Avalanche breakdown is a phenomenon that occurs in semiconductor devices, particularly in diodes and transistors, when the reverse-bias voltage across a junction becomes so high that it causes a sudden increase in current. This is due to the impact ionization process, where high-energy carriers collide with atoms in the lattice and knock loose additional charge carriers. This creates a chain reaction, leading to a sharp rise in current, known as avalanche breakdown.
In a transistor, particularly in bipolar junction transistors, avalanche breakdown usually occurs at the collector-base junction when the transistor is reverse-biased beyond its rated breakdown voltage. While this condition is typically undesirable in digital circuits, it can be intentionally used in certain analog and timing applications, such as in avalanche oscillators.
An avalanche oscillator is a type of relaxation oscillator that utilizes the avalanche breakdown behavior of a transistor. In this circuit, a transistor is reverse-biased just below its breakdown voltage using a high-resistance path. When the voltage across the junction reaches the breakdown threshold, the transistor suddenly conducts, allowing a burst of current to flow. This sudden change rapidly discharges a capacitor in the circuit, and once the voltage drops, the transistor returns to its non-conducting state. The cycle then repeats, producing a waveform, often a sawtooth or sharp pulse.
The key relation is that the oscillator exploits the fast switching and regenerative nature of avalanche breakdown. The steep onset of current during breakdown provides a very sharp transition, making avalanche oscillators useful for generating fast pulses, high-frequency signals, or noise sources in random number generators and similar applications.
In essence, the avalanche breakdown is the physical principle, while the avalanche oscillator is a circuit design that uses this principle to generate periodic signals.
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