Semiconductor Devices


 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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