Phototransistors are solid-state light detectors with internal gain that are used to provide analog or digital signals. Phototransistors are used in almost all electronic devices that depend on light including smoke detectors, laser-ranging finding devices, and optical remote controls.
They detect visible, ultraviolet and near-infrared light from a variety of sources and are more sensitive than photodiodes. This category includes photodarlingtons.
Composition of Phototransistors
Bipolar transistors are the most commonly used transistors. Phototransistors are typically bipolar NPN devices and are made of three lead components:
- 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.
While ordinary transistors have photosensitive effects when exposed to light, phototransistors are optimized for use with light. Phototransistors have larger base and collector areas than ordinary transistors. They generally have a clear or opaque casing to enhance light exposure.
Most phototransistors are made of a single material although some others may be composed of multiple materials.
- Early phototransistors had a homo-junction structure made of germanium or silicon (see photo below, left).
- Modern phototransistors may be composed of multiple material junctions of type III-V materials like gallium and arsenide (see photo below, right). The physical structure can be optimized to allow higher levels of light exposure by using an offset configuration.
Image Credit: Radio-Electronics.com
Phototransistors are made of semi-conductive materials. Although germanium has more desirable electrical properties, silicon is more commonly used because of its reliability and low cost.
How Phototransistors Work
A typical transistor consists of a collector, emitter, and base sections. The collector is biased positively with respect to the emitter and the base-collector junction is reverse biased. A phototransistor remains inactive until light falls onto the base. Light activates the phototransistor, allowing the formation of hole-electron pairs and the flow of current across the collector or emitter. As the current spreads it is concentrated and converted into voltage.
See example at 1:30. Video Credit: techtrainingonline / CC BY-SA 4.0
A phototransistor usually does not have a base connection (see diagram below). The base is left disconnected because the light is used to enable the current to flow through the phototransistor.
A phototransistor's range of operation is dependent on the intensity of applied light because its operating range is base-input dependent. 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 thousands. A phototransistor is 50 to 100 times more sensitive than a photodiode with a lower level of noise. (A photodiode is a photo-junction device that does not amplify.)
Additional amplification can be provided by a photodarlington transistor. A photodarlington is a phototransistor with an emitter output coupled to the base of a second bipolar transistor. It provides high sensitivity in low light levels because it gives a current gain equal to that of two transistors. The two stages of gain can provide net gains greater than 100,000 amps. A photodarlington has a slower response than an ordinary phototransistor.
Phototransistors can be used in common emitter and common collection circuits.
The common emitter circuit is the most commonly used circuit for a phototransistor. When light is detected, output moves from a high to low voltage state.
The common collector circuit, also known as an emitter follower circuit, produces an output that moves from a low to high state as light is detected.
Modes of Operation
Two basic modes of operation, active mode and switch mode, are used in phototransistor circuits.
- Switch mode is more commonly used. It describes a non-linear response to light; when there is no light no current flows into the transistor. Current begins to flow as exposure to light increases. Switch mode operates in an on/off system.
- Active mode, also known as linear mode, responds in a way that is proportional to the light stimulus.
There are both advantages and disadvantages to choosing a phototransistor for your application.
Advantages of phototransistors include:
- Higher current production than photodiodes
- Production of a voltage, unlike photoresistors
- Works with most visible or near infrared light sources including IREDs, neon bulbs, fluorescent bulbs, incandescent bulbs, lasers, flames, and sunlight
- Fast-acting with nearly instantaneous output
- Relatively inexpensive, simple, and small
Disadvantages of phototransistors include:
- The material used may limit voltage handling capability (silicon cannot handle over 1,000 Volts)
- Electrons do not move as freely as they do in electron tubes
- Vulnerable to electrical surges and electromagnetic energy
Phototransistor selection can be based on a number of parameters and specifications.
Collector current (IC) is a measure of the sensitivity of the phototransistor. It describes the maximum allowable current load in the collector and is measured in milliamps (mA) or amps (A). Current that exceeds this parameter may cause damage to the phototransistor.
Image Credit: EG&G Optoelectronics
Base current (Iλ) is produced when light strikes the collector base PN junction. The base current is directly proportional to the light intensity. If the size of the base region is doubled, the amount of generated base photocurrent is also doubled.
Peak wavelength is the wavelength value at which a phototransistor is most sensitive. It is measured in nanometers (nm). Phototransistors respond to light over a broad range of wavelengths from fluorescent or incandescent light sources. They perform the best when matched with infrared (IR) LED light sources. This is because phototransistors have a peak spectral response in the near IR at about 840 nm.
Collector-to-emitter breakdown voltage (VCE) is the maximum voltage allowed between the collector and emitter. Exceeding the maximum voltage can cause permanent damage to the phototransistor.
- Collect-emitter breakdown voltage (VBRCEO) typically ranges from 20 V to 50 V.
- Emitter-collector breakdown voltage (VBRECO) typically ranges from 4 V to 6 V.
Dark current (ID) is the small amount of current can flow through a phototransistor even when it is not exposed to light. Dark current is thermally generated collector-emitter leakage current. It prevents a device from ever being considered completely "off." Dark current increases with the temperature and is measured in milliamps (mA).
Power dissipation (PD or Ptot) describes power dissipation of the phototransistor measured in watts or milliwatts (mW). Actual dissipation isdeterminedby multiplying the voltage across the transistor and the current through the collector.Itis normally quoted for an ambient external temperature of 25°C.
Rise time and fall time are measures of the phototransistor's speed of response. Both are expressed in nanoseconds (ns).
- Rise time (tR) is the amount of time it takes for a pulse waveform to increase from 10% to 90% of its maximum value.
- Fall time (tF) is the amount of time it takes for the output to fall from 90% to 10%.
The chosen materials and composition play a role in the sensitivity of a phototransistor.
- Homo-structure (single material) devices have a gain level from 50 to several hundred. They are the most common phototransistors and are often made of silicon.
- Hetero-structure (multiple material configurations) devices may have gain levels up to 10,000 but are less common because of high manufacturing costs.
Electromagnetic spectrum wavelength range (nm)
Indium gallium arsenide
Chart Credit: Wikipedia
Mounting technology also plays a role in the functionality of a phototransistor.
Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board. Typically, the PCB pad is coated with a paste-like formulation of solder and flux. Elevated temperatures, usually from an infrared oven, melt the paste and solder the component leads to the PCB pads.
Through hole technology (THT), another commonly used mounting style, mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. In terms of features, some phototransistors include a cutoff filter that blocks visible light. Others have an anti-reflective coating to improve light detection. Devices with a rounded dome lens instead of a flat lens are also available.
Phototransistors can be used to detect light in a number of applications.
- Monitoring paper position and margin control in printers and copiers
- Detection in security systems
- Measuring speed and direction in encoders
- Remote meter reading for residential electric meters
- Counting coins or other items
- Remote controls for audio/visual equipment and appliances
- Shutter control for cameras
- Detection for safety shields and other protection systems
Many organizations develop standards for phototransistors. Some examples are listed here:
- BSI BS CECC 120003: Phototransistors, Photodarlington Transistors, Phototransistor Arrays: Blank Detail Specification
- DIN DIN EN 120003: Blank detail specification - Phototransistors, photodarlington transistors, phototransistor arrays; German version EN 120003:1992