COLLEGE OF ENGINEERING AND TECHNOLOGY
EC8252-ELECTRONIC DEVICES
5.9 PHOTO TRANSISTOR
Phototransistors are either tri-terminal (emitter, base and collector) or bi-terminal (emitter
and collector) semiconductor devices which have a light-sensitive base region. Although
all transistors exhibit light-sensitive nature, these are specially designed and optimized
for photo applications. These are made of diffusion or ion-implantation and have much
larger collector and base regions in comparison with the ordinary transistors. These
devices can be either homojunction structured or heterojunction structured, as shown by
Figure respectively.
Fig:5.9.1 Symbol of Photo Transistor
In the case of homojunction phototransistors, the entire device will be made of a single
material-type; either silicon or germanium. However to increase their efficiency, the
phototransistors can be made of non-identical materials (Group III-V materials like
GaAs) on either side of the pn junction leading to heterojunction devices. Nevertheless,
homojunction devices are more often used in comparison with the hetero junction devices
as they are economical.
Fig:5.9.2 Photo Transistor (a) Homo Junction (b) Hetero Junction
The circuit symbol for npn phototransistors is shown by Figure 2 which is nothing but a
transistor (with or without base lead) with two arrows pointing towards the base
