Photodiode#Other modes of operation
A picture or photo transistor is a bipolar pnp - or npn - layer sequence, the pn junction of the base-collector barrier layer is an external light source available. It is thus similar to a photodiode with an attached amplifier transistor. The transistor is realized by means of the reverse current generated by the light incident through this barrier.
The first photo-transistor was developed by John Northrup Shive at Bell Laboratories in 1948. First applications of phototransistors were in 1950 when the routing of telephone calls in automatic telephone exchanges, where phototransistors for optically reading the control information were used from punch cards.
Operation and design
The light-sensitive photo diode circuitry is parallel to the collector-base connections of the transistor. Light incident through the housing directly or through a clear lens at the otherwise closed housing can flow through the internal photoelectric effect, a small photo-current, which is amplified in the transistor to the current amplification factor to the collector current. The current amplification factor is in the range according to the type of the order of 100 to 1000, whereby the collector current is larger by the same factor as the photoelectric current of a photodiode.
Photo transistors usually have only two terminals brought out - the collector and the emitter. However, there are also versions with accessible base terminal - eg for controlling the operating point. If the base is not connected, it takes a relatively long time until the base -emitter zone is free of charge carriers. Therefore, inter alia, due the slow switch-off of the phototransistor. In addition, the Miller effect reduces the reaction rate during rapid changes in brightness. A high cutoff frequency required photodiode and transistor operation must be separated.
Areas of application
Phototransistors are much more sensitive than the photodiodes, since they simultaneously act as an amplifier. They are also used in detecting or transfer via light, for example light barriers, twilight switches, optocouplers. In the reception of remote control units, however, photo- diodes are used as photo-transistors are too slow for this application. In these, as in light barriers and optocouplers, is not carried out with visible light, but also infrared.
Receiving wavelength
The wavelength of maximum sensitivity of a silicon photo- transistor is about 850 nm (near infrared) and falls towards shorter wavelengths (visible light, ultraviolet ) from. The sensitivity of the type BP103 (metal housing, see pictures) is, for example at 420 nm for 10 % of the value at 850 nm
The reception wavelength range is limited to larger wavelengths through the energy of the band edge of silicon at about 1100 nm and ends there ( Rotgrenze ).
Designs
The most common terms of production volume phototransistors are - like light-emitting diodes - only encapsulated in transparent plastic. Usually npn transistors produced with the planar epitaxial processes are used. The chip is bonded to the collector terminal has only one leg and realized by wire bonding emitter terminal. The base terminal is not normally passed out. The housing material may be formed as a lens. The wired components can vertically or sideways "look".
Phototransistors for higher quality applications are encapsulated in metal enclosures with windows or (plastic) lens. They often have a base lead-out port.
Phototransistors in optocouplers are encapsulated and have opaque to the transmitting diode through a transparent outside often mirrored plastic body.