Jiks said:
The actual Process Involved in amplification produced by an amplifier.
I mean just by keeping a high resistance in the path of a low current,
does not seem to explain it [ more over seems wierd ] .... i am not
looking forward to a steriotypic answer... but an explanation that
explains why current will flow towards collector from base inspite of
high resistance rather than the low resistance path that allows it to
just flow back to emitter through base !
I have retrieved the description I mentioned and edited it a little,
for your enjoyment.
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 operated in reverse bias, to produce an
insulating layer between the base and collector, with no movable charges.
Lets pick a polarity... NPN.
The collector is operated with 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.
Hence an insulating layer blocking current between collector and base.
When the base-emitter junction is slightly forward biased (emitter
relatively more negative than the base), 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 disappear.
However, a well made transistor has the emitter much more highly
doped than the base, so many more electrons get pushed into the base
than holes get pushed into the emitter.
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 before they disappear. I am not implying that these
electrons have a gal, but they are bounced around randomly by the
thermal energy in the silicon, so that have no choice but to "hunt
around".
The small positive base voltage causes these electrons to wander,
slowly, toward the base lead (the most positive voltage around them,
so there is a very small electric field that makes their random
wandering have a small net drift in that direction). If the
temperature was very low, this is about all that would happen, and the
forward biased base emitter junction would produce almost no collector
current (the current gain would be very low).
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, making only slow progress
toward the base lead. And since the base layer is very thin, most of
them will never make it to an exit via the base lead. They will fall
off the cliff into the highly electric field stressed, charge-empty
reverse biased base-collector junction. There, instead of wandering
in a drunken stagger through a very small electric field (volts per
meter) they will whoosh across the reverse biased junction, and become
collector current. Note that increasing the collector voltage
produces almost no increase in the collector current, except that more
collector voltage depletes a tiny bit more of the base region,
narrowing the path the electrons are wandering along, so, slightly
increasing their chance of falling off.
The more strongly you forward bias the base emitter junction, the
higher the density of electrons pulled into the base layer, and the
more of them will drop off the cliff into the collector e-field
region, though there will also be more that make it out the base lead.
Over a wide range of collector current, the collector current be a
fairly fixed multiple of the base current. This current ratio is
called the transistor's current gain or beta.
So the electrons are like drunks being encouraged to leave the flop
house and walk down a narrow sidewalk (perhaps by a sale at the liquor
store, where the discount is analogous to base-emitter positive bias)
but there is an earthquake going on (thermal vibration), that causes
most of them to fall off the curb. The street below tilts down very
steeply (collector voltage), so that all who fall off the curb slide
into traffic, are swept away, and never make it back to the sidewalk
(base). Only a small fraction of those who attempt the walk alongside
the street, arrive at their intended destination (the liquor store
that was their original motivation). Random variations in their paths
lead most of them into the street (collector).
