This document discusses different types of field effect transistors (FETs), including their basic construction and operation. It provides details on junction FETs (JFETs) and metal-oxide-semiconductor FETs (MOSFETs), as well as depletion and enhancement MOSFETs. The document also covers the drain and transfer characteristics of enhancement MOSFETs. Finally, it describes the working and applications of relaxation oscillators using unijunction transistors (UJTs), including their voltage-current characteristics and use in timing circuits.

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