Moving electrons through material

The ability of a material to conduct current ( conductivity ) is directly proportional to the number of free ( loosely held ) electrons in the material. Good conductors, such as silver, copper, and aluminum, have large numbers of free electrons and have very low resitivies (meaning electrons can move with out much impediment in the material).

Semiconductor materials lie in the range between these two extremes. Pure germanium has a resistivity of 60 ohm-centimeters. Pure silicon has a considerably higher resistivity, on the order of 60,000 obm-centimeters. As used in semiconductor devices, however, these materials contain carefully controlled amounts of certain impurities which reduce their resistivity to about 2 ohm–centimeters at room temperature.

The introduction of carefully controlled amounts of impurities is called doping.



Doping Semi Conductor Material

To substantially reduce the resistance in silicon, we introduce other atoms. Shown below is a schematic of a silicon crystal that is "doped" with phosphorus atoms at certain locations

Particular forms of doping would look like this



Why phosphorus?

Well the phosphorus atom has five outer or valence electrons, instead of the four which silicon has. In a lattice composed mainly of silicon, the extra electron associated with the phosphorus atom has no "mating" electron and so is only weakly bound to the phosphorus atom. Just a small amount of thermal energy can liberate this electron so it can wander freely around the silicon atom. In this way, introducing few P atoms can greatly increase the amount of available free electrons, which in turn is responsible for greatly lowering the resistivity of silicon.

In our schematic theme we have been using, the cups labeled P on them are the additional allowed states that now exist due to the introduction of the phosphorus atom into our silicon crystal. The key here is that the P-cups are located at an energy scale near the top of the band gap - thus just a little extra energy allows the P-cup electrons to move to the conduction band. Now, whenever one of these electrons leaves the phosphorus atom, it leaves behind a positively charged nucleus (represented by the + sign in the figure above). In this sense, the phosphorus atoms act as donors as they donate an electron to the conduction band and leaving behind a positive charge at fixed locations in the lattice.

What makes this all work is that, the actual amount of phosphorus that needs to be added to make all this work is incredibly small.

Since there are 10²⁴ silicon atoms per cubic centimeter of silicon this means that only one in every 10 million silicon atoms has to be changed into a phosphorus one to reduce the resistance down to about 1 ohm-centimeters.



This is the real power of semiconductors. You can make dramatic changes in their electrical properties by the addition of only minute amounts of impurities. This process is called "doping" the semiconductor. It is also one of the great challenges of the semiconductor manufacturing industry, for it is necessary to maintain fantastic levels of control of the impurities in the material in order to predict and control their electrical properties. Hence, the clean room and the need to ramp up production facilities.