The light source is usually an LED, and the detector is a photodiode or phototransistor. A phototransistor is a light-sensitive transistor. The majority of opto-isolators on the market use bipolar silicon phototransistor sensors. Atomic layers of MoS have been used as a phototransistor and ultrasensitive detectors. A solaristor (from SOLAR cell transISTOR) is a compact two terminal self-powered phototransistor. A Solaristor is a two-terminal gate-less phototransistor. He also directed R&D activities on the InAs Phototransistor and P-N Junction Cold Cathodes. The phototransistor was eventually used in the nationwide direct distance dialing system.