Page 1014 - Basic Electrical Engineering

electrical signal, the liquid crystal molecules orient in a definite crystal
pattern and no twisting of liquid occurs and the cell appears dark. Liquid
crystal cell can also be made to appear bright when the background is dark.

Seven segment displays of numerical numbers, alphabets, and others are
often made from liquid crystal displays (LCDs). Due to the orientation of the

molecules of the liquid crystal, a digit or an alphabet can be shown bright
against a dark background as shown in Fig. 14.51 (b).

14.13.5 Photodiodes

When a p–n junction is reverse biased, only a very small current called the
leakage current or saturation current flows. This very small amount of current

is due to the thermally generated holes and electrons which defuse across the
p–n junction as minority charge carriers (electrons and holes are called

charge carriers). If the junction temperature increases, more and more charge
carriers are generated and the reverse current is increased. Thus, increase in

junction temperature increases the reverse current.
Similarly, when light falls on the p–n junction, due to the incident light
energy, electron–hole pairs are generated and the electrons and holes freely

move across the junction increasing the reverse current. The magnitude of
reverse current will depend on the intensity of light falling on the p–n

junction. Thus, by changing the illumination level, current flowing through
the device can be varied as shown in Fig. 14.52. Diodes which are designed
to be light (i.e., photo) sensitive are called photodiodes. Photodiodes operate

under the reverse-biased condition and are designed to be light sensitive.
There is minority charge carrier current flow when the p–n junction is

illuminated.
It is to be noted that increasing the reverse voltage will not change the

reverse current. That is why the reverse current is called reverse saturation
current. But increase in the illumination level of the p–n junction will change

the reverse current.
The resistance of the device at a different illumination level will be
different. A photodiode can work like an LDR when the light falling on its