What is a Phototransistor? | The Definitive Guide


The concept of phototransistors has long been known, but the idea for phototransistors was first proposed by William Shockley in 1951, after the discovery of the common bipolar transistor. The phototransistor was on display two years later. Phototransistors are then used in various applications and this is evolving day by day.

Phototransistors are used in various electronic circuits. Widely received from distributors of electronic components at low cost. Semiconductor devices such as phototransistors are used to detect light levels. And changes the flow of current between the emitter and collector terminals depending on the level of light received by it.

In today’s article, we will look at what a phototransistor is, why it is constructed, and what its various mineral uses are. In today’s article, we will look at an overview.

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What is Phototransistor?

What is Phototransistor?

A phototransistor is an electronic switching and current amplification component that relies on exposure to light for operation. When light falls at a junction a reverse current flows which are proportional to the luminance. Phototransistors are widely used to detect light pulses and convert them into digital electrical signals.

This is done by light instead of electric current. Which this has a huge advantage and due to its low cost, phototransistors are used in many applications. Phototransistors are capable of converting light energy into electrical energy. The phototransistor acts in the same way as the photoresistor commonly known as LDR i.e. light-dependent resistor.

Also, this is capable of producing both current and voltage. Whereas a photoresistor is capable of generating a current due to a change in resistance. A phototransistor is a type of transistor whose base terminal is open. The place to send current to the base activates the photon transistor from the striking light. This is because the phototransistor is made up of bipolar semiconductors. And it focuses on the energy that passes through it.

It is activated by light particles and is used in almost all types of electronic devices. Which somehow depends on light. All silicone photosensors respond to the entire visible radiation range as well as infrared. In fact, all diodes, Darlington, transistors, TRIACs, etc. have the same basic radiation frequency response.

The design of the phototransistor is specially adapted for photo application. Phototransistors have a larger base and collector width compared to a simple ordinary transistor and are made using diffusion or ion implants.

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Phototransistor Construction:

Phototransistor Construction

A phototransistor is nothing more than a common bipolar transistor. The construction of a phototransistor is similar to that of a normal transistor. In ancient times geranium and silicon were used in special proportions for their manufacture. A small hole is made on the surface of the collector-base junction to place the lens in it. The lens focuses the light on the surface.

Nowadays transistors are made of materials such as gallium and arsenides from high light effective materials. In this, the ammeter-base junction is kept forward bias while the collector-base junction is on the opposite bias.

When no light of any kind falls on the surface of the transistor, a small reverse saturation current is induced on the transistor. A few minority charge carriers induce a reverse saturation current. The majority of charge conductors are produced when light energy falls at the collector-base junction. Which adds the current to the inverse saturation current. The graph below shows the intensity of an increase in current with the intensity of light.

Phototransistors are widely used in electronic devices. Smoke detectors, CD players, infrared receivers, lasers, etc. for sensing lights.

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How Does a Phototransistor Work?

Any single common transistor consists of three terminals. There is an emitter, a base, and a collector. In this, the collector terminal is positively related to the emitter terminal. And BE junction is reverse biased.

A phototransistor is activated when light collides with a base terminal. And light triggers the phototransistor. So that the current flows in the emitter or collector along with the configuration of the whole-electron pair. When the current increases, it becomes concentrated and converts into voltages.

Usually, the phototransistor does not include a base connection. The base terminal is disconnected as the light is used to supply current to the phototransistor.

Types of Phototransistors:

The classification of phototransistors is as follows:

Sr. No. Types of Phototransistor
#1. BJT Phototransistor
#2. FET Phototransistor

#1. BJT Phototransistor:

Due to the lack of light, the BJT Phototransistor allows the leakage of the collector as well as the 100 NA emitter otherwise low. Once this transistor comes in contact with the beam it does up to 50 mA. This separates it from the photodiode which does not allow more current.

#2. FET Phototransistor:

This type of transistor has two terminals. Which is connected from the inside through its collector and emitter. Otherwise, source and drain inside the FET. The base terminal of the transistor responds to light and controls the current flowing between the terminals.

Phototransistor Circuit:

The function of a phototransistor is similar to that of a normal transistor. Where the base current is multiplied to give the current to the collector, except in the phototransistor, the base current is controlled by the amount of visible or infrared light. This transistor needs only two pins.

Phototransistor Circuit

Nothing is connected to the Vout in a simple circuit as shown above. The base current controlled by the amount of light determines the collector current. The current passes through the resistor. So the voltage at Vout will speed up and down depending on the amount of light. We can connect to the op-amp or directly to the input of the microcontroller to speed up the signal.

The occurrence of the output of the phototransistor depends on the wavelength of the light. These devices respond to light over a wide range of wavelengths, visible from near UV and in the nearest IR part of the spectrum. The output of a phototransistor for the illumination level of a given light source is defined by the area of ​​the open collector-base junction and the DC current gain of the transistor.

There are various configurations of phototransistors such as optoisolator, optical switch, retro sensor, etc. The optoisolator is like a transformer. In which the output is electrically separated from the input. The object is detected when it enters the gap of the optical switch and blocks the path of light between the emitter and the detector. The retro sensor produces light and detects the presence of an object and then seeks its reflection from the object to be felt.

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Difference Between Photodiode and Phototransistor:

The main differences between photodiode and phototransistor are as follows:

PhotoDiode Phototransistor
This is less sensitive This is more sensitive
The output reaction of the photodiode is fast. The output response of the phototransistor is slow
This only generates currents It produces both voltage and current
It is more reactive to incident light It is less reactive to incident light
The photodiode has a low dark current Phototransistors have a high dark current
In this, both biases are used as forward and reverse Only forward biasing is used
The linear response range of photodiodes is very wide The linear response range of a phototransistor is very low
The photodiode allows a lower current compared to a phototransistor Phototransistors allow higher currents compared to photodiodes
The photodiode is used for a battery-powered device that uses less energy The phototransistor is used as a solid-state switch, not as a photodiode.
This is used to detect UV or IR rays and measure light in solar energy production. It is used in compact disc players, lasers, smoke detectors, invisible light receivers, etc.

Characteristics of a Phototransistor:

The characteristics of a phototransistor are as follows:

  • Moderately fast response time.
  • Available with benefits ranging from 100 to 1500.
  • Visible and near-IR photodetection at a low cost.
  • The electrical characteristics were similar to those of a signal transistor.
  • Available in large packages including epoxy-coated, transfer-molded, and surface mounting technology.

Advantages of Phototransistor:

Some of the important benefits of phototransistors are as follows:

  • A phototransistor produces more current than a photodiode.
  • The phototransistor is very fast and is able to provide almost instant output.
  • Phototransistor generates voltage Photoreceptors cannot generate a voltage.
  • Phototransistors are relatively inexpensive, simple, and small enough to fit many of them on an integrated computer chip.

Disadvantages of Phototransistor:

The disadvantages of phototransistors are as follows:

  • Phototransistors made of silicon cannot handle voltages above 1000 volts.
  • Phototransistors are even more sensitive to the spikes of excess electricity as well as to electromagnetic radiation.
  • It provides low-frequency feedback.
  • Electric surges are more intense in phototransistors than photodiodes.
  • Phototransistors are affected by the difference in electromagnetic energy.

Phototransistor Applications:

The application of phototransistors is as follows:

  • Security systems.
  • IR detectors photo.
  • Computer logic circuitry.
  • Punch-card Readers.
  • Level Indicators.
  • Lighting Control.
  • Alarm Systems.
  • Electric controls.
  • Calculation systems.
  • Encoders to measure speed and direction.

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My name is Tulip, and I am a contributor to electricalgang.com! As a contributor, I write articles on various topics related to electrical engineering, including product reviews, how-to guides, and tips and tricks.

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