Actually, this is not correct. The NPN for example (see image below) has a PN junction between the collector and base as shown. This junction will conduct when the collector is taken more negative than the base voltage and if taken below the zener breakdown of the reverse emitter/base junction, there will be conduction between the collector and the emitter.
You are correct. The base junction will conduct and it will affect the transistor's conduction when it breaks down. I did not consider these atypical operating conditions - only the typical ones. Whether the BJT is operated in these extreme regimes or not, a final stage based on a single MOSFET will conduct differently...and two MOSFETS connected in series will conduct differently, too, when their body diodes never conduct simultaneously. At this point, I am not recommending putting 2 MOSFETs connected in series in place of the BJT even if that would result in a better-controlled conduction profile, but I think that its present 3-state base driving circuit is too complicated and too inflexible with regards to allowing the operator to adjust the duration of each of these 3 states (high, low, high-Z). I assume that the High-Z state is important to allow the signal form the antenna wire to jerk the BJT's base unencumbered - if it weren't, the 2-ch driver (IR2113) would be redundant in the base driving circuit (...and the delayed pulses applied to its 2 channels, too).
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