BJT Current Drive Capability vs MOSFET

[dknguyen]

There's an issue I always seem to come by when I start looking closely at gate drivers for MOSFETs. It's that you get claims that go both ways saying BJTs have higher current capability or MOSFETs that have higher current capability. it seems to me that if we are talking about heat limitations then MOSFETs win. But it might not be a heat limitation and might be something like saturation or pinch-off.

It's the same with some claims that MOSFETs switch faster because they don't have to charge depletion capacitances or wait for minority and majority charge carriers to recombine as BJTs do. But then others say BJTs switch faster becaues there is no gate capacitance.

Can anyone provide clarification to why there are claims going both ways?

 

[crutschow]

As in many arguments there's some truth in both sides.

BJTs require a large base current to switch (typically 10% of the collector current for good saturation) thus they tend to be less efficient. BJTs are also slower than MOSFETs in turn-off because of the recovery time for the minority carriers. One advantage of BJTs is that they can be turned on with a low base supply voltage, whereas MOSFETs require 5V gate-source voltage for logic level devices and 10V for standard devices to completely turn on.

MOSFETs don't require gate current but they do have a large gate capacitance that has to be rapidly charged and discharged for fast switching, which may require a dedicated gate driver circuit. This also consumes power but generally less then the BJT base power. MOSFETs can turn on and off very fast, determined primarily by how fast you can charge and discharge the gate capacitance.

So, in general, MOSFETs seem to be preferred for power switching applications due their higher speed and efficiency. For a given current rating BJTs may be cheaper than MOSFETs and thus find use in some power applications such as emitter-follower current boosters in linear power supplies.

 

[dknguyen]

Okay fair enough. THe switching time pro and con issues seems to be many issues that work together to produce an end result that might go either way.

But my larger issue was the origin of the explanation of higher current density from BJTs. Do you have any insight for that?

 

[crutschow]

But my larger issue was the origin of the explanation of higher current density from BJTs. Do you have any insight for that?

Not a lot. I believe it's related either to the BJT current flowing through a junction, which can be heavily doped to give a low resistance in a small area, or that the BJT is a minority carrier device that can generate a lower on resistance for a given area.

I think the MOSFET conduction channel may be more lightly doped or perhaps has a longer channel length then a BJT, which increases its on-resistance.

Perhaps someone with a better knowledge of the transistor fabrication physics can enlighten us on the subject.

 

[Warpspeed]

Current drive capability ?

It depends on what you actually wish to do here.
Mosfets win hands down in almost every respect, except one.

Bipolars have much better characteristics when used as linear control devices, for things such as audio amplifiers and dc power supply regulators, which is where you mostly find large bipolar power transistors these days.

If you are just switching bulk dc power on and off, (for any given package size), you can always find mosfet with a higher average current rating, lower saturation voltage, and a fatter safe operating area. It will also be vastly faster than a bipolar.

 

[Nigel Goodwin]

You need to look at what's used commercially - MOSFET's are pretty rare, almost everthing is bipolar. The performance/cost ratio is usually better for biploar, and their reliability is usually higher as well.

 

[Warpspeed]

You need to look at what's used commercially - MOSFET's are pretty rare, almost everthing is bipolar. The performance/cost ratio is usually better for biploar, and their reliability is usually higher as well.

That is certainly true for small signal devices.

But for power devices (which I believe is the topic here) mosfets greatly outnumber bipolars.

In switching power supplies mosfets are now used exclusively.