Photodiodes and Phototransistors

In most optoelectronic sensor applications with
infrared LEDs a detecting device is also needed. It is a fortunate
coincidence of nature that the spectral sensitivity of photodiodes
and phototransistors made of silicon match perfectly with the
infrared range of the electromagnetic radiation of IREDs around
900 nm. Silicon as a material for photodiodes and phototransistors
has many advantages: it is cheap, the production process is
very sophisticated and optimized and the photodetector can be
integrated into a silicon integrated circuit. In the following the
expression photodetector is valid for both photodiodes and
phototransistors.
The minimum wavelength which can be detected by silicon is around
400 nm (using special blue-enhancement technology also
shorter wavelength is possible). At about 880 nm a maximum of
sensitivity is reached and then sensitivity decreases and is
almost zero at 1100 nm. Hence silicon devices can be used for
detection of the visible range (400 nm to 800 nm) and for the
infrared range of electromagnetic radiation (800 nm to 1100
nm).
For applications where infrared LEDs are used as a light source
there is no need for the detector to be sensitive in the visible
range. In these cases photodetectors with so called “daylight filters”
are used. This is simply a black epoxy mold instead of clear
epoxy mold. It is blocking the radiation in the range of 400 nm
to 800 nm whereas the radiation above 800 nm will pass the
epoxy with almost no loss.
The above mentioned spectral behaviour is valid for both, photodiodes and phototransistors. Electrically, these two types are
quite different. The photo current of phototransistors is quite high compared to the current of photodiodes. The phototransistor is composed by a photodiode and an amplifying transistor. In most cases the amplification factor of phototransistors is between 1000 and 1500 which results in photocurrent of some 100μA or even few mA. On the other hand photodiodes are much faster than phototransistors. The raise time of photodiodes is some ns typically, whereas the raise time of phototransistors is some microseconds.
The photo current of the photodiodes is proportional to the size
of the chip. To get a high photo current in an application the chip
size has to be chosen as big as possible. Quite generally the
photodiode chips are much larger than the size of the phototransistor- dice. As a consequence many photodiode chips do not fit in the same housings as phototransistors and hence much less housings are available for photo diodes.