This guide introduces the basic building blocks of modern electronics: semiconductors, diodes, and transistors. Focus on understanding how each component works and its main applications.

1. Semiconductors: The Foundation

• Conductors: Materials that allow electricity to flow easily (e.g., copper). They have many free electrons.

• Insulators: Materials that do not allow electricity to flow (e.g., plastic). They have no free electrons.

• Semiconductors: Materials with conductivity between that of a conductor and an insulator (e.g., Silicon (Si), Germanium (Ge)). Their conductivity can be controlled.

  • Intrinsic Semiconductor: A pure semiconductor. It conducts electricity via two types of charge carriers:

    • Electrons (negative charge)

    • Holes (the absence of an electron, which acts like a positive charge).

  • Extrinsic Semiconductor: A semiconductor that has been made more conductive by adding impurities. This process is called doping.

    • n-type: Doped with a Group V element (like Phosphorus). Has an excess of negative electrons. Electrons are the majority carriers.

    • p-type: Doped with a Group III element (like Boron). Has an excess of positive holes. Holes are the majority carriers.

2. The Diode: A One-Way Street for Current

• Structure: A diode is a component formed by joining a p-type and an n-type semiconductor. This junction is called a p-n junction.

• Function: It allows electric current to flow in only one direction.

• Biasing (Connecting a voltage):

  • Forward Bias: The positive terminal of a battery is connected to the p-side and the negative terminal to the n-side. The diode conducts, and current flows.

  • Reverse Bias: The positive terminal is connected to the n-side and the negative to the p-side. The diode does not conduct, and current is blocked.

• Diode Symbol: The triangle points in the direction of conventional current flow. Anode (+) ---|>|--- Cathode (-)

3. Key Applications of Diodes

Rectification: Converting AC to DC

Rectification is the process of converting Alternating Current (AC), which changes direction, into Direct Current (DC), which flows in one direction.

• Half-Wave Rectification:

  • Uses one diode.

  • Allows only half of the AC wave to pass through, blocking the other half. The output is a pulsating DC.

• Full-Wave Rectification (Bridge Rectifier):

  • Uses a bridge of four diodes.

  • It flips the negative half of the AC wave, so the entire wave is converted into a pulsating DC. This is much more efficient.

• Smoothing:

  • A capacitor is connected in parallel with the output of the rectifier.

  • It charges up during the peaks of the pulsating DC and discharges during the troughs, "smoothing" out the ripples to create a steadier DC voltage, more like that from a battery.

Other Important Diodes

• LED (Light Emitting Diode): Emits light when it is forward biased. They are highly energy-efficient and used for lighting and indicators.

• Solar Cell: A special p-n junction that generates a voltage when light shines on it. It converts light energy directly into electrical energy.

4. The Transistor: Amplifier and Switch

• Structure: A transistor is a three-layer semiconductor device. It can be npn or pnp.

• Terminals: It has three terminals:

  • Base (B): The control terminal.

  • Collector (C): The main input terminal for current.

  • Emitter (E): The main output terminal for current.

• Symbol: The arrow is always on the emitter and shows the direction of conventional current. A simple way to remember: NPN = "Not Pointing In".

Two Main Functions of a Transistor:

1. As an Amplifier: A small current or voltage applied to the base controls a much larger current flowing from the collector to the emitter. This is how weak signals (like from a microphone) are made stronger.

2. As a Switch: A small voltage at the base can turn the main circuit (collector-emitter) ON or OFF. For a silicon transistor, a voltage of about 0.7 V at the base is enough to turn it ON. Below this, it's OFF. This is the fundamental principle behind all digital logic and computers.

Exam Tips & Tricks

1. Diode Direction: The most important thing to remember about a diode is that it's a one-way valve for current. This is the key to understanding rectification.

2. Rectifier Circuits: Be prepared to draw and recognize the circuits for half-wave (1 diode) and full-wave (4 diodes in a bridge) rectification. Don't forget to include the smoothing capacitor when asked.

3. Transistor = Control: Think of the transistor's base as a "tap" or a "gate". A small effort at the base controls a large flow from collector to emitter.

4. Know the Two Jobs: For your exam, the two key jobs of a transistor are amplifier and switch. Be ready to state these two functions clearly.

The Magic Number 0.7 V: Remember that for a transistor to act as a closed switch (turn ON), the base-emitter voltage must be approximately 0.7 V.