From what i remember when i cared about such things: It's the gate capacitance (and gates resistance) that leads to switching losses. Usually the biggest part

From what i remember when i cared about such things: It's the gate capacitance (and gates resistance) that leads to switching losses. Usually the biggest part of the switching losses is because the transistor is not fully "on" or "off" for the whole time, but is in between when in the process of switching. The gate capacitance is probably one of the biggest problems (more for FET, but also for bipolar), that influences the speed of the transistor (presented as frequency or as slew rate). Otherwise you could just put a bunch of MOSFET-s in parallel and get much lower resistance, thus greater efficiency (and it works, to a point). Another thing to consider is that FET-s let the current go back and a motor is an inductive load, while bipolar transistors do not. I forgot how much that matters, but it probably does plenty. Thankfully electronics is one of those fields of engineering where you can get all the information you need from a datasheet. Unlike from those damned headphone manufacturers (embarrassments to engineering, 99.9% of them). PS "..you will tend try to keep switching losses low relative to conduction losses.." I think you meant the other way around.