1. Silicon Controlled Rectifier (SCR) / Thyristor

The SCR is a four-layer (PNPN or NPNP), three-terminal (anode, cathode, gate) solid-state current-controlling device. It is a trade name for a type of thyristor.

• Function: Primarily used for controlling high voltage and power, making it suitable for medium- and high-power AC operations like motor control.
• Operation: An SCR conducts current like a diode but only after a gate pulse is applied.
• Modes of Operation:
  1. Forward Blocking (OFF State): The anode is positive, the cathode is negative, and the gate has zero voltage. Junctions J1 and J3 are forward-biased, but J2 is reverse-biased, blocking current flow.
  2. Conducting (ON State): Triggered by either increasing the anode-cathode voltage above the avalanche breakdown value or by applying a positive pulse to the gate. Once conducting, the gate voltage is no longer needed.
  3. Reverse Blocking: Compensates for the forward voltage drop.

1. TRIAC (Triode for Alternating Current)

A TRIAC is a three-terminal semiconductor device that can control current flow in two directions.

• Function: Unlike the unidirectional SCR, the TRIAC is bi-directional, making it ideal for AC switching applications. It can control current flow during both halves of an AC cycle.
• Structure: Can be visualized as two thyristors connected back-to-back or a DIAC with an added gate terminal.
• Applications: Commonly used in light dimmers, motor controls, and speed controls for small electric fans.
• Limitation: TRIACs exhibit non-symmetrical switching, which can cause electromagnetic interference, making them unsuitable for very high-power applications.

1. Bipolar Junction Transistor (BJT)

A BJT is a transistor whose operation relies on two types of charge carriers: holes and electrons. It can function as a switch, amplifier, or oscillator.

• Structure: Comprises two back-to-back P-N junctions, forming three regions: emitter, base, and collector.
• Key Parameters & Equations:
  • Emitter, Base, and Collector Currents: I_E = I_B + I_C
  • Transport Factor (α): The ratio of collector current to emitter current. α = I_C / I_E. It is a product of emitter efficiency (γE), base transport factor (αT), and depletion layer recombination factor (δr).
  • Current Gain (β): The ratio of collector current to base current, representing the BJT’s ability to amplify current. β = I_C / I_B = α / (1 – α).

1. Insulated Gate Bipolar Transistor (IGBT)

The IGBT is a three-terminal semiconductor device that combines the simple gate-drive characteristics of a MOSFET with the high-current and low-saturation-voltage capabilities of a BJT.

• Function: Used as a high-efficiency electronic switch in modern appliances.
• Characteristics: Fast switching, high efficiency, and low ON-state resistance.
• Operation: It is a unidirectional device, allowing current to flow from the collector to the emitter. A small positive voltage on the gate terminal turns the device ON.
• Applications: Widely used in medium- to ultra-high-power applications, including variable frequency drives (VFDs), electric cars, converters, and inverters.

1. Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

A MOSFET is a transistor used for amplifying or switching electronic signals. It has four terminals: source (S), drain (D), gate (G), and body (B).

• Structure: Features a thin layer of silicon dioxide that isolates the gate, giving it an extremely high input resistance.
• Operation: A voltage applied to the gate controls the flow of current between the source and drain.
• Forms:
  1. Depletion State: The device is normally ON (conducts with zero gate-source voltage) and requires a specific gate-source voltage to turn OFF.
  2. Enhancement State: The device is normally OFF and requires a gate-source voltage greater than the source voltage to turn ON.