andrew_h wrote:
> I am new to electronics - I have learned heaps and am enjoying every new
> thing I'm learning.
(blah blah blah)
> Something that has been confusing me no-end, and I just can't seem to
> grasp, is how a TRANSISTOR works!
> I have read many explanations, but they are confusing and vauge.
(blah blah blah)
> Any help with this would be greatly appreciated ... this is really
> proving to be a stumbling block ...
Here is the not exactly right approximate run through.
There are two PN junctions in a transistor, one is the emitter to base
junction and one is the base to collector junction. Normally, the
base to collector junction is reverse biased, to produce an insulating
layer between the base and collector with no movable charges.
Lets pick a polarity... NPN.
So the collector has a positive voltage with respect to the base, so
the doped in electrons in the collector N material are attracted away
from the base and the holes in the base are attracted away from the
collector, leaving just insulating silicon between them.
When the base emitter junction is slightly forward biased (emitter
relatively more negative and base relative more positive), the doped
in electrons in the emitter are repelled toward the base, and the
holes doped into the base are repelled toward the emitter. At about a
half volt forward bias, the holes and electrons begin to find each
other and the electrons tend to jump into the holes and both
effectively dissappear. However, a well made transistor has the
emitter much more highly doped than the base, so more electrons get
pushed into the base than holes get pushed into the emitter.
So the holes that get pushed into the emitter are annihilated very
quickly, but the electrons that get pushed into the base have to hunt
around a while beforo they dissappear.
The small positive base voltage causes these electrons to wander
toward the base lead (the most positive voltage around them). But the
base layer is very thin, and the electrons drift rather slowly in that
direction. If the temperature was very low, this is about all that
would happen, and the forward biased base emitter junction would have
almost no effect on the collector curret.
But at normal ambient temperatures (well above absolute zero) the
movement of the electrons is randomized by the thermal energy in the
silicon, so they stagger quite randomly, with only a little progress
toward the base lead. And since the base layer is so thin, most of
them will never make it to the base lead. They will fall off the
cliff into the highly stressed charge-empty reverse biased base
collactor junction. There, instead of wandering in a drunken stagger
through a very small electric field (volts per meter) they will whoosh
out of the reverse biased junction, because it is much more highy
stressed with e-field. They become collector current.
The more strongly you forward bias the base emitter junction, the more
electrons are pushed into the base layer, and the more stagger over
the cliff into the collector, though there will also be more that make
it out the base lead. Over a wide range of collector current, the
collector be a fairly fixed multiple of the base current. This is the
transistor's current gain or beta.
So the electrons are drunks being encouraged with a slightly tilted
sidewalk onto a slightly down hill, vibrating curb, next to street
that tilts away from the curb, very steeply. Most never make it to
the end of the curb, but fall onto the street where they slide into
traffic.
This is the drunken bum on a crazy street transistor analogy.
