This section provides overview, applications, and principles of phototransistors. Also, please take a look at the list of 16 phototransistor manufacturers and their company rankings.
A phototransistor is a semiconductor device for detecting light. Its structure is a combination of a photodiode and a transistor. Since they are available in various shapes depending on the package, selecting the appropriate one for the application is necessary.
When combined with a photodiode and a transistor, a phototransistor can amplify the photocurrent generated when the photodiode receives light up to about 1000 times. Therefore, phototransistors are more sensitive than photodiodes.
For this reason, phototransistors are sometimes used to compensate for the low sensitivity of photodiodes and PIN diodes. This is because phototransistors are more sensitive than photodiodes and have the characteristic of hFE times, that of photodiodes (hFE: DC amplification factor).
In fact, the sensitivity of a phototransistor is several hundred times greater than that of a photodiode, and if even higher sensitivity is required, the use of a Darlington-connected phototransistor can provide several hundred times x greater sensitivity. This makes it possible to detect the brightness of several Lux.
CDS is a photoresistor. It is also called a CDS cell or photoconductive cell.
The resistance of a CDS decreases inversely proportional to the illuminance it receives. In other words, the resistance increases when the illuminance is low, and when the illuminance is high, the resistance decreases.
The advantages of CDS are that its minute sensitivity characteristics are close to those of human vision, its structure is simple, its sensitivity is high, and its price is low.
CDS is used as a detector in various instruments. For example, they are used in illuminance meters, exposure meters for cameras, and brightness detectors for automatic flashing.
However, cadmium sulfide, the main material used as an element in CDS, is an environmentally harmful substance. For this reason, the use of CDS has been gradually decreasing in recent years.
On the other hand, phototransistors provide an output current proportional to illumination. Another advantage of phototransistors is their high sensitivity due to their structure that combines a photodiode and a transistor.
Phototransistors are widely used as light-receiving sensors. In particular, since they have a peak sensitivity of around 800 nm, they are generally used to receive infrared light.
Specific examples of phototransistor applications include light intensity measurement, infrared remote control receivers, photoelectric sensor receivers, and optical communications. For example, they are often used in combination with infrared LEDs in remote controls for TVs and air conditioners.
Phototransistors are sometimes used as sensors in automatic doors. Furthermore, they are sometimes used as switches driven by light since they generate current when light is detected.
A phototransistor is a semiconductor device with an NPN structure. This NPN structure allows the phototransistor to generate a larger output signal than a photodiode.
The NPN structure of the phototransistor specifically amplifies the output of the photodiode with a transistor. When light equivalent to the energy gap of the semiconductor enters the transistor, electrons in the valence band are excited to the conduction band.
This causes migration to the N layer, and holes move to the P layer. This transfer from the N layer to the P layer causes a forward bias at the junction and current flow.
Transistors used in phototransistors are characterized by the fact that they do not have a base electrode. However, the photocurrent generated by light reception becomes the base current, and this base current is amplified at the collector.
The amplification of the base current is hFE (transistor amplification factor) times other transistors as large as that. However, as a characteristic of phototransistors, even if the hFE times the base current is the same, relatively large phototransistors tend to be used.
The photosensitive part of a phototransistor is a collector-base junction. Because of the collector-base junction, the base-emitter junction is forward-biased when light enters the phototransistor.
The emitter current is then increased, and the current is converged to the collector, just as in a normal transistor. In other words, the base current is controlled by the intensity of the light.
This means that changes in the base current are amplified, and the collector current can be controlled. However, it should also be noted that the current is constantly leaking at the collector-base junction, and this leakage current is also amplified.
In other words, the phototransistor is in a state where a weak current is flowing even in a completely dark environment. This weak current that flows even in a dark environment is called a dark current.
The dark current that is generated by the phototransistor is internal noise as a light sensor. However, there is a way to suppress this internal noise.
The dark current has the characteristic of increasing when the temperature is high and conversely decreasing when the temperature is low. Therefore, this characteristic can suppress internal noise by cooling the device.
In addition, the photocurrent between the collector and base is slow due to carrier diffusion. In addition, note the characteristic that the response speed is slower than that of photodiodes and PIN diodes due to the presence of collector capacitance.
*Including some distributors, etc.
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