A Phototransistor is a kind of transistor that is sensitive to light. It comprises of a photodiode & a transistor which is used to detect light and convert it into an electrical signal. The phototransistor was invented by John Northrup Shive in 1950 at Bell Telephone Laboratories. Its operation is based on the concept of the photoelectric effect. That is, when light is incident on a surface, the light energy is converted into electrical energy. Phototransistors are typically made up of a semiconductor material such as silicon or germanium, along with other materials that are used to form the various layers and junctions within the device.
Figure 1: Phototransistor symbol
The symbol of a phototransistor is shown in figure 1. It is similar to that of an ordinary transistor, the only difference is the two arrows pointing toward the base region. These arrows indicate light falling on the base of a phototransistor.
Construction of Phototransistor
Figure 2: Structure of Phototransistor
A phototransistor is an ordinary bipolar transistor that contains three regions: an emitter, collector, and base. The base and collector regions are made using diffusion or ion implantation. These regions have a larger width since it is the light-sensitive area. That is, when light falls on these regions, more current will be generated. Figure 2 shows the structure of a phototransistor. Light is mainly allowed to fall on the collector-base junction. There is no external connection to the base region of a phototransistor. The collector-base (CB) junction is biased in the forward direction while the base-emitter (BE) junction is biased in the reverse direction. Light is detected in the base region, and current flowing through the base lead turns on the transistor.
Working of Phototransistor
The overall input of a phototransistor is the light energy falling on the base region. When light is incident on the base region, electron-hole pairs are generated. They are collected by the underlying semiconductor material. These collected charges then modulate or amplify the current flowing through the transistor, allowing the phototransistor to detect and convert light into an electrical signal. Like an ordinary transistor, the base current is multiplied to get the collector current in phototransistor.
For a conventional transistor with its open terminal base circuited, the collector base leakage current will act as the base current ICBO.
When the base current IB=0, the transistor is open circuited. Then the collector current is given by,
Therefore, it is clear from the above equations that the collector current increases with the base leakage current or collector base region.
Figure 3: Circuit diagram of a Phototransistor
The circuit diagram of a phototransistor is shown in figure 3. If there is no incident light, a small current will flow due to the thermally generated electron-hole pairs. Also, the output voltage will be slightly lower than the input voltage due to the voltage drop across the load resistor R. When light falls on the collector-base junction, the current flow increases.
The relation between magnitude of base current and light intensity is shown in figure 4.
Figure 4: Graph between base current and illumination intensity
The magnitude of the base current increases with illumination level of the incident light.
Characteristics of Phototransistor
Operating Modes: Phototransistors have three operating modes such as cut-off, active, and saturation just like ordinary transistors. That is, the phototransistors can be used for either switching applications with cut-off and saturation mode or for amplification using active mode operation.
Dark Current: There is a dark current or small reverse saturation current through phototransistors even if there is no light incident on them. Dark current increases with an increase in temperature. If the voltage applied across the collector-emitter junction increases above the breakdown voltage, permanent damage occurs to the phototransistors.
Spectral Response: The output of the phototransistor depends upon the incident wavelength. From the near UV region of the spectrum to the near IR region, these devices respond to light over a wide range of wavelengths.
Sensitivity: The output of the phototransistor is determined by the area of the collector-base junction and the DC current gain of the transistor. Base photocurrent gets doubled by doubling the size of the base. The photocurrent (IP) thus generated gets amplified by the DC current gain of the transistor. When there is no external base drive current applied:
where IC is the collector current and hFE is the dc current gain.
Figure 5: Voltage versus Current characteristics of a Phototransistor
The V-I characteristic of a phototransistor is represented in figure 5. Here, the x-axis represents the applied voltage at the collector-emitter region of the transistor and y-axis represents the collector current supplies throughout the device in mA. It gives an idea about how the current flow in the collector region changes with the incident light intensity. The intensity of the light increases the current in the collector terminal. The wavelength and light intensity both affect the current in the collector region. From the graph above, we can see that the current increases in intensity when light falls on the base terminal.
Types of Phototransistors
There are mainly two types of phototransistors: Bipolar Junction Transistors (BJT) and Field Effect transistors (FET).
Figure 6: Symbol of Bipolar and Field Effect Phototransistors
A bipolar junction transistor contains three terminals called emitter, collector, and base. A field effect phototransistor includes two terminals called source and drain. The base terminal of this transistor responds to light and regulates the flow of current between the terminals. Figure 6 shows the symbol of a Bipolar Phototransistor and Field Effect Phototransistor.
Configurations of Phototransistor
Figure 7: Symbol of NPN and PNP Phototransistors
Bipolar Junction Transistors are available in two different configurations: NPN and PNP. The main difference between the two is the direction of the current flow. In an NPN phototransistor, the current flows from the collector to the emitter, while in a PNP phototransistor, the current flows from the emitter to the collector. The choice between the two types depends on the specific application and the requirements of the circuit. Figure 7 shows the symbol of NPN and PNP phototransistors. Depending on the terminal which is common between the input and output terminals, they can be of common-emitter configuration or common-collector configuration. These two different configurations are shown in figure 8.
Figure 8: The common-emitter and common-collector configurations of phototransistor
The common-emitter configuration of a phototransistor is a circuit arrangement where the emitter of the phototransistor is connected to a common point (such as ground), the base is biased through a resistor, and the collector is connected to a load resistor, and the supply voltage. In this configuration, the incident light falling on the base region of the phototransistor causes a proportional increase in the base current, which in turn increases the collector current. The collector current flows through the load resistor and produces a voltage drop across it. This voltage drop can be used to drive other circuits or devices. The common-emitter configuration provides high voltage gain, which makes it suitable for applications where a small change in the input current can produce a large change in the output voltage. However, it has a low input impedance and high output impedance, which may limit its use in some applications.
In the common-collector configuration, the collector of the phototransistor is connected to a common point, the emitter is connected to a load resistor, and the base is biased through a resistor. When the base region of a phototransistor is exposed to incident light, it results in an increase in the base current that is directly proportional to the amount of light. As a result, the emitter current also increases in proportion to the base current. The emitter current flows through the load resistor and a voltage drop is produced across it. Other circuits or devices can be driven using this voltage drop. The common-collector configuration is also known as an emitter follower because the output voltage follows the input voltage with a slight voltage drop. It has a high input impedance and low output impedance, which makes it suitable for applications where a large input signal is needed and the output signal is required to drive a low-impedance load. The common-collector configuration has a low voltage gain, which may limit its use in some applications where a small change in the input current needs to produce a large change in the output voltage.
Phototransistor as a current amplifier
A phototransistor's operating range is determined by the intensity of the applied light because the base input determines its operating range. The gain of the transistor produces a current gain that ranges from 100 to 1000, which can be used to amplify the current flowing through the base terminal as a result of the incoming photons. Because the size of the base and collector region is higher, this offers better junction capacitance and they have larger gain.
Advantagesv
Disadvantages
Applications
Phototransistors are commonly used as optical sensors to detect the presence or absence of light in a circuit. They are often used in photointerrupters, which are devices that can detect changes in the amount of light that falls on a phototransistor. Photointerrupters are widely used in robotics, industrial automation, and other applications where precise detection of objects or movement is required.
They are used in remote control systems to detect signals from remote controls, such as those used in TVs, DVD players, and other electronic devices. In this application, the phototransistor is used to detect the infrared light that is emitted by the remote control, allowing the device to be operated from a distance.
Phototransistors are used to measure the intensity of light in an environment. They are often used in photographic light meters, which are used to determine the correct exposure settings for a camera. In this application, the phototransistor is used to measure the amount of light that falls on it and then convert that information into an electrical signal that can be used to determine the optimal exposure settings.
They are widely used in fiber optic communications to detect light signals that are transmitted over long distances. In this application, the phototransistor is used to detect the light signals that are transmitted through the fiber optic cable and then convert those signals into electrical signals that can be processed and amplified by other electronic components.
Phototransistors are used in a variety of automotive systems, such as automatic headlights, rain sensors, and anti-lock braking systems. In these applications, the phototransistor is used to detect changes in light levels or other environmental factors, allowing the system to automatically adjust its settings to provide optimal performance.
They are also used in Light detection and ranging (LIDAR), photo-interrupters, security systems, counting systems, level indication, computer logic circuitry, punch-card readers, a variety of industries, including telecommunications, automotive, and industrial automation, and are essential for many modern technologies.
Our Newsletters keep you up to date with the Photonics Industry.
By signing up for our newsletter you agree to our Terms of Service and acknowledge receipt of our Privacy Policy.
By creating an account, you agree with our Terms of Service and Privacy Policy.
