Fig. 4: Characteristics of NPN Power Transistors
There are four regions clearly shown: Cutoff region, Active region, quasi
saturation and hard saturation. The cutoff region is the area where base current is almost
zero. Hence no collector current flows and transistor is off. In the quasi saturation and
hard saturation, the base drive is applied and transistor is said to be on. Hence collector
current flows depending upon the load. The power BJT is never operated in the active
region (i.e. as an amplifier) it is always operated between cutoff and saturation. The
BVSUS is the maximum collector to emitter voltage that can be sustained when BJT is
carrying substantial collector current. The BVCEO is the maximum collector to emitter
breakdown voltage that can be sustained when base current is zero and BVCBO is the
collector base breakdown voltage when the emitter is open circuited.
The primary breakdown shown takes place because of avalanche breakdown of
collector base junction. Large power dissipation normally leads to primary breakdown.
The second breakdown shown is due to localized thermal runaway. This is
explained in detail later.