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 band gap will produce significant photocurrents.
Phototransistors also have slower response times. Phototransistors have a higher responsivity for light so they are not able to detect low levels of light any better than photodiodes. 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 (or hfe). A phototransistor is in essence nothing more than a bipolar transistor that is encased in a transparent case so that light can reach the base-collector junction. Phototransistors also consist of a photodiode with internal gain. 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. The leakage current of a good PIN diode is so low – < 1nAĪvalanche photodiodes have a similar structure to regular photodiodes, but they are operated with much higher reverse bias.
The photocurrent is linearly proportional to the luminance.Īlthough this mode is faster, the photovoltaic mode tends to exhibit less electronic noise. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. This increases the width of the depletion layer, which decreases the junction’s capacitance resulting in faster response times. In this mode, the diode is often reverse biased, dramatically reducing the response time at the expense of increased noise. This mode is responsible for the photovoltaic effect, which is the basis for solar cells-in fact, a solar cell is just an array of large area photodiodes. The diode becomes forward biased and “dark current” begins to flow across the junction in the direction opposite to the photocurrent. When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. 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. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a positively charged electron hole. Photocurrent flows in the opposite direction.Ī photodiode is a PN junction or PIN structure. Under forward bias, conventional current will pass from the anode to the cathode. The shorter end of the two is the cathode, while the longer end is the anode. They have two leads, coming from the bottom. Some photodiodes are similar to a light emitting diode. Photodiode Many diodes designed for use specifically as a photodiode will also use a PIN junction rather than the typical PN junction. Photodiodes are similar to typical semiconductor diodes except that they may be either exposed or packaged with a window to allow light to reach the sensitive part of the device. Photodiode is a type of light detector capable of converting light energy into either current or voltage, depending upon the mode of operation.
0 kommentar(er)
