Important Points of Semiconductors for NEET

There are two types of solids, conductors and semiconductors.There are some solids whose electrical conductivity is intermediate between conductors and insulators. These are called semiconductors. Temperature plays an important role in conductivity. At absolute zero a semiconductor becomes an ideal insulator. 

Given below are the basic and important points to be remembered from the chapter:


Energy Bands

A solid contains an enormous number of atoms which are packed closely such that when each atom is isolated, a discrete set of energy levels is found. Thus, all the atoms have completely coinciding sets of energy levels and electrons fill these energy levels in each atom independently. As the atoms approach one another to form solid, a continuous increasing interaction occurs between them which causes energy levels to split into N distinct levels. Therefore, split energy levels become so numerous and so close together that they form “energy bands”. The amount of splitting is different in different energy levels. In general, the lower levels are split less than the higher levels because the electrons in the lower levels are the inner electrons of the atoms which are not significantly influenced by nearby atoms. The energy band formed from the atomic energy levels containing valence electrons is called valence band. An empty band above the valence band into which electrons can pass is called a ‘conduction band’.


Conductors 

In some solids, there is a partially-filled band above the completely filled lower bands. The partially filled band may also be the result of overlapping of completely filled band and an empty band. The electrons in the partially-filled valence band easily acquire additional energy to move to higher unoccupied energy levels within the same band without crossing the energy gap. The additional energy is in the form of kinetic energy, and the moving electrons constitute electric current. Thus, a partially filled valence energy band is a feature of conductors.


Insulators

In some solids, the valence band (containing valence electrons) is completely separated by an energy gap of a few electron-volts that is completely empty. Such a solid is an insulator. The electrons in the valence band will not accept energy to move within the band because there are no unoccupied levels. Since, the electric field cannot give enough energy to the electrons to jump to the higher band due to the high gap. Thus, there is no flow of current electricity. 


Semiconductors

Solids having basic structure of an insulator with smaller energy gaps between valence band and conduction band are called semiconductors. Pure semiconductors are insulators at low temperatures. 


Electrons and Holes

At room temperature, valence electrons acquire thermal energy to cross over into the conduction band. A vacancy is created in the valence band in the place of electrons. This vacancy is called hole. The holes created in the valence band can move about even under small applied fields. In a semiconductor, electrons and holes both constitute current electricity.


1. Intrinsic Semiconductor - A pure semiconductor is called intrinsic semiconductor.


2. Extrinsic Semiconductor -  When a small amount of impurity is added to the intrinsic semiconductor such that the concentration of electrons and holes are unequal. There are two types of extrinsic semiconductors:

  1. N-type Semiconductor - extrinsic semiconductors doped with pentavalent impurities. The extra free electrons are called donor atoms.

  2. P-type Semiconductor - extrinsic semiconductors doped with trivalent impurities. The extra holes are called acceptor atoms. 


Semiconductor Diodes

P-n junction diode is a basic semiconductor device having excess of acceptor impurities in one region (p-type) and an excess of donors in the other region (n-type). The boundary between two regions is called ‘p-n junction’. The few electrons present in the p-region and the few holes in the n-region are called ‘minority carriers’.  


Potential Barrier

As soon as the p-n junction is formed, there is an immediate diffusion of the majority charge carriers across the junction due to thermal agitation. Electrons diffuse into p-region and holes in n-region diffuse into p-region. The diffused charge carriers combine with their counterparts in the vicinity of the junction and neutralise each other. This sets up potential differences called potential barriers. 

  1. Forward Biasing: When the positive terminal of the external battery is connected to the p-region and negative terminal is connected to the n-region of the diode.

  2. Reverse Biasing: When the positive terminal of the external battery is connected to the n-region and the negative terminal is connected to the p-region of the diode.


Diode as Rectifier

A rectifier is a device which converts an alternating current into a direct current. 

  1. P-N Junction Half-wave Rectifier: If an AC voltage is applied across a junction diode, a current flows through the diode only during the positive half cycles of the voltage, not during full cycles. Thus, a single junction diode acts as a ‘half-wave’ rectifier. 

  2. P-N Junction as Full-Wave Rectifier: In full wave rectifier, a unidirectional, pulsating output current is obtained for both halves of the AC input voltage. Essentially, it requires two junction diodes so connected that one diode rectifies one half and the second diode rectifies the second half of the input. 


Types of Diodes

  1. Zener Diode: It is a reverse biased heavily doped p-n junction diode, which is operated in breakdown regions where the current is limited by both external resistance and power dissipation of the diode. 

  2. LED: It is an important light source used in optical communication as it conbiasing electrical energy into light energy. It is a forward biased p-n junction which emits light spontaneously. 

  3. Photodiode: It is a reverse biased p-n junction made from a photosensitive semiconductor. 

  4. Solar Cell: It is a p-n junction diode that converts solar energy directly into electrical energy and is biased on photovoltaic effect.