Describe the working of a light-activated SCR (LASCR).
1 Answer
A Light-Activated SCR (LASCR), also known as a "light-triggered SCR" or "LASCR," is a semiconductor device that combines the characteristics of a Silicon-Controlled Rectifier (SCR) with the ability to be triggered or activated by light. The SCR is a four-layer solid-state device with three junctions, and it is a type of thyristor, which means it can switch large currents on and off with a small control current.
The working principle of a LASCR is based on the photoconductivity effect in certain semiconductor materials. Here's how it works:
Structure: A LASCR has a similar structure to a standard SCR, with four semiconductor layers (NPNP) and three junctions (J1, J2, and J3). The middle P-layer is called the "gate" and is the region where light activation occurs. The other layers are called the anode (A1), the cathode (A2), and the anode gate (G).
Photoconductivity: The material used for the gate region is usually photosensitive, meaning it exhibits changes in conductivity when exposed to light. In the absence of light, the gate's conductivity is low.
Off state: Initially, the LASCR is in the off state (non-conducting state). No current flows between the anode and cathode because the junctions J1 and J3 are reverse-biased, and the junction J2 is forward-biased but not conducting.
Light activation: When light is incident on the gate region (J2), it generates electron-hole pairs in the photosensitive material. This increases the gate's conductivity significantly, making it more like a short circuit.
Turn-on process: When a voltage pulse is applied between the anode and cathode (positive at anode, negative at cathode) and the gate is illuminated, the increased conductivity in the gate allows current to flow from the anode to the cathode. This triggers the thyristor action, and the LASCR enters the on state (conducting state).
On state: Once the LASCR is turned on, it latches into the conducting state, even if the light is removed. The gate continues to maintain its conductivity as long as the anode current exceeds the holding current of the device.
Turn-off process: To turn off the LASCR, the anode current must be reduced below the holding current, or the anode-cathode voltage must be reversed to forward bias the J1 and J3 junctions. This turns off the LASCR, and it returns to the off state.
LASCRs find applications in various fields, such as light-sensitive switches, optical isolators, and triggering circuits in optoelectronic devices. Their ability to be controlled by light makes them useful in situations where a light signal is more convenient than direct electrical control.
The working principle of a LASCR is based on the photoconductivity effect in certain semiconductor materials. Here's how it works:
Structure: A LASCR has a similar structure to a standard SCR, with four semiconductor layers (NPNP) and three junctions (J1, J2, and J3). The middle P-layer is called the "gate" and is the region where light activation occurs. The other layers are called the anode (A1), the cathode (A2), and the anode gate (G).
Photoconductivity: The material used for the gate region is usually photosensitive, meaning it exhibits changes in conductivity when exposed to light. In the absence of light, the gate's conductivity is low.
Off state: Initially, the LASCR is in the off state (non-conducting state). No current flows between the anode and cathode because the junctions J1 and J3 are reverse-biased, and the junction J2 is forward-biased but not conducting.
Light activation: When light is incident on the gate region (J2), it generates electron-hole pairs in the photosensitive material. This increases the gate's conductivity significantly, making it more like a short circuit.
Turn-on process: When a voltage pulse is applied between the anode and cathode (positive at anode, negative at cathode) and the gate is illuminated, the increased conductivity in the gate allows current to flow from the anode to the cathode. This triggers the thyristor action, and the LASCR enters the on state (conducting state).
On state: Once the LASCR is turned on, it latches into the conducting state, even if the light is removed. The gate continues to maintain its conductivity as long as the anode current exceeds the holding current of the device.
Turn-off process: To turn off the LASCR, the anode current must be reduced below the holding current, or the anode-cathode voltage must be reversed to forward bias the J1 and J3 junctions. This turns off the LASCR, and it returns to the off state.
LASCRs find applications in various fields, such as light-sensitive switches, optical isolators, and triggering circuits in optoelectronic devices. Their ability to be controlled by light makes them useful in situations where a light signal is more convenient than direct electrical control.