Diodes and Their Operation

Diode

• A diode is an electrical device that allows current to flow only in one direction and can convert AC to DC, making it a rectifier.
• The term “diode” is derived from “DI-ODE,” meaning a device with two electrodes.

Operation of P-N Junction Diode

• A P-N junction diode is formed by joining P-type and N-type semiconductors, creating a junction where the conventional current flows in one direction.
• It is a fundamental component in semiconductor devices.

Improving the Conductivity of Semiconductors

1. Increase in Temperature:
   • Heating causes electrons to gain kinetic energy, allowing them to move from the valence band to the conduction band, improving conductivity.
2. Doping:
   • Adding impurities to semiconductors increases the number of charge carriers, enhancing conductivity.
3. Types of Doping: i. Donor Doping:
   • Adds atoms with more valence electrons (e.g., phosphorus in silicon), creating free electrons and forming an N-type semiconductor, where electrons are the majority carriers.
4. ii. Acceptor Doping:
   • Uses atoms with fewer valence electrons (e.g., boron in silicon), creating holes (vacancies) and forming a P-type semiconductor, where holes are the majority carriers.

Examples of Semiconductors

1. Diode:
   • Can be either forward biased or reverse biased.
2. a. Forward Biased:
   • Connected positive to positive and negative to negative, allowing current to flow.
3. b. Reverse Biased:
   • When the diode or cell is reversed, preventing current flow.

P-N Junction

• Formed by P-type and N-type materials, where electrons are attracted to the P-type side, diffusing across the depletion region.

Applications of P-N Junction

• Used in diodes and transistors.

Operation of P-N Junction Diode

• A diode conducts current in one direction by allowing electron movement from N-type to P-type material, with current only flowing in a forward-biased state.

Effects of Reverse Bias in a Diode

• In reverse bias, electrons and holes move in opposite directions, widening the depletion region and increasing resistance, preventing current flow.

Structure and Operation of Bipolar Transistors

Structure and Operation of Bipolar Transistors

• A bipolar transistor is made by joining N-type and P-type semiconductor materials, forming either an N-P-N or P-N-P transistor.
• It is a three-terminal device with terminals named Base (B), Emitter (E), and Collector (C).
• The transistor works by allowing a small current in the base region to control a larger current flowing between the emitter and collector, functioning as both an amplifier and a switch.

As a Switch

• The transistor operates in two regions: cutoff and saturation.
  • Cutoff Region: Acts as an open switch with both junctions reverse-biased, offering high resistance. No current flows between the collector and emitter, and the transistor remains off.
  • Saturation Region: Acts as a closed switch with both junctions forward-biased, allowing current to flow through the collector-emitter path and turning the transistor on.

As an Amplifier

• Transistors are commonly used in radios and as low-frequency voltage amplifiers. They can amplify both voltage and current, making them essential in signal amplification.

Light Operated Switch (LOS)

• In a light-operated switch, a light-dependent resistor (LDR) is connected to a potential divider.
  • Daylight: Light reduces the resistance of the LDR, increasing the current flow to the base of the transistor, turning it on, and causing the bulb to light up.

Darkness: The resistance of the LDR increases, reducing the current flow to the base. This prevents the transistor from turning on, and the bulb remains off.