Why are n and p-type Semiconductors electrically neutral, even though their name suggests polarity?
Why are n and p-type Semiconductors electrically neutral, even though their name
suggests polarity?
Semiconductors are materials with conductivity between conductors and
insulators. Semiconductors are employed
in the manufacture of various kinds of electronic devices, including diode,
transistors, and integrated circuits. Such
devices have found wide application because of their compactness, reliability,
and low cost.
The
atoms of semiconductor elements have exactly four valence electrons. Bonding
between atoms occurs because each atom tends to fill its outermost cell with
eight electrons. Each semiconductor atom has four valence electrons, hence the
atom can share four other valence electrons of neighboring atoms to complete
eight electrons in its outermost cell. The bonding between atoms by sharing
valence electrons is called the covalent bond. This results in a covalent
crystal.
Fig A
Fig B Fig B shows
a simplified two-dimensional representation of an intrinsic (pure) silicon crystal that contains negligible impurities. Each
silicon atom in the crystal is surrounded by four of its nearest neighbors.
Each atom has four electrons in its outer orbit and shares these electrons
with its four neighbors. Each shared electron pair constitutes a covalent bond.
At low temperatures the electrons in a
semiconductor are bound in their respective bands in the crystal; consequently,
they are not available for electrical conduction. At higher temperatures,
thermal vibration may break some of the covalent bonds to yield free electrons
that can participate in current conduction. Once an electron moves away from a covalent bond,
there is an electron vacancy associated with that bond. This vacancy may be
filled by a neighboring electron, which results in a shift of the vacancy location
from one crystal site to another. This vacancy may be regarded as a fictitious
particle, called a ' Hole "
that carries a positive charge and moves in a direction opposite to that of an
electron. When an electric field is
applied to the semiconductor, both the free electrons and the holes move
through the crystal, producing an electric current. The electrical conductivity
of a material depends on the number of free electrons and holes (charge
carriers) per unit volume and on the rate at which these carriers move under
the influence of an electric field. In an intrinsic semiconductor, there exists
an equal number of free electrons and holes.
Electrical conduction in intrinsic
semiconductors is quite poor at room temperature. To produce higher conduction,
one can intentionally introduce impurities (typically to a concentration of one
part per million host atoms). This is called doping , a
process that increases conductivity despite some loss of mobility. The doping
converts intrinsic
semiconductors to Extrinsic semiconductors. Fifth
group (pentavalent ) element doping results in n-type semiconductor while third group ( trivalent
) element doping produce p-type.
N-type Semiconductor Basics:
Fig C The elements like Phosphorous, Antimony,
Arsenic from the fifth group are doped in the fourth group element like silicon to obtain the n-type semiconductor. The fifth group
elements have five outer electrons in their outermost orbital to share with
neighboring atoms and are commonly called “Pentavalent” impurities.
This allows four out of the five
orbital electrons to bond with its neighboring silicon atoms leaving one “free
electron” to become mobile when an electrical voltage is applied (electron
flow). As each impurity atom “donates” one electron, pentavalent atoms are
generally known as “donors”. ( Fig C ).
The
resulting semiconductor material has an excess of current-carrying
electrons, each with a negative charge, and is therefore referred to as
an n-type material
with the electrons
called “Majority
Carriers” while the resulting holes are called “Minority Carriers”.
Fig DIn n-type semiconductor majority of free charge carriers are negatively charged electrons, due to this fact it is called n-type. But a piece of n-type semiconductor is electrically neutral. Fig D shows this point clearly. It is seen that n-type is prepared by doping fifth group elements in fourth group elements, and these are electrically neutral. In Fig D it is shown that silicon atoms at the background, and donor + ve atoms are represented by + ve are encircled. Each donor atom makes one free electron. Thus the number of +ve donors equals the number of - ve free electrons. Besides thermally generated hole - electron pairs give the equal number of + ve and - ve charges. Thus n-type semiconductor is electrically neutral even though its name is derived from -ve charge carriers.
P-Type
Semiconductor Basics:
Fig E If we go the other way and introduce a “Trivalent”
(3-electron) impurity into the crystalline structure, such as Aluminium, Boron,
or Indium, which have only three valence electrons available in their outermost
orbital, the fourth closed bond cannot be formed. Therefore, a complete
connection is not possible, giving the semiconductor material an abundance of
positively charged carriers known as holes in the structure of the crystal
where electrons are effectively missing. (Fig E)
As there is now a hole in
the silicon crystal, a neighboring electron is attracted to it and will try to
move into the hole to fill it. However, the electron filling the hole leaves
another hole behind it as it moves. This in turn attracts another electron
which in turn creates another hole behind it, and so forth giving the
appearance that the holes are moving as a positive charge through the crystal
structure (conventional current flow).
This
movement of holes results in a shortage of electrons in the silicon turning the
entire doped crystal into a positive pole. As each impurity atom generates a
hole, trivalent impurities are generally known as “Acceptors”
as they are continually “accepting” extra or
free electrons. The doping of Boron atom causes conduction to consist mainly
of positive charge carriers resulting in a p-type material
with the positive holes being called “Majority Carriers” while the free electrons are called “Minority
Carriers”.
Fig F In p-type
semiconductor majority of free charge carriers are positively charged holes,
due to this fact it is called p-type. But a piece of p-type
semiconductor is electrically neutral. Fig F shows this point clearly.
It is seen that p-type is prepared by doping third group elements in fourth
group elements, and these are electrically neutral. In Fig F it is shown that
silicon atoms are in the background, and acceptor - ve atoms are represented by
- ve are encircled. Each acceptor atom makes one free hole. Thus the number of -
ve acceptors equals the number of + ve free holes. Besides thermally generated
hole - electron pairs give the equal number of + ve and - ve charges. Thus p-type semiconductor is electrically neutral even though
its name is derived from + ve charge carriers.
Conclusion :
1. Neutral fourth group element + Neutral fifth group element
= Neutral n-type semiconductor
2. Neutral fourth group element + Neutral third group element
= Neutral p-type semiconductor
Thus in both types of semiconductors localized fix impurity charges equal to free apposite charge carriers and thermally generated equal number of holes and electrons results that both types are electrically neutral.
Nice Article...
ReplyDeleteNice explanation...
ReplyDeleteThank You
ReplyDeleteMuch helpful and understanding..nice
ReplyDelete