The design of transistors intended for high power applications follows a different path than transistors for small signal analog or digital circuits with supply voltages of 5 volts, 3.6 volts or lower. A high collector-base breakdown voltage (BV _CBO) is desired, because the devices need to be operated at high supply voltage (24 28 volts) to achieve high output power. Furthermore, a switching transistor s collector side experiences up to two times the supply voltage during large-signal operation with inductive load. To support this high voltage, the collector epitaxial region (epi) must be thick, typically 6 8 um, to avoid reach-through breakdown, and have a resistivity value (doping level) typically in the 1 2 Ohmcm range, set by the avalanche breakdown of the junction.

The final selection of epi thickness and resistivity also affects base push out or Kirk effect (higher doping gives higher saturated power output), gain (collector-base capacitance), high frequency characteristics (f _T ), resistive power loss ( P _loss =IR ²), ballasting and linearity of the device.

Because of junction curvature, the theoretical breakdown voltage of the collector-base junction cannot be reached. However, careful geometrical layout and the use of collector depletion rings or graded diffusions at the device edges helps to reduce the sharp peaks of the electrical field and raise the breakdown voltage near its theoretical limit.

An empirical formula for the relationship of breakdown voltages in the transistor and current gain, ? or hFE,