Breakdown Voltage
Breakdown voltage is the reverse voltage at which a semiconductor junction, or an insulating layer, stops blocking current and conducts heavily. Below it, a reverse-biased diode or transistor junction passes only a tiny leakage current; at it, the current rises steeply, sometimes harmlessly and reversibly, as in a voltage-reference diode, and sometimes destructively. The value is one of the first numbers on a transistor datasheet, because it sets the highest supply voltage the device can safely face, and it is the electrical face of the material's ability to hold an electric field.
There are two physical mechanisms, avalanche and Zener, and they look different in practice. In avalanche breakdown, the strong field inside the depletion region accelerates a carrier until it hits an atom with enough energy to free another electron-hole pair; those carriers are accelerated in turn and multiply, so the current grows sharply as the voltage approaches a limit. In Zener breakdown, found in very heavily doped, thin junctions, the field is so strong that electrons tunnel directly across the gap, a process of the kind Clarence Zener described. In silicon, tunnelling dominates below roughly 5 volts and avalanche above about 7 volts, with a mixture between. The practical difference is temperature: avalanche breakdown voltage rises with temperature, tunnelling falls, and the combination near 5 to 6 volts gives reference diodes of unusual stability. Both kinds are commonly called Zener diodes, which causes needless confusion.
Field and doping set the number, which is why breakdown voltage is something designers engineer. A junction breaks down when the peak electric field reaches the critical field of the material, about 0.3 million volts per centimetre in silicon. Lightly doped material spreads the voltage over a wide depletion region, so the peak field stays low and the breakdown voltage is high; heavy doping squeezes the same voltage into a narrow region and breaks down early. Hence the standard trade-off: power devices use thick, lightly doped drift layers to block hundreds or thousands of volts, but those layers add resistance when the device conducts. Junction edges and corners concentrate the field and lower the breakdown below its ideal value, so designers add guard rings and field plates to flatten it. Wide-gap materials such as silicon carbide and gallium nitride tolerate several times the field, enabling thinner layers.
Bipolar transistor datasheets list several breakdown voltages because the junction is tested under different base conditions. BVCBO is the collector-base junction's breakdown with the emitter open, essentially a diode. BVCEO is measured with the base open, and is lower: the leakage that the collector junction generates is fed into the base and multiplied by the transistor's current gain, so avalanche multiplication starts earlier, with the result that BVCEO is typically a fraction of BVCBO that shrinks as gain rises. BVEBO, the emitter-base limit, is only a few volts in a high-frequency silicon transistor because that junction is heavily doped, a fact that makes a reverse-biased base-emitter junction a crude Zener but a damaging one if used as a signal path. Designers must keep collector-emitter voltage, including inductive spikes, below BVCEO.
Not every breakdown is avalanche, and some are destructive even below the rating. Second breakdown in a bipolar transistor is a thermal effect in which current crowds into a hot spot at high collector voltage and current, the local temperature rises, resistance falls and the filament collapses into a short, often at voltages well below BVCEO. It is the reason power-transistor datasheets include safe-operating-area curves rather than just a voltage rating. Oxide breakdown is a different event: a gate oxide that is a few nanometres thick, stressed by even a few volts or by electrostatic discharge, suffers permanent damage, since the dielectric cannot recover as a junction can. A common misconception is that the absolute maximum rating is a safe operating point; it is a limit beyond which damage may occur.
Read breakdown voltage as the electric-field ceiling of a structure: set by doping, geometry and material, dramatically lowered by gain, heat or thin oxide, and best treated as a boundary to stay well below rather than a number to approach.