Holes in semiconductor and why we need them

[lastwarrior]

Hello guys,
can you please  help me to understand why we need holes to describe the semicond. operation? I mean I know that it is a concept as many others...I'm refrashing and learning things that were passed and for that I have a lot of books that are preaty useless in explanation "what is going on".

Here the example:
A p-n junction. It has two type materials (and this is clear for me as good as how they are obtained). When combaine this we get a junction where a depletion region is formed. Where I'm stuck is when holes are sayd to flow simultaneously with electrons (of course in opposite directions).
In my understanding If I choose the concept of conventional current flow, why do I need to consider the minority electron flow from p type while  I'm using the convention? Please try to use less calculus 'cause my math skill is at begginer lvl (it will be the next skill under consideration but for now I want to get basics of the electronics to be able to design, troubleshoot and have fun with circuits at low to mid lvl of complexity). In a simple pn junction it is not to complicated to understand (my understanding is that holes flows from p to n type trough the barrier in forward bias and minority holes from n to p in reverse bias in form of saturation current), but when I'm getting to transistors and the book says that currents of two different carrier types flows simultaneously and do it's things it confuses me.

Offtopic que:
Also  there is a point in common emitter configuration that I realy cannot find any explanation. (Iceo is measured when base terminal is not connected, but how the junctions respond to the applied Vce with open base?)

[rstofer]

Where would the electron land if there wasn't a hole waiting for it.  Having moved to a hole, the electron left a hole behind for the next electron behind it.  So, if you think about electrons moving across the page from left to right, the holes are moving right to left.

Sure, there is a lot of physics behind this and I have forgotten all of it.  I use conventional current flow (plus to minus) and don't worry about the physics.

[JS]

  My semiconductor theory is a bit rusty, but I'll try it anyway.

  Depending on the type of material you have a lot of places for electrons to fall in or a lot of electrons and few empty places which need to move around, those free spaces are called holes and treated as a charged particle instead of an empty space just to make things easy and symmetric. That difference, of having mostly empty spaces for electrons to go around or mostly electrons which need to move for the empty spaces to swap places determines the difference of P and N type materials.
  Now there are some difference between holes and electrons, besides the charge sign, for example their mass, holes have higher mass than electrons. Holes themselves doesn't have any mass at all, you can't weight them if you put a lot of them in a scale, but if you look at the displacement of the empty space left by the moving electrons as opposed to an electron moving around, a hole needs more energy to reach at a certain speed compared to an electron.
E=(m*v^2)/2
  Then more energy for the same speed means more mass.

  This is a way of describing the process, rather than having a physical object called hole. If you imagine a thing called hole you can (kind of) understand what's going on and predict what would happen, it just makes the math work. So we assign properties to that imaginary object that they don't actually have (like a mass)

About the Iceo, by definition is the current from collector to emitter when the base current is zero, so you leave the base unconnected to ensure there is no current flowing at the base, nothing wrong with that. In an ideal world Ic=ß*Ib, so when Ib=0, Ic=0, as the BJT transistor isn't an ideal component there's still some current flowing at the collector even if there's no current at the base, that's what that parameter is telling you. Let's say you have a circuit that needs very low current when the transistor is off, then you'd look a device that has low Iceo so that current isn't a problem for you.

JS

[T3sl4co1l]

It's not necessarily a current flow mechanism -- electron current works just fine in metals, after all.

The trick is that a semiconductor's electrons are stuck in the valence band, immobile.  When you give them some minimum energy (the band gap energy), they are temporarily free to move (the energy promotes them to the conduction band).  As it happens, the holes thus formed in the valence band, are also free to move, though not quite as easily (hole mobility in silicon is about 2.5 times poorer than electron mobility; most materials have a much worse ratio, like GaAs where hole mobility is something like 20 times worse -- which is why GaAs FETs are always N-channel).

Dopants add new energy levels within the band gap.  This allows thermal energy (~26meV, versus gap ~1eV) to promote electrons away from the valence band, thus creating holes -- P type doping.  This also works for dopant levels near the conduction band, giving free electrons -- N type doping (though I forget why you don't need free electrons up there already?).

Holes and electrons can coexist just fine.  There is a tendency for them to recombine, of course; this occurs spontaneously (when both are present), or at dopant atoms or defects (where the energy barrier is smaller = more probable).

If nothing else, there is an intrinsic doping.  This is due to thermal energy very rarely kicking electrons to the conduction band, without dopants.  Rare, as in, a chance on the order of 1e-10 per atom.

This effect, and the diffusion rate of free charges, limits the distance they can travel, to about a dozen micrometers in silicon.  This means you can't make a BJT (or at least, a worthwhile one) over longer distances -- it's no surprise that it took so long for transistors to be developed, as there was no good way to position point-contact whiskers this precisely (you can make a schottky transistor with metal whiskers and a homogeneous doped crystal).

There's also field effect behavior, where an electric field draws charges to the surface.  This is absolutely normal, for instance in metals it causes some electrons to move away from the positive end and towards the negative end; but metals have a huge excess of free electrons, so the surface is always just as conductive as ever.  Only with semiconductors, where the charge density is small enough to be useful, can you see the field influencing material conductivity.  But only a very thin surface layer (within the Debye scattering length, give or take) is affected.  Surfaces are also sensitive to contamination, and chemical purity (not just the bulk material, but every single thing that ever touches that surface!) has to be very good.  This is why it took until the 70s to commercialize power MOS and CMOS ICs -- it took decades for the chemistry, materials science, and processing equipment to develop to the point where such fine structures could be built up to practical, macroscopic devices.

Tim

[Brumby]

The idea of a "hole" moving may seem strange - but it is as simple as this:

Where would the electron land if there wasn't a hole waiting for it.  Having moved to a hole, the electron left a hole behind for the next electron behind it.  So, if you think about electrons moving across the page from left to right, the holes are moving right to left.

Quite simply, a "hole" is a place an electron can move to.  A "hole" is "made" (left behind) when an electron leaves that place to go somewhere else.