This three-layer semiconductor device converts light levels directly into an amplified electrical current. Instead of a physical base lead, it features an exposed light-sensitive region where incoming photons generate electron-hole pairs. Because it provides internal gain, it produces a much higher output current than a simple photodiode, making it ideal for optical switches, encoders, and isolation circuits.
Heard on the job
Swap out the phototransistor on that encoder board; it's barely pulling a milliamp when fully illuminated.
Insider tip
Because of the internal amplification, phototransistors are much slower to respond to rapid light changes than photodiodes, limiting their use in high-speed data links.
Do not mix it up with
- Photodiode: A photodiode generates a small current proportional to light but has no internal amplification, whereas a phototransistor multiplies that initial current for a much stronger signal.