Figure 5.1 Phototransistors
Phototransistors is a semiconductor device that converts light into current. 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.The
phototransistor works like a photodiode, but with a much higher sensitivity 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.
Figure 5.2 Phototransistors scheme
Based on the scheme at the photo current, the electrons are amplified by the transistor and appear as a current in the collector/emitter circuit. The base is internally left open and is at the focus of a plastic lens.The actual operation of a phototransistor depends on the biasing arrangement and light frequency. For instance, if a PN junction is forward biased, the increased current through the junctions due to incident light will be relatively insignificant. On the other hand, if the same junction is reverse biased, the increase in current flow will be considerable and is a function of the light intensity. Therefore, reverse bias is the normal mode of operation.
Figure5.3 Phototransistor operation
Now, if the PN junction is the collector-base diode of a bipolar transistor, the light-induced current effectively replaces the base current. The physical base lead of the transistor can be left as an open terminal, or it can be used to bias up to a steady state level. It is the nature of transistors that a change in base current can cause a significant change (increase) in collor current. Thus, light stimulation causes a change in base current, which in turn causes a bigger increase in collector current and, considering the current gain (hfe), a rather large increase at that.
Phototransistors have several important advantages that separate them from other optical sensors. For example, phototransistors produce a higher current than photodiodes while also being able to produce a voltage, something that photoresistors cannot do. Phototransistors are very fast and are capable of providing nearly instantaneous output. Phototransistors are relatively inexpensive, simple, and small enough to fit several of them onto a single integrated computer chip. While phototransistors can be advantageous, they also have several disadvantages. For example, phototransistors made of silicon are not capable of handling voltages over 1,000 Volts. Phototransistors also do not allow electrons to move as freely as other devices do, such as electron tubes. Phototransistors are also more vulnerable to surges and spikes of electricity as well as electromagnetic energy.
Sources :
Petruzella, Frank. (1995). Industrial Electronics. Mc Graw Hill.1-2
http://theonlinetutorials.com/what-is-a-phototransistor-and-how-it-works.html
http://en.wikipedia.org/wiki/Photodiode
