What is a Photoconductor : Working & Its Applications

Generally, materials are classified into three main categories depending on their capacity to conduct electric current throughout them like semiconductors, conductors, and insulators. So the main disparity within conductivity of these materials creates from the difference within their energy-band structure. When the light falls on the materials, then the conductivity of whose material significantly increases is known as photoconductor material. The increased conductivity is called photoconductivity which is observed mainly in semiconductors. This article discusses an overview of photoconductor or photoconductivity.

What is Photoconductor/Photoconductivity?

A photoconductor or Photoconductivity is an electrical & optical phenomenon where a material turns into conductive more electrically because of the electromagnetic radiation absorption like infrared light, ultraviolet light, visible light, or gamma radiation. Once a semiconductor material absorbs light, then the number of charge carriers & also its electrical conductivity will be increased.

Once a load resistor & a bias voltage are used with the semiconductor in series, then a voltage drop across the load resistor can be measured while the change within the material’s electrical conductivity changes the current flowing throughout the circuit.

The photoconductive materials examples are the conductive polymer polyvinyl carbazole material is used widely in photocopying and lead sulfide material is used in IR detection applications like the Russian Atoll heat-seeking missiles, U.S. Sidewinder & selenium is used in early xerography & television.

Photoconductor Construction & Working Principle

The photoconductor construction is shown below. The photoconductor includes a light-sensitive material that is arranged in a long strip zigzag form across a base that is in a disc shape. The connecting terminals are fixed to the conducting material on every side of the strip. So, the light-sensitive material is a wide strip in between the two conductors and a transparent plastic cover is used for its protection.

The two materials like CdSe (cadmium selenide) & CdS (Cadmium sulfide) are used in the manufacturing of photoconductive cells. These two materials respond quite slowly to changes within light intensity. So, the response time of CdSe is approximately 10 ms, whereas, for CdS, it may be 100 ms.

Another main difference between these two materials is temperature sensitivity because there is a huge change within the resistance of a CdSe cell through changes in ambient temperature; however, the cadmium sulfide resistance remains fairly stable.

As with all other types of devices, care must be taken to make sure that the power dissipation is not extreme. The spectral response of a CdS cell is related to that of the human eye because it quickly responds to visible light.

Photoconductor Working

Once the light ray strikes the surface of a photoconductive material then it provides enough energy to cause electrons in the material to move away from their atoms. Therefore, free charge carriers like holes & electrons can be created within the material, its resistance will be decreased. So this is called the Photoconductive effect.

Photoconductivity in Semiconductors

Photoconductivity is the increased conductivity that is mainly observed within semiconductors. To understand this mechanism in detail, a basic internal semiconductor structure needs to consider. In the semiconductors energy-band diagram, the number of charge carriers like electrons within the conduction band is significantly low as compared to conductors. But, there are also charge carriers like holes within the valence band.

These are known as vacancies which are left by electrons and moved to the conduction band. The following diagram shows charge carriers within a semiconductor. The conduction of the current within a semiconductor takes place by electrons & holes within the valence band. Semiconductors are available in two types like Intrinsic & Extrinsic. Pure semiconductors are intrinsic and when impurities are included to a semiconductor for increasing its conductivity is known as extrinsic. Here, the impurity which is added can either increase the number of electrons or the number of holes.

Once the light rays drop on the semiconducting material, then the electrons in the valence band absorb the photons & jump immediately into the conduction band by leaving holes. So the increased number of electrons & holes within both the bands will increase the material’s conductivity. So the conductivity increase is mainly because of the light dropping on the material, which is known as photoconductivity.

Photoconductive Gain

The photoconductive gain can be defined as the ratio of flow of electron’s rate for each second to the rate of electron-hole pair’s generation in the device. However, the electrons flow rate for each sec is = ∆i/e. Here, the photoconductive gain can also be defined as the ratio of the lifetime of minority charge carriers & the transit time.

Advantages and Disadvantages

The advantages of a photoconductor include the following.

• It mainly depends on the light, so its resistance will be decreased once light drops on it & enhances in the dark.
• These are cheap & available in different sizes & shapes
• They need less power & voltage for their operation.
• These are used to make simple circuits

The disadvantages of a photoconductor include the following.

• These are less responsive as compared to phototransistors or photodiodes.
• When the Cadmium sulfide (CdS) is used in the construction of a photoresistor, then it is dangerous to the atmosphere. Therefore they are accessible simply in preferred countries.
• Once applied voltage increases the maximum voltage then it will cause damage to the photoconductor.
• Its response time is very slow like 10sec or 100 milliseconds.
• It is sensitive to temperature
• These characteristics are nonlinear

What is the use of a Photoconductor?

Photoconductors are used in some practical applications like the following.

Photo-diode

The photodiode is a special kind of diode where a window is formed to let the light of an appropriate frequency drop on the photodiode. Thus, new pairs of electron-hole are formed which take part within conductivity.

Photoresistor

A photoresistor is made with photoconductive materials. Once the light with appropriate frequency drops on the photoresistor, then the pairs of charge carriers are formed which enhances the resistor’s conductivity at the same time, the resistivity will be decreased. Therefore, the photoresistor is responsive to incident light.

• These materials are mainly used to sense infrared radiation within military-based applications from guiding missiles to heat-generating targets.
• Photoconductivity is used in the process of xerography or photocopying, which formerly used selenium however now it relies on photoconductive polymers.
• Some photoconductors are used in street lights, camera light meters, clock radios, nanophotonic systems, infrared detectors & photo-sensors devices with low-dimensional.
• Photoconductors are used for X-Ray image detectors
• These are used for relay control
• These are used to switch on & off transistors.
• These can be used with an op-amp-based Schmitt trigger circuit.
• These are used to control the level of current flow within an LED.

Why selenium is used in photoconductors?

Selenium is a photoconductor, so it has the capacity to change the energy of light into electricity.

Which is the high-performance photoconductor used for thermal imaging?

Mercury Cadmium Telluride is used for thermal imaging even at less temperature & with maximum precision.

What is the photoconductivity response time?

The photoconductivity response time ranges from a few nanoseconds to a few minutes and it is measured from photocurrent decay.

What is a photovoltaic mode?

The photovoltaic mode is also called zero-bias mode because no exterior reverse potential is given to the device but, the minority charge carrier flow will occur once the device is exposed to light.

How is photovoltaic energy used?

Photovoltaic energy is a renewable energy source that utilizes solar radiation to generate electricity. This energy is used to recharge a battery or power different equipment.

How is photoconductivity measured?

To measure the photoconductivity, there are two measurements are used like direct measurements using an electro-meter & indirect measurement with the time-evolution of photorefractive effects.

Photoconductor vs Photodetector

The main difference between photoconductors & photodetectors is; a photoconductor is a device or a material that uses or exhibits photoconductivity whereas a photodetector is a device used for detecting electromagnetic radiation.

Photoconductor vs photodiode

The difference between photoconductor and photodiode includes the following.

Thus, this is all about an overview of a photoconductor or photoconductivity and its working with applications. The characteristics of photoconductor are; quantum efficiency & spectral sensitivity is high, the photoconductive gain is high, response speed is high, and fewer noise materials. Here is a question for you, what is the difference between Photoconductor vs photovoltaic?