Light dependent resistor or photo resistor
- the light dependent resistor, LDR, or photo resistor
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 zig zag, 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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