Why are Power Transistors so slow?

[janneman]

Why are power transistors (bjt's) so slow, compared to small-signal transistors? Small-signal devices have bandwidths into the 100's of MHz or even GHz. Medium power devices have lower bandwidths. High power transistors are slower yet. Is it a result of the construction to make them rugged to withstand higher powers, or is there another mechanism?

jan didden

[Bonsai]

This sounds like rhetorical question Jan.

[janneman]

Originally Posted by Bonsai

This sounds like rhetorical question Jan.

Huh? Why?

jan didden

[AKSA]

Jan,

Don't you know?

[Elvee]

The answer is pretty straightforward:
Mobility

[gaetan8888]

Hello

A power transistor do have a thicker base-collector-emitor sandwitch and the electrons take more time to travel in that silicon substrat, resulting in a lower speed.

Bye

Gaetan

[DF96]

I thought it was the base thickness, not the substrate. The substrate is merely what the BJT is built on top of? A thick base stores lots of charge carriers, and they take time to cross it from emitter to collector.

[janneman]

Originally Posted by Elvee

The answer is pretty straightforward:
Mobility

So, more doping lowers mobility. Does that mean power transistors have more doping than small-signal transistors?
And does electron mobility have a relation to device bandwidth?

jan didden

[gaetan8888]

Originally Posted by DF96

I thought it was the base thickness, not the substrate. The substrate is merely what the BJT is built on top of? A thick base stores lots of charge carriers, and they take time to cross it from emitter to collector.

Hello

Substrat are not the right name, I've corrected it, I was meaning lateral thickness not vertical, like a thicker base-collector-emitor sandwitch.

Sorry for my limited english explanation, I'm french speaking.

Bye

Gaetan

[Sch3mat1c]

Actually, it's not very self-explanatory...

RF transistors often have higher doping levels, which is necessary due to the smaller feature size or reduced carrier lifetime.

If doping were the only explanation, power transistors would work very well indeed, because they usually have fairly light doping. High voltage transistors (up to 1500Vcbo in HOTs, and more in industrial use) necessarily have very light doping, on the order of 10^14, or a P-I-N structure at the collector, just like a high voltage diode, and for the same reason: to minimize electric field strength, maximizing breakdown voltage.

However, this is not the case. The reason is structure. Most transistors are planar. This worked nicely for some of the first planar transistors, like 2N2222, which boasts a ~150MHz fT (though not part of the spec). It didn't work so well for the first planar power transistors, like 2N3055, which is a poor power transistor by any measure. They're basically the same thing, so why are they so different? A few reasons. One, if they are truely the same process, then the doping and depth will be equal, but the area much wider. A thin base has a large base-spreading resistance, so it takes a long time to drive the base. Generally, the emitter is broken up into strips, giving more base contact area. The finer this structure, the more emitter "fingers" and base edges are brought out, the faster it runs.

RF power transistors are little more than monolithic arrays of 2N3904s. They may be printed on a, say, ~2um feature size, or mechanically connected, where the emitters are built as islands rather than fingers; a bond wire per emitter is required! Obviously, bond wires have lower capacitance than metallization which overlays the base and collector.

Other reasons include voltage capacity, which is generally higher. (Yes, there are a few 200, 300, even 450V small-signal transistors.) hFE goes up with doping, so the light doping required to obtain the higher breakdown voltage necessarily reduces hFE. (Those 1500Vcbo transistors have a typical saturated hFE ~= 2.5!!)

Incidentially, there are a few medium power transistors with ridiculous bandwidth. These are made for high resolution CRT drivers, and include the 2SC3597. I don't recommend using one as VAS unless you damn well know what you're doing!

Tim