Semiconductors
are the materials which have a conductivity between conductors (generally
metals) and non-conductors or insulators (such as ceramics). Semiconductors can
be compounds such as gallium arsenide or pure elements, such as germanium or
silicon. Physics explains the theories, properties and mathematical approach
governing semiconductors.
Examples of
Semiconductors:
Gallium
arsenide, germanium, and silicon are some of the most commonly used semiconductors. Silicon is used in
electronic circuit fabrication and gallium arsenide is used in solar cells, laser
diodes, etc.
Holes and Electrons in Semiconductors
Holes
and electrons are the types of charge carriers accountable for the flow of
current in semiconductors. Holes (valence electrons) are the positively charged
electric charge carrier whereas electrons are the negatively charged particles.
Both electrons and holes are equal in magnitude but opposite in polarity.
Mobility of Electrons and Holes
In
a semiconductor, the mobility of electrons is higher than that of the holes. It
is mainly because of their different band structures and scattering mechanisms.
Electrons travel in the conduction band whereas holes travel in the valence band. When an electric field is applied, holes cannot move as freely as electrons due to their restricted movent. The elevation of electrons from their inner shells to higher shells results in the creation of holes in semiconductors. Since the holes experience stronger atomic force by the nucleus than electrons, holes have lower mobility.
The
mobility of a particle in a semiconductor is more if;
Time
between scattering events is more
For
intrinsic silicon at 300 K, the mobility of electrons is 1500 cm2 (V∙s)-1 and
the mobility of holes is 475 cm2 (V∙s)-1.
The bond model of electrons in silicon of valency 4 is shown below. Here, when one of the free electrons (blue dots) leaves the lattice position, it creates a hole (grey dots). This hole thus created takes the opposite charge of the electron and can be imagined as positive charge carriers moving in the lattice.
Band Theory of Semiconductors
The introduction of
band theory happened during the quantum revolution in science. Walter Heitler
and Fritz London discovered the energy bands.
We know that the
electrons in an atom are present in different energy levels. When we try to
assemble a lattice of a solid with N atoms, then each level of an atom must
split up into N levels in the solid. This splitting up of sharp and tightly
packed energy levels forms Energy Bands. The gap between adjacent
bands representing a range of energies that possess no electron is called
a Band Gap.
Conduction Band and Valence Band in Semiconductors
Valence
Band:
The energy band involving the energy levels of valence electrons is known as the valence band. It is the highest occupied energy band. When compared with insulators, the bandgap in semiconductors is smaller. It allows the electrons in the valence band to jump into the conduction band on receiving any external energy.
Conduction
Band:
It is the lowest unoccupied band
that includes the energy levels of positive (holes) or negative (free
electrons) charge carriers. It has conducting electrons resulting in the flow
of current. The conduction band possess high energy level and are generally
empty. The conduction band in semiconductors accepts the electrons from the
valence band.
- Semiconductor acts like an insulator at Zero Kelvin. On increasing the temperature, it works as a conductor.
- Due to their exceptional electrical properties, semiconductors can be modified by doping to make semiconductor devices suitable for energy conversion, switches, and amplifiers.
- Lesser power losses.
- Semiconductors are smaller in size and possess less weight.
- Their resistivity is higher than conductors but lesser than insulators.
- The resistance of semiconductor materials decreases with the increase in temperature and vice-versa.
Types of Semiconductors
Semiconductors can be
classified as:
- Intrinsic
Semiconductor
- Extrinsic
Semiconductor
