Diode, Transistor & FET Fundamentals and Applications

Posted on Feb 14, 2026 in Technology

Very Short Notes

1. Define static and dynamic resistance

Static resistance: The ratio of voltage to current at a particular operating point.

R = V / I

Dynamic resistance: The ratio of a small change in voltage to the corresponding change in current at an operating point.

r = dV / dI

2. Explain how the diode acts as switch

ON state (forward biased): When the anode is positive with respect to the cathode, the diode conducts current with very low resistance, behaving like a closed switch.

OFF state (reverse biased): When the anode is negative with respect to the cathode, the diode blocks current flow, behaving like an open switch.

3. Write the diode applications

  • Rectification — convert AC to DC
  • Voltage regulation — Zener diodes
  • Signal demodulation — detecting information from carrier signals
  • Signal protection — protection against voltage spikes
  • Light emission — LEDs

4. Define transition capacitance

Transition capacitance: The capacitance in a diode due to the depletion region’s charge-storage effects during transitions from forward to reverse bias and vice versa.

5. Difference between transition and diffusion capacitance

  • Transition capacitance: Related to the charge stored in the depletion region during switching transitions.
  • Diffusion capacitance: Related to the charge stored in carrier diffusion processes during signal transitions.

6. Define storage time and transition time

  • Storage time: The time taken for the charge carriers to be removed from the base region of a transistor after the transistor is turned off.
  • Transition time: The time taken for the transistor to change from the on state to the off state or vice versa.

7. Define forward recovery time and reverse recovery time

  • Forward recovery time: The time taken by a diode to switch from forward conduction to blocking state.
  • Reverse recovery time: The time taken by a diode to stop conducting when the polarity is reversed (to switch from conductive to blocking state).

1. State applications of varactor diode

Applications of varactor diode:

  • Frequency modulation (FM) radio tuning circuits
  • Voltage-controlled oscillators (VCOs)
  • Phase-locked loops (PLLs)
  • Tunable RF filters

2. What are the advantages of Zener diode?

Advantages:

  • Precise and stable voltage regulation
  • Simple circuit design
  • Quick response to changes in voltage
  • Can handle high currents during breakdown (within rated limits)

3. What are the applications of photodiode?

  • Optical communication systems (fiber optics)
  • Light meters and exposure meters
  • Barcode scanners
  • Smoke detectors

4. What are the applications of LED?

  • Display screens (e.g., LED TVs, digital clocks)
  • Automotive lighting (brake lights, headlights)
  • Traffic lights and signals
  • Backlighting in LCD displays

Rectifiers, Clippers and Clampers

1. Define rectifier; types of rectifier

Rectifier:

A rectifier is an electrical device that converts alternating current (AC) into direct current (DC).

Types: Half-wave rectifier; full-wave rectifier (center-tapped and bridge rectifier).

2. Define clipper; explain series clipper

Clipper definition: A clipper is a circuit that clips off a portion of the input waveform above or below a certain level, thereby changing the shape of the waveform.

Series clipper: In a series clipper, a diode is placed in series with the load. It clips off the portion of the waveform that exceeds a certain voltage level.

3. Define clamper; types of clampers

Clamper definition: A clamper is a circuit that shifts the DC level of a waveform without changing its shape.

Types: Positive clamper and negative clamper, depending on whether the waveform is shifted upwards or downwards.

10. Define Peak Inverse Voltage

Definition: Peak Inverse Voltage (PIV) is the maximum voltage that a diode or other rectifying element can withstand in the reverse-bias direction without damage.

Transistors: Basics and Characteristics

1. What is transistor? Give its circuit symbol

Transistor: A transistor is a semiconductor device used for amplifying or switching electronic signals and power. It consists of three layers of semiconductor material forming two PN junctions.

(Circuit symbols vary for NPN and PNP; include standard transistor symbol in diagrams when using this text.)

6. What are the regions of the transistors?

  • NPN transistor: Emitter (E), Base (B), Collector (C)
  • PNP transistor: Emitter (E), Base (B), Collector (C)

7. Write any two applications of transistor

  • Amplification in audio and radio-frequency circuits
  • Switching applications in digital circuits

8. Explain various current components of transistor

  • Emitter current (IE): Total current leaving/entering the emitter terminal (depending on device polarity).
  • Base current (IB): Current entering the base terminal.
  • Collector current (IC): Current flowing through the collector terminal.

9. Why transistor is called current-controlled device?

Answer: In a BJT, the base current controls the larger collector current. The relationship between base current (IB) and collector current (IC) is defined by the current gain (β).

FETs and MOSFETs

1. Why FET is called voltage-controlled device?

Answer: A FET (Field-Effect Transistor) is called a voltage-controlled device because its operation and conductivity are controlled by the voltage applied to the gate terminal, which modulates the channel conductivity between the source and drain.

3. Important features of FET

  • High input impedance
  • Low output impedance
  • Voltage-controlled operation
  • Low power consumption

4. What is pinch-off voltage?

Pinch-off voltage: The gate-source voltage (VGS) at which the channel of a FET becomes depleted of majority carriers (electrons in N-channel, holes in P-channel), effectively reducing or cutting off the drain current.

7. What do you mean by MOSFET and its classification?

MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor): A type of FET with an insulated gate.

Classification:

  • Enhancement mode MOSFET: Normally off; requires a gate voltage to turn on.
  • Depletion mode MOSFET: Normally on; requires a gate voltage to turn off.