Q.What is leakage current?

Conduction in a bipolar junction transistor is carried by both majority and minority charge carriers. Besides the main majority-carrier collector current, a small current flows because of minority carriers crossing a reverse-biased junction; this small, temperature-sensitive current is the leakage current. Though tiny compared with the main current, it matters because it grows strongly with temperature and can trigger thermal instability.

Two leakage currents are defined for a transistor:

• Collector-to-base leakage current $I_{CBO}$ — the current that still flows through the collector when the emitter terminal is left open and the collector-base junction is reverse biased. It depends on temperature (thermal generation of electron-hole pairs roughly doubles it for every 10 °C rise in a silicon transistor) and on the reverse-bias voltage. …