Comparing Phototransistors vs Photodiodes
Phototransistors and photodiodes are sensing devices used for detecting electromagnetic radiation, especially light. Both are commonly used in various electronic applications for their ability to convert light into an electrical signal. However, these two devices differ in their composition, characteristics, and applications. In this post, we will compare phototransistors and photodiodes to find out their pros and cons over one another.
Photodiodes
Photodiodes are semiconductors that generate an electric charge when exposed to light. They operate in reverse bias mode, where the p-type region is connected to the negative terminal and the n-type region is connected to the positive terminal of a voltage source. The reverse bias voltage causes the depletion region to widen, decreasing the current flow until it reaches a certain threshold (called the breakdown voltage). When light enters the depletion region, electrons are excited into the conduction band, creating current flow proportional to the light intensity.
Pros of Photodiodes
- Very low capacitance and noise
- Fast response time (up to the GHz range)
- Higher sensitivity than phototransistors
- Simple circuit design
- High accuracy in measurement
Cons of Photodiodes
- Limited gain (usually less than 1)
- Low output current
- Narrow range of detection
Phototransistors
Phototransistors are similar to photodiodes, but they also have a transistor (pnp or npn) attached to them. The transistor amplifies the current that flows through the diode, increasing the gain of the device. The base current of the transistor is controlled by the light intensity, which means that the output current is proportional to the light intensity.
Pros of Phototransistors
- Higher gain than photodiodes (up to several hundred)
- Larger range of detection
- High output current (up to 1A)
- Easy to use in switching applications
Cons of Phototransistors
- Slower response time than photodiodes (usually less than 1 MHz)
- Higher capacitance and noise
- More complex circuit design
- Lower accuracy in measurement
Applications
Photodiodes are commonly used in optical communication, light detection and ranging (LIDAR), particle detection, and photovoltaic systems. They’re also used in cameras, medical instruments, and security systems. On the other hand, phototransistors are often used in switches, counters, encoders, and light meters. They’re also used in optical encoders, fiber optic receivers, and burglar alarms.
In conclusion, both photodiodes and phototransistors have their own advantages and disadvantages, and their suitability in any application depends on factors such as cost, speed, sensitivity, gain, and accuracy.