Bipolar Junction Transistors, BJT, replaced tubes in the early days of solid state. BJT are current controlled devices, this mind that collector is function of base current.

The following graphic shows transfer characteristic hfe of a MJ15022 transistor.

The high alinearity, specially at high current, introduce great amount of harmonic distortion. The fact that hfe  decrease  when collector current increase determinate the harmonic distortion composition, distortion has big components beyond the thirds, this components aren't ear masked. 

Drivers, in a power output stage, manage high current, is a power circuit too. The most popular configuration driver-transistor, the Darlington pair, has higher current gain, but lesser bandwidth and higher distortion.

Other BJT limitation are associated with the contribution of electrons and holes to conduction. The presence of holes, and his inherent less mobility, indicate BJT requires more time to switch, the consequence is less bandwidth.

Other BJT imitating phenomenon is called "second breakdown". This limitation impose extremely care in the manage power of each device.

BJT are terminally unstable. The unstable process is: when temperature increase Vbe decrease, then Ic increase, then temperature increase, this phenomenon is called "embalamiento térmico".

MOSFET advantage TBJ in all aspects:

-MOSFET transconductance increase with drain current, the consequence is less distortion, see PAPER .

-Less power in the driver circuit.

-Larger save operation area.

-Greater bandwidth.

-Thermal stable.

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