YEAR- (2020-2024 BATCH): SECTION A & B
Mrs.P.Sivalakshmi M.E
AP/ECE
SESSION:21
DATE: 1.06.2021
UNIT 3 FIELD EFFECT TRANSISTORS

4. Characteristics Parameters of JFET
Like vacuum tubes, a JFET has certain parameters which determine its performance in a circuit. The main
parameters of a JFET are (i) a.c. drain resistance (ii) trans conductance (iii) amplification factor.
(i) a.c. drain resistance (rd).
Corresponding to the a.c. plate resistance, we have a.c. drain resistance in a JFET. It may be defined as
follows: It is the ratio of change in drain-source voltage (ΔVDS) to the change in drain current (ΔID) at
constant gate-source voltage.
It is the reciprocal of the slope of the drain characteristics and is defined by the drain characteristics.
i.e. a.c. drain resistance, rd =(ΔVDS)/ (ΔID) at constant VGS
For instance, if a change in drain voltage of 2 V produces a change in drain current of 0.02 mA, then,
a.c. drain resistance, rd=100kΩ.
• Drain resistance of a JFET has a large value, ranging from 10 kΩ to 1 MΩ.

5. (ii)Trans conductance ( gfs).
The control that the gate voltage has over the drain current is measured by trans
conductance gfs and is similar to the trans conductance gm of the tube. It may be defined as
follows :
It is the slope of the transfer characteristics curve and is defined by,
It is the ratio of change in drain current (ΔID) to the change in gate-source voltage (ΔVGS)
at constant drain-source voltage i.e.
Transconductance, gm (or) gfs=ΔID/ΔVGS at constant VDS.
The Transconductance of a JFET is usually expressed either in mA/volt or micro mho. As
an example, if a change in gate voltage of 0.1 V causes a change in drain current of 0.3
mA, then,
Transconductance, gfs=0.3 mA /0.1 V
=3 mA/V or mho or S (Siemens)
=3 × 10−3× 106 μ mho
= 3000 μ mho (or μ S)

6. (iii)Amplification factor ( μ ).
It is the ratio of change in drain-source voltage (ΔVDS) to the change in gate-source voltage (ΔVGS) at
constant drain current
i.e. Amplification factor,
μ =ΔV DS /Δ V GS at constant ID
Amplification factor of a JFET indicates how much more control the gate voltage has over drain current than has
the drain voltage.
For instance, if the amplification factor of a JFET is 50, it means that gate voltage is 50 times as effective as the
drain voltage in controlling the drain current.
Power Dissipation: PD= ID *VDS

10. Re-draw the depletion region when the PN junction is subjected to a reverse-bias voltage:
Match the following field-effect transistor illustrations to their respective
schematic symbols:
From the “diode check” measurements taken with these two meters, identify the terminals on this
JFET, and also what type of JFET it is (N-channel or P-channel):

11. Identify which transistor terminal functions as the source and which transistor terminal functions as the drain in both of these JFET circuits:
Identify each type of JFET (whether it is N-channel or P-channel), label the terminals, and determine whether the JFET in each of these circuits will be turned

12. Identify each type of JFET (whether it is N-channel or P-channel), label the terminals, and determine whether the JFET in each of these circuits will be turned on or o

13. Determine what status the LED will be in (either “on” or “off”) for both switch positions in this JFET switching circuit: