I would like to know a simple explanation for the term 'Ohmic Contact'.I looked in wikipedia but I couldn't get what actually it means.I couldn't even get what do you mean by non rectifying junction.Could anyone help me.
Electronic – Why does an Ohmic contact contain non rectifying junction
pn-junction
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First of all, it is wrong(misleading) to refer P type as positively charge and N type as negatively charged, both P type and N type are neutral in nature, however it is right to say that P type contains free charge carriers in form of holes and N type contains carriers in form of electrons.
Secondly, a depletion region/layer is already in picture from the beginning i.e. while fabricating P-N Junction, due to abrupt change in concentration of electrons/holes in two types of materials, electrons from N type material and holes from P type material diffuses to P type and N type materials respectively. This leads to formation of depletion region/layer which contains ions (Positive and negative ions), not electrons or holes. These ions are generally immobile in nature. And in this way, region nearby p-n interface loose its neutrality and becomes charged.Since space charges in depletion region leads to an electric field which opposes further movement of electrons and holes due to process of diffusion, P-N junction reach to a state of equilibrium.
Next thing is, again, applying a positive current is somehow misleading, we apply positive voltage to P material and negative voltage (zero voltage) to N material, and when battery connected this way, its Forward Bias/Biasing.With a battery connected this way, the holes in the P-type region and the electrons in the N-type region are pushed toward the junction. This reduces the width of the depletion zone. The positive charge applied to the P-type material repels the holes, while the negative charge applied to the N-type material repels the electrons. As electrons and holes are pushed toward the junction, the distance between them decreases.Only majority carriers (electrons in N-type material or holes in P-type) can flow through a semiconductor for a macroscopic length. With this in mind, consider the flow of electrons across the junction. The forward bias causes a force on the electrons pushing them from the N side toward the P side. With forward bias, the depletion region is narrow enough that electrons can cross the junction and inject into the P-type material. However, they do not continue to flow through the P-type material indefinitely, because it is energetically favorable for them to recombine with holes. Although the electrons penetrate only a short distance into the P-type material, the electric current continues uninterrupted, because holes (the majority carriers) begin to flow in the opposite direction. The total current (the sum of the electron and hole currents) is constant in space, because any variation would cause charge buildup over time
Therefore, the current flow through the diode involves electrons flowing through the N-type region toward the junction, holes flowing through the P-type region in the opposite direction toward the junction, and the two species of carriers constantly recombining in the vicinity of the junction. The electrons and holes travel in opposite directions, but they also have opposite charges, so the overall current is in the same direction on both sides of the diode, as required.
Same analogy can be obtained/derived for Reverse Bias situation as well.
I think i answered most of the questions of yours, rest you can answer by yourself.
Though, i will also suggest you to go through some standard book (Streetman and Banerjee is good) to understand concepts fully, once you understand them, there will be no doubt in future as well, but its really difficult to understand P-N junction or physics concepts through a 1/2 hour video.
In hole movement, the particles that are moving are still electrons. When an electron moves to the conduction band (i.e. at any temp above 0 K), there is an empty state that is created in the valence band that was originally occupied by the electron. This empty state is the hole. If another electron from the valence band moves to occupy this hole, it creates another hole one atom over. A chain of such electron movements could be thought of as a hole moving.
I think this is simplifying things quite a bit, but it gets the gist, and I don't really understand the more advanced version. :)
Edit : Actually, I'll try to include the advanced version, by just quoting from the excellent Daniel Mittleman from this awesome thread on the very related topic of 'Are holes real?', since most people probably can't access that thread.
... no such thing as a lone electron or, ... a lone hole - inside any solid. Any charged particle will interact with all of the other electrons, and nuclei, in the solid ...
... [With] These interactions .. taken into account ... One ends up describing what are called 'quasi-particles', which are excitations of the solid that, in some way, resemble a lone electron or a lone hole ...
It is not just semantics - [electrons and holes] are both equally real excitations of the many-body state of the solid.
So, while the simple picture generally enough for getting the drift of things, there's more beyond it, and there's a bit more to holes than just the absence of electrons.
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Best Answer
When ever you put two material together that have a different work function, they will form a junction which acts as a diode/rectifier. So the equivalent circuit diagram becomes "A' or "B" below:
simulate this circuit – Schematic created using CircuitLab
Either of those scenarios yields a device that isn't very useful, in fact it can't be used. Ideally you want to make sure that you contact to the device looks like a resistor (ideally a zero ohm resistor, or at very least a very low value resistor) and NOT a diode. I'm making the assumption here that you'll understand why having diodes in series with your transistor is a bad, bad thing.
The interesting aspect of this question is: Just HOW is a metal attached to a semiconductor such that is does not form a shottky barrier (i.e. a diode) which a metal semi-conductor system will usually form.
The background theory is very interesting but it requires an understanding of Quantum mechanics, semi-conductor theory (fermi levels etc.) and processing knowledge.
For a N-Type contact, implants are formed in such a way that at the surface the semiconductor is degenerate (i.e. Metal like) so a Metal, Metal contact does not have a barrier. Often these system will have silicides on them (refractory Si-Metal system)- to decrease the resistance) but that is not necessary here. As the current moves into the substrate it experiences less and less doping (the band edges are gently distorted) until the carriers make it into the semiconductor.
For an P-Type contact, there are Quantum mechanical effects that allows for a tunnelling across the barrier (formed by the band gap ~ 1.12 eV), the rest of the trip from the conduction band edge up to the metal work function is explained by the same mechanism as used in a N-Type device (degenerately doped). the Qm -effect is accomplished via a hyper abrupt metal - P material junction ( a few atoms deep at most).