Phototransistor

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A photodiode is a semiconductor diode that functions as a photodetector. Photodiodes are packaged with either a window or optical fiber connection, in order to let in the light to the sensitive part of the device. They may also be used without a window to detect vacuum UV or X-rays.

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 p-n junction. The phototransistor works like a photodiode, but with a much higher responsivity for light, because the electrons that are generated by photons in the base-collector junction are injected into the base, and this current is then amplified by the transistor operation. However, a phototransistor has a slower response time than a photodiode.

Quick Reference Guide Most Photodiodes will look like the picture to the right, that is, similar to a Light Emitting Diode. They will have two leads, or wires, coming from the bottom. The shorter end of the two is the cathode, while the longer end is the anode. See below for a schematic drawing of the anode and cathode side. Current will pass from the anode to the cathode, basically following the arrow.

Principle of operation A photodiode is a p-n junction or p-i-n structure. When a photon of sufficient energy strikes the diode, it excites an electron thereby creating a mobile electron and a positively charged electron hole. 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, producing a photocurrent.

Photodiodes can be used under either zero bias (electrical engineering) (photovoltaic mode) or p-n junction#Reverse-bias (photoconductive mode). In zero bias, light falling on the diode causes a current across the device, leading to forward bias which in turn induces "dark current" in the opposite direction to the photocurrent. This is called the photovoltaic effect, and is the basis for solar cells — in fact, a solar cell is just a large number of big photodiodes.

Reverse bias induces only little current (known as saturation or back current) along its direction. But a more important effect of reverse bias is widening of the depletion layer (therefore expanding the reaction volume) and strengthening the photocurrent. Circuits based on this effect are more sensitive to light than ones based on the photovoltaic effect and also tend to have lower capacitance, which improves the speed of their time response. On the other hand, the photovoltaic mode tends to exhibit less electronic noise.

Avalanche photodiodes have a similar structure, 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.

Materials 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:

{] || 190–1100|- wrtgrhrth| Germanium ] || 800–2600|-| Lead sulfide ||

Photodiode - Wikipedia, the free encyclopedia
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.

phototransistor - definition of phototransistor by the Free Online ...
pho·to·tran·sis·tor (f t-tr n-z s t r) n. A transistor having highly photosensitive electrical characteristics. phototransistor (f t-tr n-z s t r) A transistor that ...

phototransistor definition of phototransistor in the Free Online ...
A transistor that uses light rather than electricity to cause an electrical current to flow from one side to the other. It is used in a variety of sensors that detect the presence ...

Phototransistor
The Phototransistor . With Permission, Bell Laboratories RECORD May 1950 Reprinted from SMEC 'Vintage Electrics vol#2 1990 . The new photo-electric device, shown here about nine ...

Phototransistors
Here, light striking the base replaces what would ordinarily be voltage applied to the base-- so, a phototransistor amplifies variations in the light striking it

phototransistor - What does PT stand for? Acronyms and abbreviations ...
Acronym Definition; PT: Part: PT: Point: PT: Portugal (Internet top level domain) PT: Port: PT: Patient: PT: Part Time: PT: Physical Therapy: PT: Pacific Time: PT: Physical ...

phototransistor - Definition at Your Dictionary
Converse of object. use: Detection of the laser can be done using any suitable phototransistor but the best results will be produced using a Modulated Laser Receiver.

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Phototransistor, Photo transistor :: Radio-Electronics.Com
A summary or tutorial giving information about the basics of the Photo Transistor or Phototransistor as well as the Photodarlington and PhotoFet.

phototransistor
Mozilla/4.5 [en] (Win98; I) [Netscape] ... The photo-transistor is a photo-conductive device. It has the same construction as a normal transistor plus a window and lens which ...