Why in news?
Scientists from Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), autonomous institute under the Department of Science and Technology, have fabricated an economical and energy-efficient wafer-scale photodetector (thin slice-based) using gold – silicon interface, for security applications. It could help detect weak scattered light as an indication of unwanted activity.
- Photodetectors are the heart of any optoelectronic circuit that can detect light and are employed for a wide variety of applications ranging from controlling automatic lighting in supermarkets to detecting radiation from outer galaxy as well as security-related applications.
- However, the material cost and the intricate fabrication processes involved in realizing high-performance detectors make them unaffordable for day to day applications.
- The scientists have fabricated gold (Au)– silicon (n-Si) interface, which showed high sensitivity towards light demonstrating the photodetection action.
- The Au–Si interface was brought about by galvanic deposition, a technique for electroplating of metals, wherein water-based solutions (electrolytes) are used, which contain the metals to be deposited as ions.
- In addition, a nanostructured Au film also was deposited on top of p-type silicide (having an excess of positive charges), which acts as a charge collector.
- The detector exhibits a rapid response of 40microseconds and can detect low light intensities. The device covers a broad spectral range from Ultraviolet to Infrared.
- Besides, it shows excellent uniformity throughout the entire active area with less than 5% variation in response.
What is a Photodetector?
- Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation.
- A photo detector has a p–n junction that converts light photons into current.
- The absorbed photons make electron–hole pairs in the depletion region.
- Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.
- Other optical devices similar to photo detectors are solar cells which also absorb light and turn it into energy.
- A similar but different optical device is the LED which is basically the inverse of a photodiode, instead of converting light to a voltage or current, it converts a voltage or current to light.
Applications of Photo Detectors
- Photo detectors are used in various different applications such as radiation detection, smoke detection, flame detection and to switch on relays for street lighting.
- The circuits that use photodiodes use either normally closed or normally open contacts depending on the desired operation.
- In a smoke detector circuit the photo diode is attached to a relay switch, this switch is normally closed and attached to the fire alarm. When the photo diode conducts it picks up the relay switch, this causes the normally closed switch to open preventing the alarm from activating. When the photo diode fails to conduct, the normally closed contact activates the alarm.
- Photo diodes are also used in modern oil burning furnaces as a safety feature. The photo diode is comprised of lead sulphide and is used to detect the flame from the boiler, in the event that the flame goes out or fails to occur the photo diode opens the circuit, cutting power to the motor and step up transformer.
- Another commonly used application is street lights. The photo diode in the circuit uses switch-on relays to turn on the street lights when the diode fails to conduct and turns the lights off with when sufficient light is present.
- Photodiodes are also used with lasers to form security system. When the light projected by a laser to the photodiode is broken a security alarm is tripped.