Understanding Phosphorous, Boron and Other Semiconductor Materials

Introducing Phosphorous

The process of "doping" introduces an atom of another element into the silicon crystal to alter its electrical properties. The dopant has either three or five valence electrons, as opposed to silicon's four. Phosphorus atoms, which have five valence electrons, are used for doping n-type silicon (phosphorous provides its fifth, free, electron).

A phosphorus atom occupies the same place in the crystal lattice that was occupied formerly by the silicon atom it replaced. Four of its valence electrons take over the bonding responsibilities of the four silicon valence electrons that they replaced. But the fifth valence electron remains free, without bonding responsibilities. When numerous phosphorus atoms are substituted for silicon in a crystal, many free electrons become available. Substituting a phosphorus atom (with five valence electrons) for a silicon atom in a silicon crystal leaves an extra, unbonded electron that is relatively free to move around the crystal.

The most common method of doping is to coat the top of a layer of silicon with phosphorus and then heat the surface. This allows the phosphorus atoms to diffuse into the silicon. The temperature is then lowered so that the rate of diffusion drops to zero. Other methods of introducing phosphorus into silicon include gaseous diffusion, a liquid dopant spray-on process, and a technique in which phosphorus ions are driven precisely into the surface of the silicon.

Introducing Boron 

Of course, n-type silicon cannot form the electric field by itself; it's also necessary to have some silicon altered to have the opposite electrical properties. So it’s boron, which has three valence electrons, that’s used for doping p-type silicon. Boron is introduced during silicon processing, where silicon is purified for use in PV devices. When a boron atom assumes a position in the crystal lattice formerly occupied by a silicon atom, there is a bond missing an electron (in other words, an extra hole). Substituting a boron atom (with three valence electrons) for a silicon atom in a silicon crystal leaves a hole (a bond missing an electron) that is relatively free to move around the crystal.

Other semiconductor materials.

Like silicon, all PV materials must be made into p-type and n-type configurations to create the necessary electric field that characterizes a PV cell. But this is done a number of different ways depending on the characteristics of the material. For example, amorphous silicon's unique structure makes an intrinsic layer or “i layer” necessary. This undoped layer of amorphous silicon fits between the n-type and p-type layers to form what is called a "p-i-n" design.

Polycrystalline thin films like copper indium diselenide (CuInSe2) and cadmium telluride (CdTe) show great promise for PV cells. But these materials can't be simply doped to form n and p layers. Instead, layers of different materials are used to form these layers. For example, a "window" layer of cadmium sulfide or another similar material is used to provide the extra electrons necessary to make it n-type. CuInSe2 can itself be made p-type, whereas CdTe benefits from a p-type layer made from a material like zinc telluride (ZnTe).

Gallium arsenide (GaAs) is similarly modified, usually with indium, phosphorous, or aluminum, to produce a wide range of n- and p-type materials.