Light Dependant Resistors

Notes on light dependant resistors for GCSE double award Science, Electricity module

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Light Dependant Resistors
The light dependent resistor, LDR, is known by many names including the
photoresistor, photo resistor, photoconductor, photoconductive cell, or
simply the photocell. These devices have been seen in early forms since
the nineteenth century when photoconductivity in selenium was
discovered by Smith in 1873. Since then many variants of
photoconductive devices have been made.
Other light dependent resistors, or photo resistors have been made using
materials including cadmium sulphide, lead sulphide, and the more
commonly used semiconductor materials including germanium, silicon and
gallium arsenide.
The photo resistor, or light dependent resistor, LDR, finds many uses as a low cost photo
sensitive element and was used for many years in photographic light meters as well as in
other applications such as flame, smoke and burglar detectors, card readers and lighting
controls for street lamps.
Basic Structure
Although there are many ways in which light dependent resistors, or photo resistors can
be manufactured, there are naturally a few more common methods that are seen.
Essentially the LDR or photoresisitor consists of a resistive material sensitive to light that
is exposed to light. The photo resistive element comprises section of the material with
contacts at either end. Although many of the materials used for light dependent resistors
are semiconductors, when used as a photo resistor, they are used only as a resistive
element and there are no pn junctions. Accordingly the device is purely passive.
A typical structure for a light dependent or photo resistor uses an active semiconductor
layer that is deposited on an insulating substrate. The semiconductor is normally lightly
doped to enable it to have the required level of conductivity. Contacts are then placed
either side of the exposed area.
In many instances the area between the contacts is in the form of a zigzag, or
interdigital pattern. This maximises the exposed area and by keeping the distance
between the contacts small it enhances the gain.
It is also possible to use a polycrystalline semiconductor that is deposited onto a
substrate such as ceramic. This makes for a very low cost light dependent resistor.
In order to ensure that the resistance of the light dependent area of the device is the
major component of the resistance, all other spurious resistances must be minimised. A
major contributor could be the resistance between the contact and the semiconductor. To
reduce this component of resistance, the region around the metal contact is heavily
doped to increase its conductivity.

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