Here is a reference that goes over the theory in some detail: http://ecee.colorado.edu/~bart/book/book/chapter5/ch5_1.htm
excellent reference. he really throws in the kitchen sink ( including things like 'early effects' and all the other oddities most people have never heard about)
now to come back to the debate.
for the sake of clarity can we define a few things ?
- an electron has a charge
- the core (nucleus) of an atom also has a charge
- for a condustor ( and most atoms): the charge of the electrons cancels out the charge of the nucleus so the net charge is zero.
- a moving charge is a current .
- the number of charge carriers per second is a quantity we call ampere.
right ?
now, before we delve into transistors let's take a look at something more simple.
: an electric conductor , simplified, a wire. we will start with an ideal wire first ( no resistance)
A wire is made from a material with movable electrons. a wire' in rest ' has enough electrons to fill the outer shell of all the atoms in the wire.
a wire in rest has no charge across it. There are charges moving internally as the electrons spin around their nuclei but, since the sum of the electron charge is equal to the sum of the nuclei charges the net effect is zero. it all cancels each other out.
now, let's push in an electron on side of the wire shall we ?
what happens : we unbalance the system. there is now an extra, unbound electron, in this stub of wire. so something has to give... none of the existing atoms want anything to do with this electron. so , as soon as there is a pathway for it to be shoved out of the wire it will be.
close the loop and the wire will spit out an electron for every electron shoved in the other end.
as electrons are charge carriers and moving charge carriers is called 'current '.... you got the idea right ?
of course in order to get an electron to move we need to apply a field.
now. step two. let's take a resistor. ( or , a not so ideal wire if you would like )
the principle still stands : one electron in is one electron out. electrons are particles , matter. laws of physics state you cannot create or destroy matter. ( unless in nuclear reactions). if i send 20 electrons in that resistor , 20 will come out. none extra are created and none are destroyed in this process.
Electrons are accelerated by the applied electric field. strenghten the field ( up the voltage) and you up the acceleration. Electrons only move at the speed of light in a vacuum. in material the run slower.
so an electron entering the resistor is accelerated by the applied field. so it gains kinetic energy. as it starts traveling into the wire it will sooner or later hit an atom and lose some of its kinetic energy. enrgy also cannot be created or destroyed , only converted. The electron is still propelled by the initial push it got from the external field so it keeps going towards the exit , all the while slamming into several atoms and losing some of its kinetic energy to the atom. this causes the atoms of the wire to jiggle faster thus producing ... heat !
but, eventually all twenty electrons sent in , will exit as none of the atoms int he resistor want them ! their charges are in balance.
In short we have been dealing with materials called 'conductors' : materials where the charges are in balance. Every proton has an electron so the net charge is zero. shoe in an electron and it needs to come out.
Now it gets complex :Semiconductor.
a semiconductor is a material where the charges, for a single atom, are NOT in balance. there are not enough electrons to satisfy the number of protons (sometimes called holes) in the core.
Now, atoms are pretty clever little buggers and, in an INTRINSIC semiconductor, they will arrange themselves in a crystal like lattice so they can 'share' some of the electrons on their outer shell. As the electrons are spinning they no longer run around a single core but actually run in figure eight patterns around two cores. so now the atom cores 'see' enough electrons.
if you take a such a material and shove in an electron on side , one will roll out the other side. in an intrinsic semiconductor material the charge is not balanced absolutely ( if you were to count the number of electrons and number of protons there is a mismatch ) , but on a per-atom level they are balanced. as each atom has enough electrons for its number of protons. just not all the time as the electrons keep circling mulitple cores. note : this is heavily simplified. in reality it is more a brownian motion , 'electron soup' if you will.
a lump of intrinsic semiconductor material has no net charge and behaves like a wire. shove an electron in and one will come out. there is no need for the extra electron. the atom pairs sharing have no use for it. Sure, one of the two atoms sharing a single electron could absorb it and be content but then the other atoms is left with an electron missing so it is going to seek another sharing partner. Resulting in there being now one electron too many in the newly formed sharing couple. So that electron is spit back out.
Now we are going to alter this intrinsic material by shoving in impurities. called 'doping'. we can shove in a material that has spare electrons or a material that doesn't have enough electrons.
doping the intrinsic semiconductor starts an electron exchange process.
if you feed extra electrons, one of the paired atoms will absorb it temporarily causing its bonded twin to be unhappy now and seek another partner. three is company .. , also in electron land and The newly partnered atoms now have an excess electron being spat out . This process goes on and on. These 'traveling electrons' hop from bonded atom pair to bonded atom pair and are called free electrons. this is N doped material
if you feed a material with shortage of electrons ( holes ) the same happens, only now does the 'vacant spot' ( the hole) travel around. the lacking material snatches an electron from a bonded atom pair , one atom in the bond is now unhappy and goes in search for an electron it can borrow from an adjacent pair. causing a marital rift in that pair , divorce and another atom going in search for an electron ... so the 'vacant spot' also travels.
now. if we take such a doped piece of material and we shove in electrons things happen differently. i forgot the metallic - oped semiconductor process operation. it is the shottky effect ( a shottky diode is essentially a diode made from one doped material ( p material) and a simple conductor not relevant for this discussion. i;d have to read up on that again.
but , if we take a lump of P material and a lump of N material and we slam them together something happens. in the contact surface the free electrons from the N material will 'fall' in the holes of the P material making atomic bonds (the recombination zone) . since in that region there are now no mobile charge carriers, this is effectively an isolator !
now, if we apply a field to such a diode. , in the right direction , we will be able to send electrons into it , but none will come out the other side ! you will need to keep increasing the external field until you hit the point where the recombination zone is gone. essentially this zone travels in the material. add some electrons in the N material by applying a field and they will push the recombination zone to the exit ( other end) once you hit enough potential the recombination zone is gone and electrons flow freely. this potential is called the forward voltage. ( 0.6 volts in typicla silicon diode) and is depending on the levels in the valence bands of the used material.
now. if we make a bipolar transistor we get two such recombination zones. by applying pressure to the base-emittor (biassing) we get that thing to go in conduction. now we have established an electron flow there. ( a current). that electron flow now pushes out the second recombination zone and allows collector current to flow. ( i have described that process already once in detail here on the forum, i'm not going to repeat it here )
change the intensity of the emittor-base current and the intensity of emittor-collector current changes. ( electron model currents )
so , to put the dot's on the 'i': for a properly biased bipolar transistor ( bipolar as it used both electrons and holes as charge carriers) the collector current has a relation to the base current. once base current flows, both recombination zones are gone and collector current flows. send in more electrons to the base and more will flow toward collector. depending on the strength of the doping there is an amplification factor.
ergo : such transistors are current controlled.
internal material resistance cause losses, so you get all kinds of side effects such as temperature dependency of certain factors . there is also charge accumulation due to uneven material properties ( microcracks in the lattice , uneven distribution of the impurieties etc ) so you get all other kinds off effects. if you flood in enough electrons to completely collapse the recombination zone you are in 'saturation' and then other effects kick in as well. these are well understood and described in various models and equations. but none of that erases the base principle :
make base current flow to get collector current. increase base current to get more collector current. so : current controlled current ampliefier.
now do you get it ?
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