Light-Activated Thyristor or LASCR or LAT or Light-Activated Silicon-Controlled Rectifiers or Light-Activated SCR or Photothyristor or PhotoSCR

A light-activated thyristor or LASCR or LAT or Light-Activated Silicon-Controlled Rectifiers or light-activated SCR, also known as a photo thyristor or Photo SCR, is a specialized semiconductor device that combines the principles of a thyristor and a photodetector. Unlike conventional thyristors, which rely on electrical signals to trigger switching actions, a light-activated thyristor utilizes light as a control input. When exposed to specific wavelengths of light, it undergoes a transition from a high-impedance state to a low-impedance state, allowing current to flow freely through it.

Note : Light Activated Thyristors

• The circuit symbol and V-I characteristics of light-activated thyristor, also called LA SCR.
• Light-activated SCR or (LASCR) or a Photo SCR is just an ordinary SCR except that it can also be light-triggered.
• LASCR is a semiconductor device that turns ON when it is exposed to light.
• The LASCR is a type of thyristor which is triggered by photons present in the light rays.
• It is a three-terminal device, consists of cathode, anode and gate terminal.
• The gate terminal is used when the electrical triggering is supplied to the LASCR.
• LA SCRs are turned on by throwing a pulse of light on the silicon wafer of thyristor. The pulse of appropriate wavelength is guided by optical fibres to the special sensitive area of the wafer.
• If the intensity of light exceeds a certain value, excess electron-hole pairs are generated due to rediation and forward-biased thyristor gets turned on.
• LASCR works on the principle of photoconduction that is conduction due to photon striking the semiconductor surface.
• The advantage of using light triggering for the thyristor is the prevention of electrical noise disturbances. Thus, LASCR is considered to be one of the best devices.
• The primary use of light-fired thyristors is in high-voltage high-current applications, static reactive-power compensation etc. 
• A light-fired thyristor has complete electrical isolation between the light-triggering source and the high-voltage anode-cathode circuit. 
• LASCR is basically a thyristor; it is made up of semiconductor material.
• The light rays falling on the device are focused at one place to intensify it.
• The more the intensity of light, the more will be the current through the LASCR.
• The primary function of a LASCR is to control electric power and current by acting as a switch.
• It is used as a rectifier because it can switch rapidly from a state of conducting current to a state of non-conduction.
• LASCR is used in a wide range of electric circuits.
• Light-activated thyristors are available up to 6 kV and 3.5 kA, with on-state voltage drop of about 2 V and with light-triggering requirements of 5 mW.

What is a Light-Activated Thyristor?

Photothyristors are actuated by light. The advantage of photothyristors is their insensitivity to electrical signals, which can beget defective operation in electrically noisy surroundings. A light- touched off thyristor( LTT) has an optically sensitive region in its gate, into which electromagnetic radiation( generally infrared) is coupled by an optic fiber. Since no electronic boards need to be handed at the eventuality of the thyristor in order to spark it, light- touched off thyristors can be an advantage in high- voltage operations similar as HVDC. Light- touched off thyristors are available with in- erectedover-voltage( VBO) protection, which triggers the thyristor when the forward voltage across it becomes too high; they've also been made with in- erected forward recovery protection, but not commercially. Despite the simplification they can bring to the electronics of an HVDC stopcock, light- touched off thyristors may still bear some simple monitoring electronics and are only available from a many manufacturers. 

Two common photothyristors include the light- actuated SCR( LASCR) and the light- actuated TRIAC. A LASCR acts as a switch that turns on when exposed to light. Following light exposure, when light is absent, if the power isn't removed and the oppositeness of the cathode and anode haven't yet reversed, the LASCR is still in the" on" state. A light- actuated TRIAC resembles a LASCR, except that it's designed for interspersing currents. 

Construction of light-activated thyristor

The light-activated thyristor is made up of silicon material, and the glass lens in the light-activated thyristor is used to focus the light from the external source on the semiconductor material. The silicon pellet is used in the bottom of the device, and the light intensity dislodges electrons in the semiconductor crystal and contributes to conduction.

Working Principle and Operation of light activated thyristor 

The working principle of the light-activated thyristor involves the interaction between incident light and the semiconductor material. The light-sensitive gate structure is typically made up of materials like silicon, gallium arsenide, or indium phosphide, which exhibit photoconductivity.

When light strikes the surface of the light-activated thyristor, photons with sufficient energy create electron-hole pairs within the semiconductor material. These generated carriers contribute to the photocurrent produced by the photodiode. The optically controlled switch, connected to the photodiode, responds to this photocurrent and controls the conductivity of the thyristor.

The optically controlled switch can be implemented using various mechanisms, such as a metal-insulator-semiconductor (MIS) structure, a heterojunction, or a p-i-n junction. These structures employ the photo-generated carriers to modify the electrical properties of the switch, thereby controlling the thyristor's conduction characteristics. The switch's conductivity can be modulated by the light's intensity, enabling precise control over the thyristor's on/off states.

To ensure efficient light-to-current conversion, the light-activated thyristor is often designed with anti-reflection coatings and light-trapping structures. These enhancements maximize the device's sensitivity to incident light, enabling optimal performance in a wide range of lighting conditions.

VI characteristics of light-activated thyristor

The VI (voltage-current) characteristics of a light-activated thyristor (LAT) are similar to a conventional thyristor, but with an additional light-activation feature.

In the forward-biased condition, the LAT behaves like a normal thyristor. Initially, the device remains in the blocking state until the forward voltage exceeds a certain threshold known as the forward breakover voltage (VBO). Once the VBO is exceeded, the LAT turns on and enters the forward conduction state. The forward voltage drop across the LAT is relatively low, typically a few volts, similar to a regular thyristor.

The unique characteristic of a light-activated thyristor is the ability to trigger and control its conduction state using light. When the LAT is exposed to a light source, typically in the form of a pulse or modulated light, it can be activated or deactivated depending on the light intensity. This light activation allows for optoelectronic control of the thyristor's conduction state.

In the reverse-biased condition, the LAT behaves similarly to a regular thyristor. It remains in the blocking state until the reverse voltage exceeds the reverse breakdown voltage (VRBM). Once VRBM is surpassed, the LAT can experience a reverse breakdown and enter the reverse conduction state. The reverse voltage drop across the LAT is typically higher than in the forward conduction state.

The VI characteristics of a light-activated thyristor exhibit similar behavior to a conventional thyristor, but with the added capability of light-triggered activation and control of its conduction state.

The VI characteristics of a light-activated thyristor involve its behavior in the forward conduction mode, holding current requirements, reverse blocking capability, and the additional capability to trigger the thyristor using light.

Note:- The specific VI characteristics of a light-activated thyristor can vary depending on the device's design, construction, and operating conditions. These characteristics may be provided in the device's datasheet or technical specifications for accurate information.

Applications of the Light Activated Thyristor

• Power Electronics: The light-activated thyristor finds applications in power electronics, particularly in areas such as motor control, lighting systems, and energy conversion. Its light sensitivity allows for fine-grained control over power distribution, resulting in improved efficiency and reduced energy wastage.
• Light sensors: The light-activated thyristor can serve as a photodetector, converting light signals into electrical signals for applications in light sensing, spectroscopy, and imaging systems.
• Renewable Energy: With the increasing focus on renewable energy sources, the light-activated thyristor can play a vital role in improving the efficiency and reliability of solar energy systems. By enabling precise control of solar panel output and facilitating advanced power conversion techniques, this technology can enhance the integration of solar power into the grid.
• Optoelectronic: The ability to control and amplify light signals makes the light-activated thyristor an ideal candidate for optoelectronic applications. It can be utilized in optical communication systems, optical switches, and optoelectronic integrated circuits, enabling faster data transfer rates and improved performance.
• Biomedical Devices: In the field of biomedicine, the light-activated thyristor opens up new avenues for optical stimulation of neural activity, facilitating advancements in neuroprosthetics, deep brain stimulation, and optogenetics. Its precise control of light signals can help researchers better understand the complexities of the human brain and develop targeted therapies.
• Low Power Applications: The Light activated SCR is generally used for the application which requires low power to operate. This is because the power generated by SCR is low in magnitude.
• Motor Control: The Light Activated SCR finds applications in the working of Motor Control.
• Computer Applications: Components used in the computer system also require LASCR for meeting power requirements.