What is a Phototransistor : Circuit Diagram & Its Working

The phototransistor concept was known for the past many years. The first idea was proposed by William Shockley in the year 1951, after the discovery of a normal bipolar transistor. After two years, a phototransistor was demonstrated. After that, it was used in different applications, and day by day its development was continued. Phototransistors are extensively obtainable with low cost from the distributors of electronic components to use in different electronic circuits. A semiconductor device like a phototransistor is used to detect the light levels and changes the flow of current among emitter & collector terminals based on the light level it gets. This article discusses an overview of phototransistors.

What is Phototransistor?

A Phototransistor is an electronic switching and current amplification component which relies on exposure to light to operate. When light falls on the junction, reverse current flows which are proportional to the luminance. Phototransistors are used extensively to detect light pulses and convert them into digital electrical signals. These are operated by light rather than electric current. Providing a large amount of gain, low cost and these phototransistors might be used in numerous applications.

Phototransistor Symbol
Phototransistor Symbol

It is capable of converting light energy into electric energy. Phototransistors work in a similar way to photoresistors commonly known as LDR (light dependent resistor) but are able to produce both current and voltage while photoresistors are only capable of producing current due to change in resistance.

Phototransistors are transistors with the base terminal exposed. Instead of sending current into the base, the photons from striking light activate the transistor. This is because a phototransistor is made of a bipolar semiconductor and focuses on the energy that is passed through it. These are activated by light particles and are used in virtually all electronic devices that depend on light in some way. All silicon photosensors (phototransistors) respond to the entire visible radiation range as well as to infrared. In fact, all diodes, transistors, Darlington’s, TRIACs, etc. have the same basic radiation frequency response.

The structure of the phototransistor is specifically optimized for photo applications. Compared to a normal transistor, a phototransistor has a larger base and collector width and is made using diffusion or ion implantation.


A phototransistor is nothing but an ordinary bi-polar transistor in which the base region is exposed to illumination. It is available in both the P-N-P and N-P-N types having different configurations like common emitter, common collector, and common base but generally, common emitter configuration is used. It can also work while the base is made open. Compared to the conventional transistor it has more base and collector areas.


Ancient phototransistors used single semiconductor materials like silicon and germanium but now a day’s modern components use materials like gallium and arsenide for high-efficiency levels. The base is the lead responsible for activating the transistor. It is the gate controller device for the larger electrical supply. The collector is the positive lead and the larger electrical supply. The emitter is the negative lead and the outlet for the larger electrical supply.

Photo Transistor Construction
Photo Transistor Construction

With no light falling on the device there will be a small current flow due to thermally generated hole-electron pairs and the output voltage from the circuit will be slightly less than the supply value due to the voltage drop across the load resistor R. With light falling on the collector-base junction the current flow increases. With the base connection open circuit, the collector-base current must flow in the base-emitter circuit, and hence the current flowing is amplified by normal transistor action.

The collector-base junction is very sensitive to light. Its working condition depends upon the intensity of light. The base current from the incident photons is amplified by the gain of the transistor, resulting in current gains that range from hundreds to several thousand. A phototransistor is 50 to 100 times more sensitive than a photodiode with a lower level of noise.

How Does a Phototransistor Work?

A normal transistor includes an emitter, base, and collector terminals. The collector terminal is biased positively relating to the emitter terminal & the BE junction is reverse biased.

A phototransistor activates once the light strikes the base terminal & the light triggers the phototransistor by allowing the configuration of hole-electron pairs as well as the current flow across the emitter or collector. When the current increases, then it is concentrated as well as changed into voltage.
Generally, a phototransistor doesn’t include a base connection. The base terminal is disconnected as the light is used to allow the flow of current to supply throughout the phototransistor.

Types of Phototransistor

Phototransistors are classified into two types namely BJT and FET.

BJT Phototransistor

In the deficiency of light, BJT phototransistor allows leakage among collectors as well as an emitter of 100 nA otherwise low. Once this transistor is exposed to the beam, it performs upto 50mA. This distinguishes it from photodiode which cannot allow much current.

FET Phototransistor

This kind of phototransistor includes two terminals that connect inside through its collector & emitter otherwise source & drain within FET. The transistor’s base terminal reacts to light & controls the current flow among the terminals.

Phototransistor Circuit

A phototransistor works just like a normal transistor, where the base current is multiplied to give the collector current, except that in a phototransistor, the base current is controlled by the amount of visible or infrared light where the device only needs 2 pins.

Phototransistor Circuit
Phototransistor Circuit Diagram

In the simple circuit, assuming that nothing is connected to Vout, the base current controlled by the amount of light will determine the collector current, which is the current going through the resistor. Therefore, the voltage at Vout will move high and low based on the amount of light. We can connect this to an op-amp to boost the signal or directly to an input of a microcontroller.

The output of a phototransistor is dependent upon the wavelength of the incident light. These devices respond to light over a broad range of wavelengths from the near UV, through the visible, and into the near IR part of the spectrum. For a given light source illumination level, the output of a phototransistor is defined by the area of the exposed collector-base junction and the dc current gain of the transistor

Phototransistors are available in different configurations like optoisolator, optical switch, retro sensor. Optoisolator is similar to a transformer in that the output is electrically isolated from the input. An object is detected when it enters the gap of the optical switch and blocks the light path between the emitter and detector. The retro sensor detects the presence of an object by generating light and then looking for its reflectance off of the object to be sensed.


The operation range of a phototransistor mainly depends on the applied light intensity because its operating range is dependent on the input of the base. The current of the base terminal from the incident photons can be amplified through the transistor’s gain, which results in a current gain that ranges from 100 to 1000. A phototransistor is more sensitive as compared to a photodiode through a less noise level.
Extra amplification can be supplied through a photodarlington-type transistor.

This is a phototransistor including an emitter output that is connected to the base terminal of the next bipolar transistor. It gives high sensitivity within the levels of low light as it provides a current gain that is equivalent to the two transistors. The gain of the two stages can offer net gains higher than 100,000A. A photodarlington transistor includes less response as compared to a normal phototransistor.

Modes of Operation

In phototransistor circuits, the basic modes of operation include two like active & switch where the commonly used mode of operation is switch type. It explains a non-linear response toward the light; once there is no light then there is no flow of current into the transistor. Current starts to supply like exposure toward light increases. The switch-mode works in an ON/OFF system. Active mode is also called a linear that reacts in such a way that, it is proportional toward the light stimulus.

Performance Specifications

The selection of Phototransistor can be done depending on different parameters as well as specifications like the following.

  • Collector Current (IC)
  • Base Current (Iλ)
  • Peak Wavelength
  • Collector-to-Emitter Breakdown Voltage (VCE)
  • Collect-emitter breakdown voltage (VBRCEO)
  • Emitter-collector breakdown voltage (VBRECO)
  • Dark current (ID)
  • Power dissipation (PD or Ptot)
  • Rise time (tR)
  • Fall time (tF)

Design Parameters

The selected materials, as well as composition, play an essential role in the sensitivity of this type of transistor. The gain level of Homo-structure or single material devices ranges from 50 to several hundred. These are normal phototransistors that are frequently designed with silicon. The heterostructure devices or several material configuration devices may include gain levels up to 10k but they are less common dues to high production costs.

  • The electromagnetic wavelength range of different materials include the following,
  • For Silicon (Si) material, the electromagnetic wavelength range is 190 to 1100 nm
  • For Germanium (Ge) material, the electromagnetic wavelength range is 400 to 1700 nm
  • For Indium gallium arsenide (InGaAs) material, the electromagnetic wavelength range is 800 to 2600 nm
  • For Lead sulfide material, the electromagnetic wavelength range is <1000 to 3500
  • For the proper function of a phototransistor, mounting technology plays a key role.

The SMT or surface mount technology uses components to a PCB (printed circuit board) by connecting the component terminals through soldering otherwise to the top face of the board. Usually, the printed circuit board pad can be coated using a paste such as a solder & flux formulation. High temperatures usually from an infrared oven will dissolve the paste to solder the terminals of component toward the PCB pads.

THT or through-hole technology is a commonly utilized mounting style. The arrangement of components can be done by placing component terminals using holes within the PCB & these components can be soldered in the opposite face of the PCB. The features of phototransistors mainly include a cutoff filter, used to block observable light. The light detection in others can be improved through an anti-reflective coating. Devices including a round dome lens in place of a flat lens are also obtainable.

Photodiode Vs Phototransistor

The difference between photodiode and phototransistor includes the following.



The photodiode is a PN-junction diode, used to generate electric current once a photon of light strikes on their surface. The phototransistor is used to change the energy of the light into an electrical energy
It is less sensitive It is more sensitive
The output response of photodiode is fast The output response of the phototransistor is low
It produces current It produces voltage and current
It is used in solar power generation, detecting UV otherwise IR rays & also for light measuring, etc. It is used in compact disc players, smoke detectors, lasers, invisible light receivers, etc.
It is more reactive to incident lights It is less reactive
The photodiode has a less dark current Phototransistor has high dark current
In this, both the biasing is used like forward and reverse In this, forward biasing is used
The linear response range of photodiode is much wider The linear response range of phototransistor is much lower
Photodiode allows low current as compared to a phototransistor Phototransistor allows high current as compared to the photodiode
The photodiode is used for battery-powered devices that use less power. The phototransistor is used as a solid-state switch, not like a photodiode.


The characteristics of a phototransistor include the following.

  • Low-cost visible and near-IR photodetection.
  • Available with gains from 100 to over 1500.
  • Moderately fast response times.
  • Available in a wide range of packages including epoxy-coated, transfer-molded, and surface mounting technology.
  • Electrical characteristics were similar to that of signal transistors.

Advantages of Phototransistor

Phototransistors have several important advantages that separate them from another optical sensor some of them are mentioned below

  • Phototransistors produce a higher current than photodiodes.
  • Phototransistors are relatively inexpensive, simple, and small enough to fit several of them onto a single integrated computer chip.
  • Phototransistors are very fast and are capable of providing nearly instantaneous output.
  • Phototransistors produce a voltage, that photo-resistors cannot do so.

Disadvantages of Phototransistor

  • Phototransistors that are made of silicon are not capable of handling voltages over 1,000 Volts.
  • Phototransistors are also more vulnerable to surges and spikes of electricity as well as electromagnetic energy.
  • Phototransistors also do not allow electrons to move as freely as other devices do, such as electron tubes.

Applications of Phototransistors

The Areas of application for the Phototransistor include:

  • Punch-card readers.
  • Security systems
  • Encoders – measure speed and direction
  • IR detectors photo
  • electric controls
  • Computer logic circuitry.
  • Relays
  • Lighting control (highways etc)
  • Level indication
  • Counting systems

Thus, this is all about an overview of a phototransistor. From the above information finally, we can conclude that phototransistors are widely used in different electronic devices for detecting light such as infrared receivers, smoke detectors, lasers, CD players, etc. Here is a question for you, what is the difference between phototransistor and photodetector?

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