n-TYPE AND p-TYPE
MATERIALS
Because
Si is the material used most frequently as the base (substrate) material in the
con- struction of solid-state electronic devices, the discussion to follow in
this and the next few sections deals solely with Si semiconductors. Because Ge,
Si, and GaAs share a similar covalent bonding, the discussion can easily be
extended to include the use of the other materials in the manufacturing
process. As indicated earlier, the characteristics of a semiconductor material
can be altered sig- nificantly by the addition of specific impurity atoms to
the relatively pure semiconductor material. These impurities, although only
added at 1 part in 10 million, can alter the band structure sufficiently to
totally change the electrical properties of the material. A semiconductor
material that has been subjected to the doping process is called an extrinsic
material. sureable importance to semiconductor device fabrication: n-type and
p-type materials. Each is described in some detail in the following There are
two extrinsic materials of immea subsections.
n-Type Material
Both
n-type and p-type materials are formed by adding a predetermined number of
impuri atoms to a s ilicon base. An n-type material is created by introducing
impurity elements th have five valence electrons (pentavalent), such as
antimony, arsenic, and phosphorus. Each a member of a subset group of elements
in the Periodic Table of Elements referred to as Gro V because each has five
valence electrons. The effect of such impurity elements is indicated Fig. 1.7
(using antimony as the impurity in a silicon base). Note that the four covalent
bor are still present. There is, however, an additional fifth electron due to
the impurity atom, wh is unassociated with any particular covalent bond. This
remaining electron, loosely boun its parent (antimony) atom, is relatively free
to move within the newly formed n-type mater Since the inserted impurity atom
has donated a relatively "free" electron to the structure: Diffused
impurities with five valence electrons are called donor atoms. It is important
to realize that even though a large number of free carriers have been es lished
in the n-type material, it is still electrically neutral since ideally the
number of tively charged protons in the nuclei is still equal to the number of
free and orbiting negat charged electrons in the structure.
The effect of this doping process on the
relative conductivity can best be described through the use of the energy-band diagram.
Note that a discrete enenerg level (called the donor level) appears in the forbidden
band with an Eg significantly less than that of the intrinsic material. Those free
electrons due to the added impurity sit at this energy level and have less difficulty
absorbing a sufficient measure of thermal energy to move into the conduction band at
room temperature. The result is that at room temperature, there are a large number of
carriers (electrons) in the conduction level, and the conductivity of the material increases significantly. At room temperature in an intrinsic Si material there
is about one free electron for every 1012 atoms. If the dosage level is 1 in
10 million (107),the radio 1012/107= 105
indicates that the carrier concentration has increased by a ratio of 100,000:1.
p-Type Material
The p-type material is formed by doping a pure
germanium or silicon crystal with impurity atoms having three valence electrons. The
elements most frequently used for this purpose are boron, gallium, and indium. Each
is a member of a subset group of elements in the odic Table of Elements
referred to as Group III because each has three valence electrons. The effect of one
of these elements, boron, on a base of silicon is indicated.
Note
that there is now an insufficient number of electrons to complete the covalent band of the newly formed lattice. The resulting vacancy is called a hole and is
represented by a small circle or a plus sign, indicating the absence of a negative
charge. Since the resulting vacancy will readily accept a free electron:
The diffused
impurities with three valence electrons are called acceptor atoms.
p-type material
The resulting
p-type material is electrically neutral, for the same reasons described for the
n-type material
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