Sri Ramakrishna College of Arts & Science, Coimbatore, Department of Physics

1. Field
Effect
Transistor
Unit II

2. FET
• The field-effect transistor (FET) is a type of transistor that uses an
electric field to control the flow of current in a semiconductor.
• FETs are devices with three terminals:
• source
• gate and
• Drain
• The Field Effect Transistor, FET, is a three terminal active device
that uses an electric field to control the current flow and it has a
high input impedance which is useful in many circuits.

3. Types
• There are two types of field-effect transistors,
• The Junction Field-Effect Transistor (JFET) and
• The “Metal-Oxide Semiconductor” Field-Effect Transistor (MOSFET) or
Insulated-Gate Field-Effect Transistor (IGFET).

4. FET

5. n- Channel and p- Channel

6. Comparison

7. V-I or Transfer Characteristics

8. V-I or Drain Characteristics

9. Transfer Characteristics

10. Comparison between FET and BJT
SL
No
FET BJT
1 Unipolar Device Bipolar Device
2 Voltage – Controlled Device Current – Controlled Device
3 High Input Resistance – Few M-ohms Low Input Resistance – Few K-ohms
4 TCR - Negative TCR - Positive
5
No – Minority carrier Storage effect – high
switching Speeds and Cutoff frequencies
Suffers Minority carrier Storage effect –
Lower switching Speeds and Cutoff
frequencies
6 Less Noisy More Noisy
7 Simpler to Fabricate Difficult to Fabricate
8
Immune to radiation – excellent signal
chopper
Susceptible to radiation – stability is
disturbed
9 Lower Gain BW Higher Gain BW
10
Susceptible to overload – required spl
handling during installation
Does not require spl handling during
installation

11. MOSFET
MOSFET – Two Basic Types
Depletion Type – D-MOSFET
Enhancement Type – E-MOSFET
Difference – Difference in
Construction

12. D-MOSFET
• Gate to Source
Voltage -
Negative

13. E-MOSFET
• Positive Gate to Source Voltage

14. Drain & Transfer Characteristics of E-MOSFET

15. Uni Junction Transistor (UJT)

16. UJT Working
𝑉𝑃 = 𝜂𝑉𝐵𝐵 + 𝑉𝐷
𝑅 =
1000
𝜂𝑉𝐵𝐵

17. V-I Characteristics of UJT

18. Applications of UJT
• Trigger Device for SCR’s and TRIACs
• Non-Sinusoidal Oscillators
• Saw-Tooth generators
• Timing circuits

19. Relaxation Oscillator
• An oscillator is a device that produces a waveform by its own,
without any input. Though some dc voltage is applied for the
device to work, it will not produce any waveform as input.
• The UJT relaxation oscillator is called so because the timing
interval is set up by the charging of a capacitor and the timing
interval is ceased by the rapid discharge of the same capacitor.
• In electronics a relaxation oscillator is a nonlinear electronic
oscillator circuit that produces a nonsinusoidal repetitive output
signal, such as a triangle wave or square wave.

20. • This waveform depends generally upon the charging and
discharging time constants of a capacitor in the circuit.

21. Relaxation Oscillator - Construction
Construction:
• The emitter of UJT is connected with a resistor and capacitor.
• The RC time constant determines the timings of the output waveform
of the relaxation oscillator.
• Both the bases are connected with a resistor each.
• The dc voltage supply VBB is given.

22. Relaxation Oscillator - Working
Working:
• Initially, the voltage across the capacitor is zero.
• The UJT is in OFF condition. The resistor R provides a path
for the capacitor C to charge through the voltage applied.
V=V0(1−e−t/RC)
• The capacitor usually starts charging and continues to
charge until the maximum voltage VBB.
• But in this circuit, when the voltage across capacitor
reaches a value, which enables the UJT to turn ON (the
peak voltage) then the capacitor stops to charge and
starts discharging through UJT.
• Now, this discharging continues until the minimum voltage
which turns the UJT OFF (the valley voltage).
• This process continues and the voltage across the
capacitor, when indicated on a graph, the following
waveform is observed.
Relaxation oscillators are widely used
in function generators, electronic
beepers, SMPS, inverters, blinkers,
and voltage controlled oscillators