Physics · Ch 14 — Electronic Devices
p-n Junction Diode under Reverse Bias
p-n Junction Diode under Reverse Bias
Connecting the bias. In reverse bias, the external battery's positive terminal is connected to the n-side of the junction and its negative terminal to the p-side -- the applied voltage connected in the SAME sense as the junction's own built-in field, reinforcing rather than opposing it.
Widening the depletion region. The applied field pulls majority holes on the p-side further AWAY from the junction, and majority electrons on the n-side further away too, exposing MORE fixed ions on both sides and WIDENING the depletion region, which increases the effective barrier the majority carriers would have to cross -- so, for all practical purposes, majority-carrier current across the junction is now blocked almost entirely.
The small reverse (leakage) current. A very small current does still flow under reverse bias, but it is carried not by majority carriers (which are now firmly blocked) but by the small population of MINORITY carriers on each side -- the few holes thermally generated in the n-region and the few electrons thermally generated in the p-region -- which the reverse field actually assists across the junction, rather than opposing. Because this current depends only on the RATE of thermal generation of minority carriers, and not on how large the reverse voltage itself is, the resulting reverse saturation current stays nearly CONSTANT over a wide range of reverse voltage -- it is nearly independent of the applied reverse voltage, but rises sharply with temperature, since thermal generation is a strongly temperature-dependent process, exactly like the intrinsic carrier generation of Section 9.7.
Breakdown at large reverse voltage. If the reverse voltage is increased far enough, the diode eventually reaches its BREAKDOWN VOLTAGE, at which the reverse current rises abruptly and steeply while the voltage across the diode stays almost constant (the near-vertical portion of the characteristic curve). An ordinary rectifier diode is never operated anywhere near this regime, since sustained breakdown current would destroy it -- but a Zener diode (Sections 9.16-9.17) is purpose-built specifically to operate safely in exactly this regime. …
What this figure shows. A single graph is drawn with applied voltage on the horizontal axis (positive values, forward bias, to the right of the origin; negative values, reverse bias, to the left) and diode current on the vertical axis (positive/upward for forward current, drawn on a much LARGER scale than the reverse-current axis, which is drawn separately, greatly magnified, in the third quadrant). In the FORWARD (right) region, the curve stays essentially flat along the voltage axis for small voltages, then bends sharply upward (a distinctly NONLINEAR, exponential-looking knee) once passes a threshold labelled the KNEE or CUT-IN VOLTAGE (about V for silicon, V for germanium), after which current rises very steeply for only a small further increase in . In the REVERSE (left) region, the curve stays very close to the horizontal axis -- a tiny, almost constant REVERSE SATURATION CURRENT (drawn on the magnified inset scale, in the micro- or nano-ampere range) -- flowing essentially flat as reverse voltage increases, until, at a large reverse voltage labelled the BREAKDOWN VOLTAGE, the curve turns sharply and steeply downward (a near-vertical drop), showi …