By Phototransistor 283
Main Parameters of a Phototransistor
Features, Advantages, and Disadvantages
Applications and Circuit Design
Comparison: Photodiode vs Phototransistor
A phototransistor is a semiconductor device that converts light signals into electrical signals. It combines the amplification function of a Bipolar Junction Transistor (BJT) and the light sensitivity of a photodiode.
This device can directly detect light changes and convert them into amplified electrical signals. It is widely used in industrial control, communication, and consumer electronics.
The working principle of a phototransistor is based on the internal photoelectric effect.
When light with a suitable wavelength shines on the base-collector junction, the photon energy excites electron-hole pairs. These photo-generated carriers are separated by the built-in electric field in the PN junction, forming a photocurrent.
Unlike a normal transistor, this photocurrent acts as the base current.
Under the collector-emitter voltage, this small "photo base current" is amplified by the transistor’s current gain (β). As a result, a much larger output current appears at the collector.
In simple terms, a phototransistor can be seen as a photodiode combined with a transistor, where the photodiode’s photocurrent drives the transistor’s base.
Most phototransistors use an NPN structure. To improve light absorption, the base area is made large.
They are usually packed in a case with a lens or transparent window to focus light.
According to the number of leads, there are two types:
Collector current is the current flowing between the collector and emitter under a given light intensity. It shows how well the device converts light into electricity.
For example, under 1000 lux, a typical silicon phototransistor can produce 1–10 mA.
Dark current is the small leakage current flowing between the collector and emitter when there is no light. It indicates the noise level of the device.
Good phototransistors have dark currents below 100 nA.
Responsivity means how much photocurrent is generated per unit of incident light power. The unit is A/W (ampere per watt).
It shows the light-to-electricity conversion efficiency. Typical values are 0.1–0.5 A/W.
The peak wavelength is the light wavelength where the device’s responsivity is highest.
Most silicon phototransistors have peak wavelengths in the near-infrared region (850 nm or 940 nm), different from visible light.
These parameters show how fast the phototransistor reacts to light changes.
Rise time is the time for the output current to go from 10% to 90%.
Fall time is the opposite.
Typical phototransistors respond in a few to tens of microseconds.
Typical Parameter Comparison Table
| Parameter | Typical Range | Test Condition |
| Collector Current | 1–10 mA | 1000 lux, 5V CE voltage |
| Dark Current | 10–100 nA | No light, 5V CE voltage |
| Responsivity | 0.1–0.5 A/W | Peak wavelength, 5V CE voltage |
| Response Time | 2–20 μs | Standard condition |
Phototransistors have several strong advantages:
However, they also have some limits:
Type Performance Comparison
| Type | Sensitivity | Speed | Typical Use |
| Standard Silicon | Medium | Medium | General detection |
| Darlington | High | Slow | Weak light detection |
| Three-lead | Adjustable | Adjustable | Precision use |
Phototransistors are used in many areas:
There are two main circuit types:
Common-Emitter Mode
Most common type. Collector connects to power through a pull-up resistor. Output is from the collector.
Without light → output is high.
With light → transistor conducts → output low.
The resistor value affects sensitivity and speed.
Emitter-Follower Mode
Output is from the emitter. It gives almost no voltage gain, but high current gain.
Used when driving low-impedance loads.
In practice, phototransistors perform better than photodiodes in weak light. They can improve the signal-to-noise ratio (SNR) by about 20–40 dB.
| Feature | Photodiode | Phototransistor |
| Sensitivity | Low | 100–1000× higher |
| Speed | Nanoseconds | Microseconds |
| Linearity | Good | Poor |
| Best Use | High-speed, precise | Sensitive, low-cost |
Phototransistors are better for low-speed, high-sensitivity uses. Photodiodes are better for high-speed or precise measurement.
You need:
Dark current test:
To test response time:
Good phototransistors show 3 μs rise time and 5 μs fall time with 10 kΩ load — much faster than photoresistors, which take tens of milliseconds.
These tests help you choose the right device for your application, balancing sensitivity and speed.
The phototransistor is an optoelectronic device that integrates photosensing and transistor amplification to transform optical input into an electrical output signal.
In devices like smoke detectors, laser rangefinders, and optical remote controls, phototransistors serve as a key component for detecting light and transforming it into an electrical signal.
A phototransistor is an optoelectronic device that detects light intensity and generates a corresponding electrical signal (current or voltage), utilizing its semiconductor structure for both light conversion and signal amplification.
Photodiodes and phototransistors belong to the family of semiconductor photoelectric devices, which function by converting light into electrical signals. The key distinction lies in their operation: photodiodes produce current in response to light, while phototransistors employ a transistor to perform the light-to-current conversion.
Phototransistors are highly sensitive devices capable of detecting extremely low-level signals ranging from picowatts to nanowatts, thanks to their internal amplification mechanism. One example is the MTD8600N4-T, an NPN phototransistor from Marktech Optoelectronics, which responds to wavelengths from 400 nm to 1100 nm, covering both the visible and near-infrared spectral regions.