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From: email@example.com (chris)
Subject: Re: Thermal runaway: please settle this dispute!
Date: 3 Jan 2003 04:39:57 -0800
NNTP-Posting-Date: 3 Jan 2003 12:39:57 GMT
"Phil Allison" wrote in message news:...
> "Paul Burridge" wrote in message
> > Hi all,
> > I ended up having a bit of a disagreement with a friend of mine over
> > the holiday concerning thermal runaway in a BJT.
> > I maintain that an increase in temperature across the emitter diode
> > leads to a fall in that junction's barrier height, making the
> > so-called 'energy hill' easier to overcome for the forward biased
> > current flowing through it. The causes increased current flow, giving
> > rise to further heating of the emitter diode, thereby lowering the
> > barrier potential still further and so on until the device is
> > destroyed. The fall in barrier height is around 2mV per degree C., if
> > I recall correctly.
> ** That is the usual figure for Silicon BJTs.
> > My friend, OTOH, maintains that it's nothing to do with the
> > emitter/base junction at all, but solely arises due to an increase in
> > temperature at the collector, causing the same cycle of
> > heating/increased current until the device is destroyed.
> ** If your friend was right there would be no way to stabilise a BJT's
> collector current with temperature.
> BJT amplifier OP stage bias stability schemes correct the output
> devices Vbe at the rate of - 2 mV per junction per degree C, assuming that
> Vbe is in the bias determining loop (ie the OP stage is a darlington
> emitter follower) That is what the "Vbe multiplier" stage in the bias cct
> .............. Phil
Hi, vbe does fall at 2mv/degC but that has nothing to do with thermal
runaway, which is caused by the increase in collector-base leakage
current, which subsequently gets multiplied by the devices gain
(hfe).Modern silicon devices dont really suffer much from this
problem, its an issue of yesteryears germainium transistors.
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