How are modern photodiodes different from vacuum-tube phototubes?

Modern photodiodes differ from traditional vacuum-tube phototubes primarily in their construction, operating requirements, and performance characteristics.

While phototubes are vacuum tubes that rely on the emission of electrons from a cathode into a vacuum, modern photodiodes are solid-state devices made from thin semiconductor wafers.12

Key differences include:

• Voltage and Size: Photodiodes are compact, rugged, and operate at low voltages comparable to their bandgaps, whereas phototubes require a large positive voltage relative to the cathode to collect emitted electrons.12
• Efficiency: Photodiodes offer significantly higher quantum efficiency—converting up to 80 or 90 percent of incident photons into electron-hole pairs—compared to the 15 to 20 percent efficiency typical of photocathodes in vacuum tubes.2
• Signal Amplification: Unlike photomultiplier tubes, which use a series of dynodes to enormously amplify the initial signal, standard photodiodes do not provide internal charge amplification. Consequently, photodiodes may face challenges with electronic noise in pulse-mode applications, though they are highly effective in current mode for intense radiation fluxes.12