To improve analog performance of device, we need high input impedance and large transconductance. This requires large collector current and large value of β.

For high performance of BJT in ac applications, required device characteristics include:

i) High gain without low early voltage or punch-through

ii) Very low capacitances

iii) Low parasitic resistances

iv) Short base width

v) A high Kirk effect onset current

In case of digital circuits, to improve the speed we must reduce collector capacitance ($C_C$). To reduce $C_C$ one must reduce capacitance associated with collector base junction, collector substrate junction, the input of emitter follower and the wire capacitance.

Scaling the BJT is more complicated than scaling MOSFETs hence in bipolar ECL, one must scale capacitances. There are limits on doping of base and on band gap narrowing while applying scaling rules.