2.2 Introduction to the mos transistor and mosfets

We now move on to another three terminal device - also called a transistor. This transistor, however,works on much different principles than does the bipolar junction transistor of the last chapter. We will now focus on adevice called the field effect transistor , or metal-oxide-semiconductor field effect transistor or simply MOSFET.

In [link] we have a block of silicon, doped p-type. Into it we have made two regions which are doped n-type. To each of those n-typeregions we attach a wire, and connect a battery between them. If we try to get some current, I , to flow through this structure, nothing will happen, because then-p junction on the RHS is reverse biased, i.e., the positive lead from the battery going to the n-side of the p-n junction.If we attempt to remedy this by turning the battery around, we will now have the LHS junction reverse biased, and again, nocurrent will flow. If, for whatever reason, we want current to flow, we will need to come up with some way of forming a layerof n-type material between one n-region and the other. This will then connect them together, and we can run current in oneterminal and out the other.

To see how we will do this, let's do two things. First we will grow a layer of SiO ₂ (silicon dioxide or silica, but actually refered to as "oxide") on top of the silicon. To do this the wafer is placed in an oven under an oxygen atmosphere, andheated to 1100 °C. The result is a nice, high-quality insulating SiO ₂ layer on top of the silicon). On top of the oxide layer we then deposit a conductor, which we call the gate. Inthe "old days" the gate would have been a layer of aluminum; hence the "metal-oxide-silicon" or MOS name. Today, it is much more likely that a heavily doped layer of polycrystallinesilicon (polysilicon, or more often just "poly") would be deposited to form the gate structure. Polysilicon is made from the reduction of a gas, such as silane (SiH ₄ ), [link] .

The silicon is polycrystalline (composed of lots of small silicon crystallites) because it is deposited on top of theoxide, which is amorphous, and so it does not provide a single crystal "matrix" which would allow the silicon to organizeitself into one single crystal. If we had deposited the silicon on top of a single crystal silicon wafer, we would have formed asingle crystal layer of silicon called an epitaxial layer. This is sometimes done to make structures for particularapplications. For instance, growing a n-type epitaxial layer on top of a p-type substrate permits the fabrication of a veryabrupt p-n junction.