Making
p- and n-type
Material
T
he most common way of making p-type or n-type silicon material is to add an element that has an extra electron or
is lacking an electron. In silicon, we use a process called “doping.” We’ll use silicon as an example because
crystalline silicon was the semiconductor material used in the earliest successful PV devices; it’s still the most widely
used PV material, and, although other PV materials and designs exploit the PV effect in slightly different ways, knowing
how the effect works in crystalline silicon gives us a basic understanding of how it works in all devices.
An Atomic Description of Silicon
All matter is composed of atoms. Atoms, in turn, are composed of positively charged protons, negatively charged
electrons, and neutral neutrons. The protons and neutrons, which are of approximately equal size, comprise the closepacked central “nucleus” of the atom, where almost all of the mass of the atom is located. The much lighter electrons
orbit the nucleus at very high velocities. Although the atom is built from oppositely charged particles, its overall charge
is neutral because it contains an equal number of positive protons and negative electrons.
The electrons orbit the nucleus at different distances, depending on their energy level; an electron with less energy orbits
close to the nucleus, whereas one of greater energy orbits farther away. The electrons farthest from the nucleus interact
with those of neighboring atoms to determine the way solid structures are formed. The silicon atom has 14 electrons, but
their natural orbital arrangement allows only the outer four of these to be given to, accepted from, or shared with other
atoms. These outer four electrons, called “valence” electrons, play an important role in the photovoltaic effect.
The Silicon Molecule
Large numbers of silicon atoms, through their valence electrons, can bond together to form a crystal. In a crystalline
solid, each silicon atom normally shares one of its four valence electrons in a “covalent” bond with each of four
neighboring silicon atoms. The solid, then, consists of basic units of five silicon atoms: the original atom plus the four
other atoms with which it shares its valence electrons.
The solid silicon crystal, then, is composed of a regular series of units of five silicon atoms. This regular, fixed
arrangement of silicon atoms is known as the “crystal lattice.”
Introducing Phosphorus and Boron
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 (because phosphorus 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.
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.
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, boron, which has three valence electrons, is 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).
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. 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 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, phosphorus, or aluminum, to produce a wide range of
n- and p-type materials.
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