Why does an Ohmic contact contain non rectifying junction?
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 &ndash; 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).