Reverse Bias
Reverse bias is the polarity of voltage applied across a pn junction with the positive side connected to the n-type material and the negative side to the p-type material, which raises the potential barrier and widens the depletion region so that almost no current flows. It is the off state of a diode, and it is also a state in which junctions do useful work by holding voltage, storing charge and detecting light.
A reverse voltage adds to the built-in potential, so the barrier grows and the junction becomes a better insulator. The applied voltage pulls mobile holes away from the p side and electrons away from the n side, exposing more fixed ions, and the depletion region widens roughly as the square root of the total voltage across it. Majority carriers can no longer climb the barrier, so the only current is a small leakage made of minority carriers swept across by the field and carriers thermally generated inside the depletion region. In a small silicon diode this reverse current is often only nanoamperes or less, and the ideal diode equation says it levels off at the negative of the saturation current. Leakage rises with temperature, roughly doubling for every 8 to 10 degrees Celsius, and grows with junction area and with the density of crystal defects and contamination.
Breakdown is what happens when the reverse field becomes strong enough to force carriers across, and it comes in two distinct mechanisms. In Zener breakdown, studied by Clarence Zener in 1934, a very high field in a narrow, heavily doped depletion region lets electrons tunnel directly from the valence band on one side to the conduction band on the other. In avalanche breakdown, the field in a wider, more lightly doped region accelerates carriers until they knock new electron-hole pairs loose by impact ionization, and each new carrier does the same, multiplying the current. Zener behavior dominates below about 5 volts and avalanche above roughly 7 volts, with a mix in between; the first has a negative temperature coefficient and the second a positive one, which is why roughly 5 to 6 volt reference diodes are especially stable. Neither mechanism is destructive by itself, and voltage-regulating and surge-protection diodes are built to run in breakdown safely, provided heating is limited by an external resistance.
Light turns a reverse-biased junction into a detector. A photon with energy above the band gap creates an electron-hole pair inside or near the depletion region, and the field sweeps the two apart, producing a current proportional to the light intensity. Photodiodes are therefore run with reverse or zero bias: reverse bias widens the region, lowers capacitance and speeds the response, which is why fiber-optic receivers and camera sensors use it, while zero bias gives the lowest noise. A solar cell is the same junction used in the power-producing quadrant, where light generates a voltage of about 0.5 to 0.7 volts and current flows against that voltage, rather than a diode held under bias. Avalanche photodiodes and single-photon avalanche detectors, used in lidar, run close to or beyond breakdown so that one photon triggers a measurable pulse.
The widened depletion region also makes the reverse-biased junction a voltage-controlled capacitor and an isolation wall. A varactor diode uses the fact that capacitance falls as reverse voltage rises. In early integrated circuits, Kurt Lehovec's 1959 junction isolation idea used reverse-biased pn junctions to keep neighboring transistors electrically separate on one chip, and every well and substrate contact in a CMOS chip still relies on a reverse-biased junction to prevent unwanted current. The same off-state leakage is a central concern in modern processors, where it contributes to standby power, and in DRAM, where leakage across a reverse-biased junction drains the stored charge and forces periodic refresh.
The common misconception is that reverse bias means zero current, so the junction is a perfect switch when off. In reality the leakage is nonzero, rises with temperature and voltage, and is sensitive to defects, which is why leakage specifications are a major part of diode and transistor data sheets. Another confusion treats breakdown as failure; designed correctly, it is an operating mode, while a diode not rated for it can still be damaged by the heat if the current is not limited.
Read reverse bias as pushing a junction into its blocking state, where the widened depletion region holds off the voltage until a tiny leakage current or a deliberate breakdown shows up.