10. (definition)

17. HBT
Heterojunction Bipolar Transistor

20. Gain as a function of current.

21. Effect of recombination currents.
High injection effects also shown.
Note: recombination does not contribute to Ic!

23. General behavior of β (hFE) as a function of collector current (from Sze).
Low currents: Recombination currents dominate (just as in diode).
High currents: High injection effects (increases effective base doping) and series resistance effects increase.

25. This looks good, but:
Only part of the emitter is active, but the whole emitter contributes capacitance.
High current density reduces current gain.