"james" wrote in message
...
On Mon, 25 Sep 2006 19:06:49 -0400, "Leland C. Scott"
wrote:

+++Believe it or not I've seen many of the old Motorola RF devices use
+++internal emitter resistors. Those took the form of many small tungsten
+++bonding wires from different areas of the emitter structure to the
emitter
+++terminal. The main idea there was the many wires, resistors, in
parallel
+++resulted in a very small overall emitter resistor. Also they found that
a
+++problem called "second break down" would occur if they didn't do this.
+++What it amounted too was local hot spots, thermal runaway, in tiny
areas
+++of the transistor's emitter structure. I think the term they used for
RF
+++devices built this way was "emitter ballasting".
*********

I have never seen tungstun bonding wire. All I have ever seen is gold.

Correct. That's a mistake on my part.

You need a soft malible metal to bond to the die pads on any
semicondcutor. The bond wire is sonic heated to the aluminum metal die
pad. This forms the nice ball on the die pad that is a weld of the
aluminum and gold. Tungstun is far to hard a metal for bonding.

I did read in the Motorola manual they did, or tried, to use it in a stack
up of metals since they saw a problem with electromigration of the gold used
in the bonding wires with the silicon base material.


Emmitter ballasting is done on the die within the emmitter matrix.

Correct. The two methods I did read about are polysicicon resistors or
Nichrome resistors made on the chip.

There are several metods of fabricating an RF transistor. Major
factors are power, frequency and device operating point. For most
transistors operating below 50 MHz use an interdigitated emmitter
geometry. Incorparated within are current balancing resistors in the
emmitter matrix. This does increase die size and reduces gain. It does
spread heat and current more evenly through the die. Interdigitated
emmitters will have multiple bond wires.

For those really interested in the details I found the PDF document below
that goes in to it a bit more.

rfwireless.rell.com/pdfs/AN_IRFPT.pdf


--
Regards,
Leland C. Scott
KC8LDO