What is a Photoconductive cell?

Photoconductive cells are light-sensitive resistors in which resistance is inversely proportionated to light intensity when illuminated. These devices consist of a thin single-crystal or polycrystalline film of composite semiconductor materials. Cadmium sulfide (CdS) is used to make the most commercially available photoconductive particles which are sensitive to light in the visible spectrum. Other less commonly used substances respond to infrared light, but not to the visible spectrum. Such substances are lead sulfide (PbS), lead selenide (PbSe), and lead telluride (PbTe), and so on.

A photoconductive cell is a two-terminal semiconductor device. The terminal resistance of such cells varies with the intensity of the incident light. Therefore it is often referred to as a photo-resistive device.

The most commonly used photoconductive materials are cadmium sulfide (CdS) and cadmium selenide (CdSe). Both materials respond slowly to changes in light intensity. The maximum spectral response times of CdS units are approximately 100 ms and 10 ms for CdSe cells. Their temperature sensitivity is another important difference between the two materials. The resistance of the cadmium selenide cell changes greatly with the change in ambient temperature, while the cadmium sulfide resistance is relatively constant. The spectral response of the cadmium sulfide cell is closely matched to the human eye. Therefore the cell is often used in applications such as street light control or automatic iris control for cameras, which is a human focus factor.

Ceramic substrate important element of a photoconductive cell is a layer of photoconductive material. The metal electrodes connect the device to the circuit and moisture-resistant enclosure.

Photoconductivity

Photoconductivity is an electrical phenomenon where due to the absorption of electromagnetic radiation a material becomes more conductive. Such radiation includes visible light, ultraviolet light, infrared light, gamma radiation, and so on.

In material such as a semiconductor when light is absorbed, there is an increase in the number of free electrons and holes, increasing electrical conductivity. To produce excitation, the light that approached the semiconductor should have enough energy to accelerate the electrons from the bandgap and to excite the impurities in the bandgap. When the bias load resistor and voltage are used in linear series with the semiconductor. When the current is changed by the change in electrical conductivity of the material, the voltage drop across the load resistors can be measured.

Some materials exhibit a decrease in photoconductivity when exposed to light. A metastable reduction in photoconductivity can be observed, a prime example is hydrogenated amorphous silicon (a-Si: H). Other substances reported to exhibit negative photoconductivity include molybdenum disulfide, graphene, indium arsenide nanowires, ornate carbon nanotubes, and metal nanoparticles.

Features of photoconductive cells

Light-dependent resistors (LDRs) are often referred to as CdS photoconductive cells (CdS cells). They operate within the same general spectral range as the human eye and are widely used in applications that require this type of spectral response

Photoconductive cells operate by absorbing light energy and in the semiconductor material. They release electrons from their valence bonds. At room temperature, the number of free charges in the semiconductor is relatively limited. The addition of light-free electrons increases the conductivity which reduces the resistance of cells. The embedding of the conductor path in the zig-zag pattern in the semiconductor substrate increases the resistance levels. In addition to increasing the dark resistance, it also reduces the current and changes the output current to one milliampere (mA) per lumen.

Characteristics of photoconductive cells

The photosensitive material is arranged as long and zigzagged at the disc-shaped base with protective shoulders. For extra protection, a glass or plastic cover may be included. Both ends are brought to the connecting pin under the base.

The luminous properties of a typical photoconductive cell, from which it is clear that when the cell is not illuminated, its resistance exceeds 100 kilo ohms. This resistance is called dark resistance. When the cell is bright, the resistance drops to a few ohms. Note that the criteria on the brightness characteristic are logarithmic to cover the widest range of possible resistance and brightness. Cell sensitivity can be expressed in terms of cell current for a given voltage and a given level of brightness

The main drawback of photoconductive particles is that the temperature variation causes a significant variation in the resistance to a certain light intensity, which in turn causes a significant variation in resistance. Therefore, such a cell is not suitable for analog applications.

The photoconductive cell is used for relay control. When the cell is illuminated, the relay current is maximum and resistance is low. When the cell is dark, its high resistance reduces the current to a very low level to activate the relay. Resistance R is added when the resistance of the cell is low to limit the relay current to the desired level. Photoconductive cells are used to turn on and off transistors as shown in the figure. When the cell shown in the picture is dark, the base of the transistor is higher than its emitter level and the device turns on. When the cell is bright, the lower resistance of the cell in series with R is less than the transistor base voltage below its emitter level. Therefore, the device is turned off.

Context and Applications

A photoconductive material is added as a part of a circuit. It acts as a resistor depending on the light intensity. In this case, the material is called a photoresistor (also known as a light-based resistor or photoconductor). The most common application of photoresistors is photodetectors. Those devices measure light intensity. Photoresistors are one of the types of photodetectors. Other types include photodiodes and phototransistors and also they are the most common. Some photodetector applications that frequently use photoresistors are clock radios, infrared detectors, street lights, and low dimensional photo-sensor devices. Photoconductive cells are generally small in size and are less expensive. Their ease of use makes them popular in many applications.

Some of them are:

• Automatically turn street lights on and off according to daylight level
• In point-of-sale and inventory bar code reading electronic components
• In security devices such as motion-sensing cameras and lights
• In the alarm system

This topic is important for professional exams in both undergraduate and postgraduate courses like:

• Bachelors of Technology in Electrical Engineering
• Masters of Technology in Electrical Engineering

Practice Problems

Q1. In which of the following the photoconductive cell is used?

Answer: Option b

Explanation: Photoconductive cells are light-sensitive resistors that are used in low-frequency devices. and thus cannot be used in devices with high and medium frequency.

Q2. Which of the following materials is used in photoconductive cells?

Answer: Option c

Explanation: Selenium is used in photoconductive cells. These cells use selenium to capture and convert photons into electrical charges.

Q3. Solar cell is an example of which of the following?

Answer: Option a

Explanation: Solar cells are an example of photovoltaic cells. These cells excite by solar energy. It is a device that converts light energy into electricity directly through a photovoltaic effect.

Q4. Due to which of the following mentioned effects, the increased conductivity of an illuminated semiconductor junction?

Answer: Option c

Explanation: Photoconductive effect means the increased conductivity of an illuminated semiconductor junction. This is the event of a change in semiconductor material conductivity caused by light changes.

Q5. Which of the following devices consists of a film of compound semiconductor substances?