# Transport Saturation Current (Is) in a BJT

Discussion in 'Electronic Basics' started by Paul_M, Apr 16, 2004.

1. ### Paul_MGuest

I'm trying to find out exactly what the Transport Saturation Current
(Is) is that the Ebers Moll equation relates to. Typical values are
10e-16, this value is so small that it can't possibly be the Collector
or Emitter current when the transistor is in saturation. All materials
I've read give hardly any explanation and I've searched the internet
for some days now. The Art of electronics book just says that it's the
Saturation current of the particular transistor (depends on T) and in
a different paragraph it says that it represents the reverse leakage
current.
I find this very confusing. Can anyone enlighten me?

Regards

Paul

2. ### Jim ThompsonGuest

Approximately...

Ie = Is*(e^(q*vbe/k/T)-1)

*Roughly*, Is is the reverse *leakage* current.

...Jim Thompson

3. ### Paul_MGuest

Thanks for the info Jim,
I did see this equation, in the "Art of electronics" book, but in a
slightly different form. I can see now why it would be the reverse
leakage current due to the Base-Collector junction being reverse
biased in the Forward Active region and a base-emitter voltage
increase corresponding to a collector current increase (60mV to a 10
fold increase in Collector current).
Also I've found something that states that :
Is = Js.A
where Js is the Transport current density and
A is the emitter area.
Can you recommend anything that goes into this a little deeper?

Regards

Paul

4. ### MantraGuest

For far more detail, especially the "next model up" (Gummel-Poon)
which is a superset of Ebers-Moll:

http://eesof.tm.agilent.com/docs/ic...MODELING/3TRANSISTORS/1GummelPoon/GP_DOCU.pdf

6. ### John LarkinGuest

Seems to me that actual device leakage currents are somewhat larger
(as in, say, 1e6 times larger) than is suggested by a value of Is that
explains the forward curves.

John

7. ### Jim ThompsonGuest

That's why I put "leakage" in quotes... it's called "leakage" in the
literature, but real "leakage" is due to crystal misalignments,
extraneous dopants and just plain dirt.

...Jim Thompson