Maximum rapidly-switchable power density in junction triodes

The maximum power density which may be switched at (switching time/current gain) quotients comparable to 1/2πfαis shown to be 10⁵- 4 × 10⁵watts/cm²for p-n-p germanium transistors. This result is derived first for junction triodes in which the collector depletion layer at peak reverse voltage lies largely in a collector body of conductivity type opposite to that of the base; e.g., diffused base transistors of the mesa type. The limitation arises from the linear dependence of maximum (scattering limited) current density on collector-body impurity concentration and from the approximately reciprocal dependence of breakdown voltage on the same parameter. It is shown that space-charge limitation of current dennsity leads to a somewhat lower limit for intrinsic collector barriers of the same maximum width and, a fortiori, to a lower value for collector barriers lying largely in material of the same conductivity type as the base layer. Similar limits for n-p-n germanium and for silicon transistors are higher but generally comparable.