Bipolar Transistors (BJTs) can be connected in parallel if balancing resistors are placed in series with the emitter.
BJTs generally tend to become more conductive as their temperature increases. The example below from MMBT2222A datasheet shows one how this device’s typical gain can change by a factor of 3 to 5 times as its temperature varies over the allowed operating range. This causes thermal instability in BJTs operated in parallel, because even if the two transistors and their associated base resistors are perfectly identical (which is impossible), any slight temperature difference between them will cause one of them to begin conducting a greater portion of the shared current, which will cause it to become hotter, and carry even more of the shared current, and become even hotter still, until something eventually breaks.
Typically, BJTs can be more reliably operated in parallel if balancing resistors are placed in series with the emitter, rather than the base. Even a small resistance here will strongly counteract the tendency for imbalance, because an increase in current flowing through any one transistor causes the applied base-emitter voltage to decrease. This in turn reduces the amount of base current that flows, and therefore the portion of the shared current that the affected transistor carries. This technique is commonly known as “Emitter Degeneration.”
