text: Leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in collector-to-emitter leakage current. This is frequently seen in a push&ndash;pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.
  One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that V ≤ 1/2V
  Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor.  This in turn causes the thermal feedback transistor  to turn on at a slightly lower voltage,  reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.
  If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur.  Special measures must be taken to control this characteristic vulnerability of BJTs.
  In power transistors (which effectively consist of many small transistors in parallel), current hogging can occur between different parts of the transistor itself, with one part of the transistor becoming more hot than the others. This is called second breakdown, and can result in destruction of the transistor even when the average junction temperature seems to be at a safe level.