From: Winfield Hill
Subject: Re: AOE question.
Date: 10 Dec 2002 05:10:27 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
> On page 80 (ed2), where the Ebers-Moll model is being explained, Is
> is brought in, with little explanation, "Is is the saturation current
> of the particular transistor (depends on T)".
> Is it explained earlier? (I was going over my knowledge of transistors,
> to make sure I knew it all :) )
As you see we don't have a lot to say about Is. That's because it's
a parameter that's determined from the specific semiconductor physics
for a given device type. It's a parameter over which circuit designers
have no influence and which is not given on the transistor's data sheet.
Despite these facts, we do mention Is, because it lies at the base of the
valuable Ebers-Moll equations, and its temperature coefficient ends up not
only dominating the Vbe tempco, but changing its sign! Despite the large
influence of the parameter, Is, our advice to the novice circuit designer
is basically to forget about Is, per se, and instead use the Ebers-Moll
formulas to predict the changes in Vbe under conditions of current and
temperature, using the manufacturer's Vbe values as a starting point.
If you want to know a typical value of Is for a given transistor type,
look up its Vbe voltage spec at a given current, or better yet read the
typical value from a graph on the data sheet. Re-arrange the formula
on page 80 to calculate Is, viz, Is = Ic e^(-Vbe/Vt), where Vt = 25mV
at room temp. For example a transistor's Vbe is 650mV at 1mA, and we
get Is = 5.1 x 10^-15. That's a pretty small current! It's sometimes
called the reverse leakage current, but it's not closely related to the
actual leakage currents you'll externally observe for the transistor,
as these are generally somewhat higher.