Q.Explain collector-to base leakage current and collector-to- emitter leakage current.

Leakage currents are the small minority-carrier currents that flow under reverse bias; the two kinds are the collector-to-base leakage $I_{CBO}$ (emitter open) and the collector-to-emitter leakage $I_{CEO}$ (base open). …

[!TLDR]

Both are small minority-carrier (leakage) currents: $I_{CBO}$ is measured with the emitter open, $I_{CEO}$ with the base open; both grow with temperature and reverse voltage, and $I_{CEO} = (1+\beta)I_{CBO}$.

In a BJT current flows by both majority and minority carriers. The minority-carrier flow across the reverse-biased collector-base junction is the leakage current — small, but temperature sensitive.

(I) Collector-to-base leakage current $I_{CBO}$: This is the current that flows through the collector when the emitter terminal is left open and the collector-base junction is reverse biased. It is due to minority carriers and depends on (a) temperature — thermal generation of electron-hole pairs roughly doubles $I_{CBO}$ for every 10 degree Celsius rise in a silicon transistor, and (b) the reverse bias voltage.

(II) Collector-to-emitter leakage current $I_{CEO}$: This is the current that flows in the collector-emitter path when the base terminal is left open and the collector is reverse biased. It also depends on temperature and reverse voltage, and is much larger than $I_{CBO}$. …