Phototransistor ---is a combination of photodiode and transistor, it has higher sensitivity because it has gain. But it requires power source to make it works.
photodiode is not a solar cell. it is used in reverse bias to detect current increase (when illuminated). phototransistor is more sensitive because it also amplifies the signal.
A diode has two semiconductor regions (NP)
A transistor has three (NPN, or PNP)
Photodiodes and Phototransistors behave like diodes and transistors respectively.
A photo transistor is a transistor where the CB junction is photo sensitive and exposed, ie the CB junction is a photo diode.
Both require a bias voltage to operate. A photo diode is not a solar cell.
