Schottky barriers and ohmic contacts

Not all diode junctions involve contacts between different doping types of the same semiconductor ( homojunctions ). The remainder, junctions between dissimilar materials or heterojunctions , are generally divided into metal/semiconductor and semiconductor/semiconductor junctions. We consider metal/semiconductor heterojunctions first. A sufficiently degenerate semiconductor behaves essentially as a metal and produces results nearly identical to the metal/semiconductor behavior. Metal/semiconductor junctions turn out to have either linear (ohmic) or diode-like current voltage characteristics and are called ohmic contacts or Schottky diodes, respectively. 

Experimental measurements generally determine the work function or other related doping-dependent quantities. Careful conversion to the doping-independent value is therefore needed to determine the electron affinity. The measurements are difficult and many factors can affect the outcome. Therefore, electron affinity values have often been the source of significant debate in the experimental literature. Real device 

When the direction of electron flow is into n-type material or out of p-type material, it increases the majority carrier concentration of the semiconductor. This increases the conductivity of the semiconductor near the junction and is described as ohmic because the junction acts only as a resistor. Such junctions are desired for contacts to microelectronic devices. Unfortunately, it is often impossible to find a metal of sufficiently high or low work function to arrange the electron flow in the correct direction to obtain an ohmic contact, especially to wide-gap semiconductors and organic electronic materials. 

The current voltage curve for the resulting metal-semiconductor junction has the same form as a homojunction diode  

Note that nowhere in Equations 3.38 or 3.39 do minority carrier properties appear. A very significant advantage of this type of diode is that its operation does not depend upon the minority carrier properties of the semiconductor. The only carriers injected across the junction are majority carriers. 

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