it really depends on the application. with progress made in cmos past 15 years or so. bjt is only used in rf front end, with decreasing application space for bjt and even for bicmos.
Strengthening its point of view, almost all modern cpu / gpu are limited power or operate with the given thermal design power (a.k.a tdp). even for chips with cmos circuits, clock speed can be increased (a.k.a overclocking) if the extra heat generated by increased power can be eliminated.
For front end rf, si processed with bipolar ge transistor offer better performance than fet at much higher power consumption. as long as only a few sige bjt are used, the extra heat generated by this bjt can be handled.
However, if the level of integration is higher, there is no way to use the bjt device extensively and keep the heat (and ic temperature) in the thermal budget.
Once the level of integration and power levels are taken into account, modern twists outperform bjt and bipolar-cmos (bicmos) in almost all cases.
Which one is faster: BJT or MOSFET?: This depends on application.
- For current signal mode processing, bjt will be faster because current amplification does not involve big changes in connection potential.
- To switch the application where the collector / drain voltage should be close to zero and backup, the mosfet will be faster, because in the case of bjt there will be minority carrier storage on base in saturation which will take time to flush out.
- For typical ciruits in modern process (0.18um and lower) MOSFETs will be faster because they can be much smaller than bjt and therefore have smaller connection capacitances.
For bjt
Transconductance dic / dvbe which is an exponential function that depends on the vbe (input signal), Transconductance of mosfets equals to un cox (w/l) (vgs-vth)
Which is a linear function of vgs
And as ft (transit transit frequency) is transconductance over some equivalent capacitance and if ft is high then the transistor is faster.
So if high transconductance, high and transistors are higher in speed and So bjt is faster than mosfets But now after all the big investment in cmos technology, mosfet speed is now proportional to bjts
Bjt disadvantages:
- High alignment, especially at high currents, introduces a large number of harmonic distortions. the fact that HFE decreases when the collector increase determines the composition of harmonic distortion.
- while drivers in the power output stage manage high currents, they work in lower bandwidth and with higher distortion.
- The presence of holes requires more time to switch, consequently less bandwidth.
- A phenomenon called second damage. This limitation requires attention in managing the power of each device.
- bjt terminal is unstable: when temperature rises, vbe decreases, then ic increases, then temperature increases even more.
- Mosfet advantage:
- transconductance increases with drain current, the consequence is less distortion .
- less power requirement in the driver circuit.
- larger save operation spectrum.
- greater bandwidth.
- thermally stable.
