Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Thermal runaway: please settle this dispute!
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Thu, 2 Jan 2003 16:06:07 -0000
NNTP-Posting-Date: Thu, 02 Jan 2003 16:06:11 GMT
> "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.
>> 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.
>> So what is the panel's view on the subject? I always believed I was
>> right, but this chap is regarded as pretty eminent in electronics so
>> I can't simply dismiss his views out of hand.
> I think your friend is right.
Not really. Paul is basically correct.
>The way I understood thermal runaway,
>especially with Germanium transistors. Is that the hotter the
>gets the more current it conducts, and so gets even hotter. The cycle
>continues until the transistor destroys itself.
Yes, this is what happens, but it is not due to a temperature increase
across the collector junction, although that obviously happens.
The collector current is determined by the emitter current, i.e. Ic~Ie.
The basic equation is:
Is is a strong function of temperature, it doubles for every 10 deg. C.
With a fixed Vbe, Ie will increase due the Is(T), this increase will
increase the temperature etc.
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