P-N JUNCTION DIODE

P-N junction is obtained by P-type semiconductor with an N semiconductor. The figure shows the P-N junction diode before the formation of the junction.

There are excess holes, shown as small circles, in the P-section which exist in the covalent bond between the host atoms and the impurity atoms. The figure shows two impurity atoms of Aluminum near the junction.

There are excess electrons in the N section obtained from the pentavalent impurity atoms. The figure shows two Arsenic impurity atoms near the junction. Both N and P sections are electrically neutral.

The electron defuse from N to P section as the N section has excess of electrons as compared to P section. These electrons occupy holes of P side near the junction. A small amount of holes also diffuse from P to N section.

The adjoining figure shows the situation after some diffusion has occurred. Two electrons of Arsenic are shown to occupy the two holes near the Aluminum atoms. This leaves Arsenic atoms as positive ions and Aluminum atoms as negative ions. As the diffusion progresses, more and more Arsenic and Aluminum atoms become positive and negative ions respectively.

This result in the steady electric field near the junction due to the charges on the ions direction of which is from N to P region. The electrons have to overcome this increasing electric field to diffuse from N to P side. The diffusion of electrons stops when the electric field is sufficiently established to oppose the diffusion. This situation is shown in the following figure.

Two points are noteworthy:

(1) Electrons are no longer the majority charge carriers in the small region of the N type material near the
junction and the holes are not the majority charge carriers in the small region of the P-type semiconductor near the junction. These regions are known as depletion region as they are deplete of their majority charge carriers. The width of the depletion region is approximately 0.5 μm.

(2) The varying electric potential at the region near the junction is called the depletion barrier. Its value is about 0.7 V for Si and 0.3 V for Ge.

It can be seen from the band diagram of the P-N junction shown that the charge carriers need about qVB energy to cross the junction and go into the other region of the diode.

Less the amount of impurity atom added to the P and N type semiconductors, wider is the depletion region and weaker the electric field intensity near the junction.

The depletion region contains immobile positive and negative charges which constitute a capacitor having depletion capacitance or transition capacitance, C d. The width of the depletion region increases with the increase in the reverse bias which decreases the value of the capacitance. Such diode in which value of the capacitance varies with voltage is known as varactor diode or variable diode.

N AND P TYPE SEMICONDUCTORS

N-TYPE SEMICONDUCTORS: -

N-type semiconductors is prepared by adding pentavalent impurities like Antimony or Arsenic in pure semiconductor. P-type semiconductor is prepared by adding trivalent impurities like Aluminium, Gallium or Indium.
The figure shows N-type semiconductor in which two Arsenic atoms have replaced two Germanium atoms in the lattice structure of Ge crystal. Four of the five valence electrons of As atom are used up in forming covalent bonds and the fifth electron can act as a free electron with 0.01 eV energy. This energy is 0.05 eV in case of Silicon atom. This much energy is easily available at room temperature as thermal energy.

The pentavalent impurity is called donor impurity as it denotes the electric charge carrier electron, to the host atom. It is added in proportion of 1 in 106 pure atoms. Hence in one mole of crystal, about 1017 impurity atoms and 1017 free electrons are present. A good conductor like copper contains nearly 1023 free electrons per mole. Besides these, some more free electrons and equal number of holes result from breaking of covalent bonds. As their number is very small as compared to the free electrons from the impurity atoms, electrons are the majority charge carriers and holes minority charge carriers in the case of N-type semiconductors ( n e > n h ).

P-TYPE SEMICONDUCTORS: -

If trivalent impurities like Aluminum is
added to Ge or Si, then three free electrons of this impurity atom form covalent bonds with its neighbouring three Ge or Si atoms. Thus there is a deficiency of one electron in the formation of the fourth covalent bond. This deficiency of electron can
considered as a hole which is present in one of the bonds between the aluminium and Ge or Si atoms. This hole has a tendency to attract electron. Hence aluminium atom is known as acceptor impurity. Here, holes which behave as positively charged particles are majority charge carriers and electrons are minority charge carriers. Hence such a semiconductor is known as P-type semiconductors  (n e > n h). The figure shows symbolic representation of aluminium impurity added to Ge crystal lattice.

CONDUCTORS, INSULATORS AND INTRINSIC SEMICONDUCTOR

The elements in the first three groups of the periodic table like alkali metals, noble metals, Aluminum, etc. are good conductors due to the presence of free electrons. Non-metals are bad conductors of electricity due to lack of free electrons. The elements in the fourth group of the periodic table like Si and Ge have greater resistance than good conductors but less than bad conductors. They are known as semiconductors. They behave as bad conductors at absolute zero temperature in their pure form.

The resistivity of the good conductors increases with temperature, while the resistivity of the semiconductors decreases on increasing the temperature unto a certain limit. The conductivity of the semiconductors is changed by making radiation of suitable frequency incident on them.

Two very important semiconductors Ge and Si are discussed here. Both have diamond crystal structure. If an atom of Si is considered at the center of the tetrahedron, then its four nearest neighbours are at the vertices's of a tetrahedron as shown in the figure. Diamond crystalline structure is obtained on extending this arrangement in a three dimensional space.

The electronic arrangement of Si is 1s² 2s² 2p⁶ 3s² 3p². The electrons in 1s² 2s² 2p⁶ completely occupy the K and L shells. 3s² 3p² electrons are the valence electrons. These 2 s orbitals and 2 p orbitals combine to form 4 sp³ complex orbitals. These orbitals combine with similar such orbital’s of the neighbouring atoms and form covalent bonds. Thus, each of the four valence electrons of the silicon forms a covalent bond with its four neighbouring atoms as shown in the figure.

At absolute zero temperature, Si and Ge behave as insulators as the valence electrons are bound in covalent bonds. At room temperature, these bonds break due to thermal oscillations of atoms freeing the electrons which increase conductivity. Deficiency of electron in a bond produces a vacant space which is known as a hole. The hole has the ability of attracting electrons and the randomly moving free electron can get trapped in a hole. Thus hole behaves as a positive charge though it is neither a real particle nor has any positive charge.

On applying p.d. between two ends of a crystal as shown in the figure, electric current gets set up. Now, thermal oscillations and external electric field cause covalent bonds to break and the free electrons produced get trapped in the holes during their motion. Simultaneously, new holes are produced by electrons breaking free from the covalent bonds. The free electrons move towards the positive end and the holes to the negative end. The motion of holes towards the negative end is equivalent to the motion of bound electrons towards the positive end. Thus current in a semiconductor is due to ( i ) motion of free electrons and ( ii ) motion of bound electrons. Both these currents are in the same direction.

The number density of free electrons n_e and holes n_h in a pure semiconductor are equal. Pure semiconductor is called intrinsic semiconductor. Hence electrons and holes are called intrinsic charge carriers and their number density is indicated by n_i. n_e = n_h = n_i.