Department of Physics, Swami Shraddhanand College (University of Delhi) Delhi, INDIA Brijender Dahiya and Vinod Prasad Department of Physics, Swami Shraddhanand College (University of Delhi) Delhi, INDIA Layout Introduction Principle of Operation Photovoltaic Mode Photoconductive Mode Material/ Construction Applications Conclusion INTRODUCTION A photodiode is a type of photo- detector capable of converting light into either current or voltage, depending upon the mode of operation. The common, traditional solar cell used to generate electric solar power is a large area photodiode. Silicon photodiodes are semiconductor devices responsive to high energy particles and photons. Photodiodes operate by absorption of photons or charged particles and generate a flow of current in an external circuit, proportional to the incident power. Photodiodes are similar to regular semiconductor diodes except that they may be either exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fiber connection to allow light to reach the sensitive part of the device. Symbol of a Photodiode Many diodes designed for use specifically as a photodiode use a PIN junction rather than a p-n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias. PIN Junction or PIN Diode A PIN diode is a diode with a wide, lightly doped 'near' intrinsic semiconductor region between a p-type semiconductor and an ntype semiconductor region. The p-type and n-type regions are typically heavily doped because they are used for ohmic contacts. Other Importance of PIN Junction The wide intrinsic region is in contrast to an ordinary PN diode. The wide intrinsic region makes the PIN diode an inferior rectifier (one typical function of a diode), but it makes the PIN diode suitable for attenuators, fast switches, photo- detectors, and high voltage power electronics applications Principle of Operation There are three major parts of the principle of operation Photovoltaic Mode Photoconductive Mode Other Modes of Operation Principle of Mechanism A photodiode is a p-n junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a free electron and hole. (photoelectric effect). If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in field of the depletion region. (Holes anode, Electrons cathode) The photocurrent is the sum of both the dark current (without light) and the light current, so the dark current must be minimized to enhance the sensitivity of the device. Photovoltaic Mode When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. This mode exploits the photovoltaic effect, which is the basis for solar cells. Photoconductive mode In this mode the diode is often reverse biased, dramatically reducing the response time at the expense of increased noise. This increases the depletion layer, decreasing the junction's capacitance resulting in faster response times. For a given spectral distribution, the photocurrent is linearly proportional to the illuminance. The Two modes Other modes of operation Avalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. This allows each photo-generated carrier to be multiplied by avalanche breakdown, resulting in internal gain within the photodiode, which increases the effective responsivity of the device. A phototransistor is in essence a bipolar transistor encased in a transparent case so that light can reach the base-collector junction. The electrons that are generated by photons in the base-collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β. If the emitter is left unconnected, the phototransistor becomes a photodiode. Phototransistors also have significantly longer response times. Material/ Construction The material used to make a photodiode is critical to defining its properties, because only photons with sufficient energy to excite electrons across the material's bandgap will produce significant photocurrents. Materials commonly used to produce photodiodes include Material λ Range (nm) Material λ Range (nm) Silicon 190-1700 Germanium 400-1700 InGaAs 800-2600 Pb(II)Sulfide <1000-3500 Because of their greater bandgap, Si-based photodiodes generate less noise than Ge-based photodiodes. Applications Photodiodes have wide range of applications Consumer Electronics Auto- circuits Light measurements Medicals Optical Communication Scientific Instruments Consumer Electronics CD Player Smoke Detector Receivers for IR remote control Devices Camera light meters Street Light Switches etc. Auto-circuits Combined with a light source photodiode is used for auto circuits such as : Various types of Safety alarms Counters Slotted optical Switch Auto electric switches for low power consumption, etc. animation Light measurements Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors. Medicals They are also widely used in various medical applications, such as detectors for computed tomography (coupled with scintillators), instruments to analyze samples (immunoassay), and pulse oximeters. Optical Communication PIN diodes are much faster and more sensitive than p-n junction diodes, and hence are often used for optical communications and in lighting regulation. Scientific Instruments P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, avalanche photodiodes, intensified charge-coupled devices or photomultiplier tubes are used for applications such as astronomy, spectroscopy, night vision equipment and laser rangefinding. Conclusion Silicon photodiodes are semiconductor devices responsive to high energy particles and photons. Photodiodes can be used to detect minute quantities of light and can be calibrated for extremely accurate measurements. Silicon photodiodes are utilized in such diverse applications as spectroscopy, photography, analytical instrumentation, optical position sensors, beam alignment, surface characterization, laser range finders, optical communications, daily life consumer devices and medical imaging instruments. Hence making them the Multipurpose Device.