Light dependent resistor 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.

Luiggi Escalante

CI 18.878.611

CRF

Fuente:
http://www.radio-electronics.com/info/data/resistor/ldr/light_dependent_resistor.php

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