A thyristor is a four-layer (PNPN) semiconductor device that consists of three series-connected PN junctions and three terminals (anode, cathode, and gate). Like a diode, a thyristor is a unidirectional device, but unlike diodes, it can also function as a switching device. It acts as a switch in high-power circuits and is commonly used in AC-DC conversion applications.

Structure and Operating Principles

A thyristor is made of P-type and N-type semiconductor materials, and its structure is similar to that of two transistors connected in reverse. The connection between the anode and cathode forward biases the first and last junctions (J1 and J3), while the J2 junction is reverse-biased. However, when a triggering signal is applied to the gate terminal, the reverse bias of J2 is broken, and the device switches to the conducting state.

Thyristors operate only in fully ON or fully OFF states, meaning they are always in one of these two conditions. This characteristic makes them unsuitable for analog amplifiers but highly efficient as switching devices.

^{Thyristor Structure}

Differences Between Diodes and Thyristors

1. Diode: A diode is a two-layer semiconductor device that allows current flow in only one direction. It is typically used in a single operation and permits current flow only in the forward direction.
2. Thyristor: A thyristor is a four-layer device that can conduct current in one direction but also switches ON and OFF with triggering. This feature allows a thyristor to function like a diode while also acting as a transistor-like switch.

Operating Modes

A thyristor has three main operating modes, determined by the state of its junctions and the influence of external voltages:

1. Forward Blocking Mode (Off-State): Without a gate signal, when the anode is positive and the cathode is negative, junctions J1 and J3 are forward-biased, while J2 is reverse-biased, and the device does not conduct.
2. Forward Conduction Mode (On-State): When a sufficient positive triggering signal is applied to the gate terminal, the device conducts in the forward direction, allowing a large current flow from the anode to the cathode. In this state, the thyristor can maintain conduction even after the triggering signal is removed.
3. Reverse Blocking Mode: If the device is reverse-biased, only a very small reverse leakage current flows, and the device does not conduct.

^{Operating Structure}

Types of Thyristors

1. Silicon Controlled Rectifier (SCR): A four-layer, three-junction semiconductor device capable of conducting current in one direction. It is used for controlling high currents and is often found in high-frequency switching circuits.
2. Applications: Power control, lighting control, motor control, etc.
3. Gate Turn-Off Thyristor (GTO): This type of thyristor can be turned off with a negative gate current, making it a fully controllable switch.
4. Applications: Inverters, AC drives, induction heaters, etc.

Advantages and Disadvantages

Advantages:

1. High voltage and current handling capacity.
2. High efficiency in power control.
3. Fast switching characteristics.
4. Low conduction losses.

Disadvantages:

1. Limited switching speed: Slower switching compared to transistors.
2. Control challenges: Gate triggering must be precise.
3. Reverse blocking limitations: The device can be damaged under high reverse voltages.
4. Sensitivity to noise and voltage spikes: Electrical noise or voltage spikes can cause false triggering.

Applications

Thyristors are widely used in high-voltage and high-current applications. They are particularly favored in AC and DC motor speed control, phase control circuits, lamp switching systems, and inverters.