Intrinsic Semiconductor
Intrinsic semiconductor is a pure semiconductor. Silicon and germanium are
the semiconductor materials. TO form a stable covalent bond, 8 valence electrons
are required. The silicon atom at the centre has 4 valence electrons. It shares
4 electrons from the neighbour atoms to form a covalent bond. The symbol of
the covalent bond is shown in Fig
At absolute zero temperature, no energy is supplied to the crystal. All the
electrons are engaged in forming covalent bond and no free electrons are
available. Hence there is no conduction. Thus the semiconductor acts as an
insulator at 0 degree Kelvin. The Energy band diagram for this condition is shown in Fig.
l. I(b). The conduction band is empty as no conduction electrons are available.
When thermal energy is supplied to the semiconductor, some of the covalent
bonds are broken due to the energy supplied. These electrons jump from valence
band to the conduction band as shown in Fig. l.l(c).
Extrinsic Semiconductor
Extrinsic semiconductor is also called impure semiconductor. They are classified
as N-type and P-type. N stands for negative and P stands for positive.
(a) Negative Type or N-Type
When the intrinsic semiconductor is doped with pentavalent impurity, negative
type semiconductor is formed. The pentavalent impurities are antimony, arsenic
and bismuth. The pentavalent atom at the centre has 5 valence electrons. This
atom shares four electrons from the neighbour atoms. For the formation of
stable covalent bond, only 8 electrons need to rotate in the valance orbit. Thus
one excess electron is produced by each impurity atom. Several impurity atoms
donate several electrons. Since the impurity atoms donate electrons, they are
known as donors. The energy band diagram is shown in Fig. 1.2(b). A few
covalent bonds are broken at room temperature due to the thermal energy
supplied by the nature. The vacancies are shown as holes in the valence band.
The majority carriers are electrons and the minority carriers are holes.
Ec is the lowest energy level of the conduction band and Ev is the highest
energy level Of the valence band. Ef is the Fermi Energy level. Fermi level
corresponds to the centre of gravity of the electrons and holes. In the case of
intrinsic semiconductor, the number of electrons are equal to the number of
holes. The Fermi level lies midway between the valence band and conduction
band. In the N-type semiconductor the fermi level is lifted towards the conduction
band as the conduction electrons are the majority carriers.
(b) Positive Type or P-Type
When the intrinsic semiconductor is doped with trivalent atoms, positive type
semiconductor is formed. The trivalent atoms are indium, gallium, boron and
aluminum. The trivalent atom at the centre has 3 valence electrons. This atom
shares 4 electrons from the neighbour atoms. 8 electrons are required to form
the valence orbit. In Other words the trivalent atom can accept one electron.
This vacancy is known as a hole. The holes have positive charge. Millions of
impurity atoms Can accept millions Of electrons. Hence they are called as
acceptors. The majority carries are holes and the minority carriers are electrons.
The electrons jumped from the valence band to the conduction band due to
thermal energy are represented in the conduction band. Valence orbit of each
impurity atom has one hole. Thus, holes in the valence orbits of the impurity
atoms are represented in the valence band as shown in the energy band diagram.
The Fermi level is shifted down as the majority carriers are the holes in the
valence orbits.
PN-DIODE
Diode is a two layer device as shown in Fig. 1.5(a). The junction is formed
using P and N layers. The two electrodes of the diode are anode and cathode.
When positive terminal Of the battery is connected to the anode and negative
terminal to the cathode, the diode is said to be forward biased. A stream of
electrons start from the negative terminal of the battery and they flow through
the N layer as conduction electrons. Near the junction, electrons fall into the
holes and they become valance electrons. They travel through the P-layer as
valance electrons and later these electrons are attracted by the positive terminal
of the battery. This explains the conduction of diode when it is forward biased.
From the forward characteristic it can be seen that there is no current till the
applied voltage is less than the knee voltage (Vk). When the voltage is more
than the Vk, the current through the diode increases with the increase in the
applied voltage as shown in Fig. 1.5(b).
When the anode is connected to the negative terminal and cathode to the
positive terminal Of the battery, the diode is said to be reverse biased. Current
of the order of micro Amperes flows due to the minority camers.
When the voltage is increased beyond VA, electric field increases. The electron
entering this field experiences more force and this electron can knock off
another electron from the covalent bond. Again these two can knock off another
two and this process continues. A large current flows through the diode. This
effect is called avalanche effect. If the current is not limited, the device gets
damaged.
The diode can be represented as a closed switch when it is forward biased
and a open switch when it is reverse biased.
Signal diodes are rated for low voltage and low current. The wattage will
be 1/2 watt or I watt. Power diodes are rated at high voltage and high current.
They are of the order Of Kilovolts and Kiloamperes. The frequency of operation
of the signal diodes will be higher than that of power diodes.
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