Doped Materials

Intrinsic or pure semiconductors are useless. They are neither
good conductors nor insulators, and their resistance is dependent on
temperature. properties of the intrinsic material can be modified by introducing foreign
material or impurities into the intrinsic crystal. These impurities are called as
dopants. A crystal with an added dopant is referred to as an extrinsic semiconductor
or doped material. The amount of impurity added is generally small, perhaps in the
neighborhood of one part per million. The dopant may be added through a gaseous
diffusion process where the crystal is heated in an oven and the dopant added in
gaseous form. Over a period of time the impurities will diffuse or “seep into” the
target crystal. An alternate approach is ion implantation. In this method the
impurities are accelerated and quite literally smash into the target, dislodging and
replacing some of the original atoms in the crystal.

There are two different types of semiconductors possible. One is called N-type
material, and the other, P-type material. Unsurprisingly, the N stands for Negative
and the P stands for (you guessed it) Positive. N-type material is created by adding
pentavalent impurities, that is, a dopant with five electrons in its outer shell.
Examples include phosphorus, arsenic and antimony. In contrast, P-type material is
created by adding a trivalent impurity, one with three electrons in its outer shell.
Possible trivalent impurities include boron, gallium and indium.

N–Type Material

Figure 1.13 shows a model of a silicon crystal with a pentavalent impurity at its
center. Compared to an ordinary silicon atom that would have four electrons in its
outer shell, the pentavalent impurity creates an extra, or donor, electron. Thus, the
crystal has a net negative charge and is referred to as N-type material. The energy
level of the donor electrons is just below the bottom of the conduction band. In other
words, the difference between the donor level and the bottom of the conduction band
is much, much smaller than the band gap itself. Therefore it is relatively easy for
these donor electrons to jump into the conduction band, becoming free ionized
electrons and leaving behind ionized holes2.

An ion is an atom or molecule that does not have a neutral net charge, i.e., the numbers of
protons and electrons are not equal. If it loses electrons, resulting in a net positive charge,
it is called a cation. If it gains electrons resulting in a net negative charge it is called an
anion.

Compared to the undoped intrinsic crystal, the doped extrinsic crystal exhibits a
relatively high number of free electrons. As you might surmise, this enhances the
conductivity of the material, and the greater the doping level, the greater the
enhancement. Earlier it was mentioned that both electrons and holes can serve as
charge carriers. Because the number of free electrons is significantly larger than the
number of holes in N-type material, electrons in N-type material are referred to as
the majority charge carrier (or more simply, the majority carrier) while holes are
referred to as the minority charge carrier (or minority carrier).

The extra electrons add to the number of filled energy states and, being of higher
energy than the valence electrons, push the Fermi level to a higher value.
Remember, the Fermi level represents the point where 50% of states would be filled,
so if we add states above this, then the new 50% point must be higher than the
former level. This is illustrated in Figure 1.14. Note how close the donor level is to
the conduction band and that the Fermi level has been pushed up, away from the
valence band and closer to the conduction band. This will be of great significance in
up-coming discussions on semiconductor devices.

P–Type Material

In a similar manner, if we introduce a trivalent impurity, our crystal model now
features a hole; a location where an electron is lacking. For this reason, trivalent
impurities are sometimes called acceptors. The resulting crystal model is illustrated
in Figure 1.15.

The resulting situation is essentially the reverse of that of the N-type material. Figure
1.16 shows the energy band diagram for our new P-type material. In this case, the
Fermi level has been pushed down, closer to the valence band.

In P-type material, holes out number free electrons. Consequently, holes are referred
to as the majority carrier in P material while electrons take on the role of minority
charge carrier.

As with N-type material, the greater the amount of trivalent impurity added, the
greater the overall effect. By itself, a doped crystal can be used to create a resistor.
The resistivity of the material is a function of the doping level. By setting the cross-sectional
area, length and doping level, we can create well-defined resistor values. If
this was all we could do with semiconductors then we could say two things: first, the
solid state semiconductor revolution would not exist; and second, this text would be
very short. The interesting bits arrive when we combine both N- and P-type materials
into a single device, as we shall begin to see in the next articles.

conclusion

In this chapter we have examined the basic structure of atoms. This includes the
concept of electron shells and permissible energy states. We have used both the Bohr
model of the atom and the corresponding energy band diagrams.
Crystals such as silicon show a very ordered three dimensional structure that relies
on strong covalent bonds. The crystal tends to “fuzz” or broaden the permissible
energy levels into thicker energy bands. Further, the crystal exhibits a modest energy
gap, or band gap, between the valence band and conduction band. This gap is much
smaller than the gap seen in insulators, and therefore the material is referred to as a
semiconductor, being somewhere between a true conductor and a true insulator.
The electrical characteristics of a pure, or intrinsic, semiconductor crystal can be
altered by adding impurities or dopants. A doped crystal is referred to as an extrinsic
crystal. If a pentavalent dopant is added, there will be a surplus of electrons and a
raising of the Fermi level. The new crystal is called N-type material. In contrast, if a
trivalent dopant is added, there will be a surplus of holes and a lowering of the Fermi
level. The new crystal is called P-type material. In N-type material, electrons are the
majority charge carrier and holes are the minority charge carrier. In P-type material,
holes are the majority carrier while electrons serve as the minority carrier.

Reviews Questions

1. Describe the differences between a conductor, an insulator and a
   semiconductor.
2. Define the terms Fermi level, valence band, conduction band and band gap.
3. What is the fundamental difference between an intrinsic crystal and an
   extrinsic crystal?
4. What is meant by the term doping?
5. What is the effect of donor and acceptor impurities on the Fermi level?

References