Which photodetector type requires no external voltage?

• A. Barrier Layer Cell
• B. Phototube
• C. Photodiode
• D. Photomultiplier tube

Answer: (A)

The key idea is that some photodetectors generate and collect charge without any external power, thanks to a built‑in electric field. A barrier layer cell uses such a built‑in field at its junction, so light creates carriers that are swept to the electrodes and produce a current even with zero applied voltage. In contrast, a phototube and a photomultiplier tube are vacuum devices that need external high voltage to move electrons and, in the case of PMTs, to amplify the signal. A photodiode can operate without bias only in special photovoltaic mode, but in most practical instruments it’s used with some reverse bias to improve speed and linearity, so it doesn’t inherently require no external voltage. Hence, the barrier layer cell is the one that requires no external voltage.