forward voltage, reverse current

[Jacky Luk]

I'm trying so hard to imagine of such a transistor circuit when it is driven
into saturation."The collector-base pn junction acquires a forward voltage
bias even though there is still a reverse current flowing through it", I'm
trying to visualize it, how can a foward voltage applied across to the
terminals while there is a reverse current over it. Any hints would be
greatly appreciated!
Thanks
Jack

 

[John Larkin]

On Fri, 27 Aug 2004 19:27:29 +0800, "Jacky Luk" <jl@knight.com> wrote:

I'm trying so hard to imagine of such a transistor circuit when it is driven
into saturation."The collector-base pn junction acquires a forward voltage
bias even though there is still a reverse current flowing through it", I'm
trying to visualize it, how can a foward voltage applied across to the
terminals while there is a reverse current over it. Any hints would be
greatly appreciated!
Thanks
Jack

In "transistor action" the collector-emitter current is arguable
backwards as far as the collector is concerned. The transistor action
shoots charge into what would have been a back-biased c-b diode. So a
normal non-saturated conducting transistor has a back-biased c-b
junction but still has collector current. So somebody applies the
words "reverse current" to this situation (personally, I wouldn't,
because there's no c-b junction current. There's transistor current
from c to e.)

OK, let Vc get very low, in saturation. Now the collector voltage is
below the base voltage, so the b-c junction gets forward biased. Now
base current is flowing into *both* the emitter and the collector. So
some wise guy can claim the the collector is getting forward and
reverse current at the same time. But the b-c current is positive
(talking NPN here, of course) in saturation, and there is positive c-e
current, too, but the statement

"The collector-base pn junction acquires a forward voltage
bias even though there is still a reverse current flowing through it"

is silly.


John

 

[Joe McElvenney]

Hi,

I'm trying so hard to imagine of such a transistor circuit when it is driven
into saturation."The collector-base pn junction acquires a forward voltage
bias even though there is still a reverse current flowing through it", I'm
trying to visualize it, how can a foward voltage applied across to the
terminals while there is a reverse current over it. Any hints would be
greatly appreciated!

Digging into my creaky old memory banks, I seem to remember that under
saturation minority carriers pile up in the base region. The distribution of
these being proportional to the emitter current at one end and the collector
current at the other. As the first is greater than the latter, a density
gradient forms across the base region down which the carriers diffuse into the
collector. In other words, diffusion currents are not necessarily respecters
of bias polarity.


Cheers - Joe