Phototransistors are semiconductor devices that are sensitive to light. When light falls on the active area of a phototransistor, it generates electron-hole pairs in the semiconductor material, which can modulate the transistor's conductivity or current flow. The amount of current generated by the phototransistor is proportional to the intensity of the incident light.

Irradiance is an important parameter of a phototransistor. It determines how the phototransistor responds to different light levels. Irradiance refers to the amount of light energy falling on the phototransistor's active area per unit area. It's usually measured in units of watts per square meter (W/m²) or microwatts per square centimeter (µW/cm²).