Q.Explain the output V-I characteristics of a power transistor.

The output characteristics of a power transistor plot $I_C$ against $V_{CE}$ for a family of base currents, showing cut-off, active, quasi-saturation and hard-saturation regions. …

[!TLDR]

Plot $I_C$ vs $V_{CE}$ for increasing $I_B$; as a switch the power transistor works between cut-off (OFF) and hard saturation (ON), with active and quasi-saturation regions in between.

The output (collector) characteristics of a power bipolar junction transistor (PBJT) show the collector current $I_C$ as a function of the collector-emitter voltage $V_{CE}$, drawn as a family of curves for increasing values of base current $I_B$. The collector current increases as the base current increases.

The regions are:

• Cut-off region: with $I_B=0$ (or negative), both junctions are reverse biased; only a small leakage current flows and the transistor is OFF.
• Active region: the emitter-base junction is forward biased and the collector-base junction reverse biased; the collector current is controlled by the base current, $I_C\approx\beta I_B$, and the curves are nearly flat ($I_C$ almost independent of $V_{CE}$).
• Quasi-saturation region: at reduced $V_{CE}$ the lightly doped collector drift region (present in a power transistor to withstand high voltage) is not yet fully conductivity-modulated, so the curves bend as $V_{CE}$ decreases. …