This document discusses the structure, operation, and characteristics of bipolar junction transistors (BJTs) used as power switching devices. It describes the common collector-emitter (CE) configuration used for power BJTs and explains how the output characteristics and switching times are affected by operating regions like saturation. Common techniques like Darlington connections, Baker's clamping circuit, and base drive isolation using optocouplers are summarized. The document also provides an example power BJT specification sheet and lists some key demerits of using BJTs for power switching like high base drive needs, susceptibility to second breakdown, and difficulty in parallel operation. It concludes that MOSFETs and IGBTs are now