Unit 5 Part – C

Q1. UJT as Relaxation Oscillator

1️⃣ Components: UJT, resistor (R), capacitor (C), supply (VBB).
2️⃣ Capacitor charges through R exponentially.
3️⃣ When VC = VP (peak voltage), UJT turns ON → capacitor discharges rapidly.
4️⃣ Generates sawtooth waveform.
5️⃣ Frequency: f=1RCln⁡(11−η)f = \frac{1}{R C \ln(\frac{1}{1-\eta})}f=RCln(1−η1​)1​

6️⃣ Applications: Trigger circuits, timing circuits.

Q2. N-Channel Enhancement MOSFET

1️⃣ Construction: P-substrate with N+ source and drain; insulated gate (SiO₂).
2️⃣ With VGS = 0 → no channel.
3️⃣ Apply +VGS ≥ Vth → induces N-channel → current flows (ID).
4️⃣ Increasing VGS → higher ID (enhancement).
5️⃣ Used in amplifiers, switching devices.

Q3. N-Channel Depletion MOSFET

1️⃣ Channel exists at VGS = 0 (conducts IDSS).
2️⃣ Apply –VGS → depletes carriers → current decreases.
3️⃣ Apply +VGS → enhances current.
4️⃣ Hence both depletion & enhancement possible.
5️⃣ Used in variable resistors, amplifiers.

Q4. N-Channel JFET

1️⃣ Construction: N-type bar with P-gate junctions on both sides → terminals S, D, G.
2️⃣ With VGS = 0 → max current (IDSS).
3️⃣ Negative VGS → depletion region widens → channel narrows.
4️⃣ At pinch-off (VP), channel closes → ID ≈ constant.
5️⃣ Regions: Ohmic, saturation, cutoff.
6️⃣ Application: Amplifier, buffer, mixer circuits.

Unit 5 Part – C Tamil

Q1. UJT as Relaxation Oscillator – Circuit, Working & Waveform

Definition:
Unijunction Transistor (UJT) ஒரு negative resistance device, இது sawtooth waveform generate பண்ணப் பயன்படும்.

1️⃣ Circuit Description:

• Components: UJT, Resistor (R), Capacitor (C), Supply voltage (VBB).
• UJT terminals → Emitter (E), Base1 (B1), Base2 (B2).
• Capacitor C emitter terminal-இல் connect பண்ணப்படும்.

2️⃣ Working Principle:

1. Power ON செய்தவுடன் capacitor C charge ஆகத் தொடங்கும் through resistor R.
2. Capacitor voltage (VC) gradually increase ஆகும்.
3. VC = Peak voltage (VP) (UJT turn-ON voltage) ஆகும் போது, UJT conduct பண்ணும்.
4. Capacitor C instant discharge through emitter → base1 path.
5. VC drop ஆனவுடன், UJT OFF ஆகும் → C மீண்டும் charge ஆகும்.
6. இது repeated charging & discharging cycle produce பண்ணும்.

3️⃣ Output Waveform:

• Capacitor voltage = Sawtooth waveform.
• Emitter voltage pulses appear during discharge.

4️⃣ Frequency of Oscillation: f=1RCln⁡(11−η)f = \frac{1}{R C \ln\left(\frac{1}{1 – \eta}\right)}f=RCln(1−η1​)1​

where η = intrinsic stand-off ratio of UJT.

5️⃣ Applications:

• Trigger circuits (SCR, TRIAC).
• Timing & pulse circuits.
• Sawtooth generators.

Q2. N-Channel Enhancement MOSFET – Construction, Working & Characteristics

Definition:
MOSFET = Metal Oxide Semiconductor FET,
Enhancement type → normally OFF device (no current when VGS = 0).

1️⃣ Construction:

• Built on P-type substrate.
• Two heavily doped N+ regions → Source (S) & Drain (D).
• Thin SiO₂ insulation layer separates metal gate (G) from channel.

2️⃣ Working:

1. VGS = 0 → No N-channel → ID = 0.
2. Apply +VGS ≥ Vth → electrons attracted under gate → inversion layer (N-channel) form.
3. Current flow from Drain → Source via induced channel.
4. Increasing VGS → channel conductivity ↑ → ID ↑.

3️⃣ Characteristics:

• Transfer curve (ID vs VGS) → starts at threshold voltage Vth.
• Output curve (ID vs VDS) → shows Ohmic & saturation regions.

4️⃣ Applications:

• Amplifiers, Switching circuits, Digital ICs (CMOS).

Q3. N-Channel Depletion MOSFET – Construction, Working & Characteristics

Definition:
Depletion MOSFET normally ON at VGS = 0, current flow already present.

1️⃣ Construction:

• P-type substrate, N-channel already formed between Source (S) & Drain (D).
• Gate insulated by SiO₂ layer.

2️⃣ Working:

1. VGS = 0 → full channel available → maximum current IDSS flows.
2. Apply –VGS → negative voltage repels electrons → channel narrow → ID ↓.
3. Apply +VGS → attracts more electrons → channel widen → ID ↑.
4. Thus, both depletion & enhancement modes possible.

3️⃣ Characteristics:

• ID decreases with –VGS (depletion mode).
• ID increases with +VGS (enhancement mode).

4️⃣ Applications:

• Analog amplifiers, Variable resistors, Automatic gain control circuits.

Q4. N-Channel JFET – Construction, Working & Characteristics

Definition:
JFET (Junction Field Effect Transistor) என்பது voltage-controlled unipolar device, current control gate voltage மூலம் பண்ணப்படும்.

1️⃣ Construction:

• N-type semiconductor bar with two P-gate junctions on sides.
• Terminals → Source (S), Drain (D), Gate (G).
• Gate reverse biased → control channel width.

2️⃣ Working:

1. VGS = 0: channel full open → max current IDSS flows.
2. Negative VGS: depletion region widen → channel narrow → ID ↓.
3. At Pinch-off voltage (VP): channel completely close → ID ≈ constant (saturation).
4. Further VDS ↑ → ID almost constant → saturation region.

3️⃣ Characteristics:

• Transfer curve (ID vs VGS) → parabolic decrease.
• Output curve (ID vs VDS) → shows ohmic, saturation & cutoff regions.

4️⃣ Applications:

• Amplifier circuits, Buffer stages, Mixer circuits, Oscillators.

Summary:

Device	Control Type	Mode	Normally	Major Use
JFET	Voltage	Depletion	ON	Amplifier
D-MOSFET	Voltage	Both	ON	Variable resistor
E-MOSFET	Voltage	Enhancement	OFF	Digital switch
UJT	Negative resistance	Relaxation	—	Trigger oscillator