Unit-3 Semiconductors and Devices
Introduction
Semiconductors and Devices
SCs are materials having electrical conductivity considerably greater than that of insulators but
significantly lower than that of conductor.
Of all the elements in periodic table, 11 elements are semiconductors. Ge and Si are elemental
semiconductors and are widely used in semiconductor devices. GaAS, InP etc are compound
semiconductors which are formed from the combination of the elements of groups III & V or II
& VI and are widely used in fabrication of optoelectronic devices such as lasers and LEDs.
The unique and interesting feature of SCs is that they are bipolar and two charge carriers, namely
electron and holes transport the current in these materials.
The electrical conductivity of a pure sc is significantly low and not be used in device fabrication.
Through the technique of doping the conductivity of a sc can be increased in magnitude to a
desired value and can be independent of temperature in a certain temperature interval.
Doped scs are known as extrinsic semiconductors. The remarkable feature of extrinsic
scs is that current is transported I charge carriers elections and holes and through two
different processes drift and diffusion.
Extrinsic scs are widely used in fabrication of solid-state devices. An understanding
mechanism of conduction in intrinsic and extrinsic scs helps us the working solid state
devices. A p-n junction diode is also known as sc diode. The p-n junction diode allows
current in one direction and resists in opposite direction. Sc diodes are widely used in
rectifiers. Practically, all sc devices contain at least one p-n junction. With the
understanding of PN junction diode, we study Zener diode and transistor.
Chemically pure scs are known as intrinsic scs. A sc is considered to be pure when
there is less than one impurity atom in a billion host atoms. In intrinsic sc, thermally
generated electron-hole pair causes electrical conduction.
An intrinsic sc is a sc crystal in which electrical conduction arises due to thermally
excited electrons and holes.
Significance of band gap
The band gap Eg is the minium amount of energy required to
excite an electron from VB to CB. Eg is the characteristic of
the material. The energy required to break the covalent bond
in Ge is about 0.72 eV at 300 K and that in silicon is 1.12
eV at 300 K.
Carrier concentration or density of charge carriers
The number of electrons in the CB per unit volume and the
number of holes per unit volume of the material is known as
carrier concentration or density of charge carriers
Limitations of intrinsic semiconductors
We summarize the limitations as follows
Conductivity is low. Germanium has conductivity of 1.67
S/m which is nearly 107 times smaller than that of copper.
Conductivity is a function of temperature and increases
exponentially as the temperature increases.
Conductivity cannot be controlled from outside.
A controlled amount of impurity added into an intrinsic semiconductor is
known as doping. The impurity which is introduced known as dopant. A
semiconductor doped with impurity atom is called an extrinsic
semiconductor. The impurity produced electrons are not temperature
dependent but are voltage dependent and they will be under control.
Typical doping level range from 1020 to 1027 impurity atoms/m3. The
substitutional impurities do not cause any distortion in the original
crystal structure.
Intrinsic concentration in Ge- at room temperature 1.5*1016
carriers/m3 Si- at room temperature 2.5 *1019 carriers/m3
Advantages of extrinsic semiconductors
•Conductivity is high
•Conductivity can be tailored to the desired value
through control of doping concentration
•Conductivity is not a function of temperature
As the temperature further, the valence band contains holes that have
been generated by two different processes namely
•Acceptor impurity ionization
•Intrinsic process
The intrinsic process causes electrons to appear in the conduction band.
At high enough temperatures, the p-type semiconductor behaves as an
intrinsic semiconductor. The number of majority carriers is independent
of temperature.
Diffusion length is defined as the distance covered by excess carriers
between its generation and recombination. Average distance travelled or
covered by excess carriers during its lifetime
Variation of Fermi level with temperature
To start with, with increase of temperature EF decreases slightly as per equation. As the
temperature is increased, more and more acceptor atoms are ionized. For a particular
temperature all the acceptor atoms are ionized. Further increase in temperature results
in electron hole pair generation due to breaking of covalent bonds and the material
tends to behave in intrinsic manner. The Fermi level gradually moves towards the
intrinsic Fermi level Ei as shown in fig.
Variation of Fermi level with acceptor concentration
When we compare the behavior of a p-type ot n-type semiconductor of higher acceptor
concentration with lower one, we find that raising of Fermi level from EF when EF=
Ev+Ea/2 to intrinsic Fermi level Ei with rise of temperature is slow in the case of
highly doped one. This is because the highly doped semiconductor will behave in
intrinsic manner only after all acceptor atoms are ionized as shown in fig
