40 BJT Structure

This is the 40th video in a series of lecture videos by Prof. Tony Chan Carusone, author of Microelectronic Circuits, 8th Edition, covering chapters 1 - 7 of the text: Devices and Basic Circuits. Find more resources at analogicdesign.com.
In this video, we will go over the basic structure of the bipolar junction transistor (BJT) and its operation in active mode.

[@adriaanberger1930]

Nice, clear video. Do you know why superposition is allowed in the derivation of this model of the BJT? I mean, ignoring the space charge layer width differences. In particular, the derivation whereby first V is assumed zero and then V, which allows the assumption that there are no minority charge carrier at the boundary (of the region between B and the opposite n-type region) to be made, and then leads to two expressions for both i and i which are finally summed? My book briefly mentions that: it's okay because each junction has only one non-zero voltage applied to it. But, I'm not convinced, or more accurately don't understand it well enough. Any help? Thanks.

[@Microelectronics-ChanCarusone]

Charge conservation requires that the emitter current equal the sum of the base and collector currents in steady-state.