11.3:
Diode: Reverse bias
A diode is reverse-biased when its n-type region is connected to the positive terminal of the supply voltage and its p-type region is connected to the negative terminal.
At lower reverse bias voltages, the small leakage current through the diode is primarily due to minority carriers, and the IV curve in this region is almost flat.
As the reverse bias voltage increases beyond a threshold voltage, known as the breakdown voltage or the knee voltage, a steep rise in the current is observed for a small variation in the voltage.
In heavily doped diodes at high reverse voltages, the electric field across the depletion region is strong enough for the valence electrons to break free from their bonds. This creates electron-hole pairs, resulting in a significant increase in current.
In lightly doped diodes, high reverse voltages accelerate the minority carriers across the depletion region. They collide with atoms, creating additional electron-hole pairs. This process cascades, leading to a rapid increase in current.
In lightly doped diodes, the current increases gradually compared to the heavily doped diodes.
11.3:
Diode: Reverse bias
A diode is reverse-biased when the positive terminal of an external voltage source is connected to the n-type material and the negative terminal to the p-type material. This configuration opposes the natural direction of current flow through the diode, effectively increasing the width of the depletion region and the barrier potential. The reverse bias condition produces a minimal leakage current, primarily due to minority charge carriers. This leakage becomes significant when the reverse voltage surpasses the thermal voltage under standard room conditions, leading to a flattened current-voltage (I-V) response curve. Unlike the exponential current increase observed in forward bias, the increase in reverse bias is negligible.
However, in practice, the reverse current in diodes often exceeds the predicted saturation current. For instance, diodes designed for small signals with femtoampere-level reverse saturation currents may exhibit nanoampere-level reverse currents. While this reverse current slightly increases with the reverse voltage, these changes are too small to affect the I-V curve noticeably. This reverse current originates from thermal carrier generation within the junction, depending on the diode junction's physical dimensions.
A sharp increase in reverse current occurs when the applied reverse voltage reaches a critical threshold known as the breakdown voltage, specific to each diode. This phenomenon, represented by the knee on the I-V curve, signifies a substantial current rise with minimal voltage increase.
It's essential to recognize that diode breakdown is not inherently damaging, provided the current stays within its safe operating area, typically defined by its maximum power dissipation capacity in the datasheet. External circuitry, designed to limit reverse current to safe levels, is necessary to prevent potential damage. Zener diodes, engineered to function within the breakdown region for voltage regulation, exemplify diodes that operate safely under these conditions.