Physics · Ch 14 — Electronic Devices
Photodiode
Photodiode
Structure. A photodiode is a p-n junction diode built with a thin, transparent window positioned directly over its junction region, so that light from outside the package can reach the junction itself, and, unlike almost every other diode in this chapter, it is deliberately operated under REVERSE bias rather than forward bias.
Why reverse bias. Under reverse bias, the depletion region is WIDENED (Section 9.11) and the diode's own reverse current would ordinarily be a tiny, nearly constant leakage current , set only by the RATE at which minority carriers are thermally generated. Shining light on the junction provides an ADDITIONAL, controllable way of generating electron-hole pairs, right within or near the depletion region: each absorbed photon of sufficient energy can break a covalent bond and create one new electron-hole pair, exactly as thermal energy does in Section 9.7, except now the rate of generation is set by the intensity of the incident light rather than by temperature alone. Because reverse bias already sweeps the resulting minority carriers rapidly across the junction (the depletion region's field points exactly the right way to assist THIS carrier flow, the same reason a small thermally-generated reverse leakage current exists at all, Section 9.11), every photon-generated carrier pair reliably adds to the reverse current -- so the reverse current of an illuminated photodiode rises in close, linear proportion to the intensity of the light falling on it. This linear, directly light-controlled current is the whole point of the device -- a photodiode is a LIGHT-CONTROLLED CURRENT SOURCE, and would not work nearly as usefully under forward bias, where the (already large) forward current would swamp any small photo-generated contribution and vary only weakly and nonlinearly with light intensity. …
What this figure shows. A p-n junction, enclosed in a small package with a transparent window directly over the junction region, is drawn connected in a circuit to a battery oriented so that the p-side is joined to the battery's negative terminal and the n-side to its positive terminal -- i.e. REVERSE bias -- in series with a microammeter. Several parallel wavy arrows, labelled INCIDENT LIGHT (photons), are drawn striking the junction region through the transparent window from outside the package. Inside the depletion region, a few electron-hole pairs are shown being newly generated at the point where the light arrows strike, with small curved arrows showing the resulting electron swept toward the n-side and the resulting hole swept toward the p-side by the junction's own (reverse-bias-widened) electric field, adding to the small reverse current the microammeter reads. Alongside, the standard circuit symbol for a photodiode is drawn: an ordinary diode symbo …