Electrical Conductivity Of Semiconductors

Semiconductors are those materials having very lower conductivity as compared to metals. These elements are known as weak insulators rather than weak conductors; it is only due to their respective atomic structure. Though some semiconducting substances had been identified and studied but  their properties could not be  properly explained till the mid-twentieth century, when the principles of  modern quantum-mechanics  were applied for  the analysis of  both metals and semiconductors materials, that theoretical measures of conductivity agreed with the  results of experimental measurements.

 However in an insulator, the negatively charged particles cannot move because nearly all orbits have already been occupied. External energy must then be provided to remove an electron from an outermost orbit to a higher possible allowed state. It creates new space or vacancy into which another bounded electron can jump with effect of an electric field. Thus, both electron and its vacant space can easily be moved. The vacant space behaves like a positively charge particle, called a hole, It moves opposite to electrons. Electrons and holes are normally called charge carriers.

Insulators are having very high activation energy so as their availability requires quite high temperature. Where as in  weak  insulators, normally called semiconductors, activation become effective  at temperatures more than 80°F .Each substance has a  specific characteristics accordingly they behave.

A part from elements many compounds can also be classified as semiconductors. Some of elements in column IV of the Modern periodic table, having covalent bonding like carbon (C), Germanium (Ge), and Silicon (Si). For Carbon, Graphite shows semiconductor properties; diamond is the best example of insulator. One more element down in column IV of  Modern Periodic Table is Tin (Sn),During  transition  period from semiconductor to metal around  60 Degree Farenheit  comparatively in cool  surroundings ,it  shows   quite low activation energy. Some elements of column VI also show semiconductor behavior, specifically Selenium (Se) and Tellurium (Te).

There are mainly two principal groups of compounds having  semiconducting properties, first group consists of Gallium Arsenide (GaAs) and Indium Anti monide (InSb), ,while the second one is of  Zinc Sulphide (ZnS) and some  more Oxides. In many ways these compounds deviate with the behavior of column IV elements. Normally compounds have Mixed Covalent as well as Ionic Chemical Bondings. A part from above some organic compounds also behaves like semiconductors.

As we know semiconductor is called intrinsic or pure semiconductor, if its conductivity is the result of equal contributions from its own electrons and holes. Then the equation can be expressed as n_e = n_h, and e has the same numerical value for an electron (-) and the hole left behind (+).  As usual the numeric value of mobilization of electron and hole are absolutely different. As we already know that the opposite charges move in opposite directions, resulting in a pair of like signs in each product.

When we discuss about the application of semiconductor materials in a devices, they are rarely used in their pure composition. Under specific controlled conditions, impurities are doped with the help of Doping process which contribute either an excess or a deficit of electrons. Excess electrons neutralize holes so that only electrons are available for conduction. The resulting extrinsic material is called N-type semiconductor, N for negative carrier.  Si is an example of n-type material, where we used Sb as Dopant , a column IV element with a column V impurities also known as a Donor. In N-type material, donor remain constant and positively charged, That’s why N type semiconductors are called Donor type semiconductor. When an element of column III impurities or dopant is doped into a column IV element, electrons are bound and holes made available. That material is called P-type, P for positive carrier. Column III impurities are known as Acceptors; in the material acceptor atoms remain constant and negatively charged. Due to lack of electrons this type of semiconductor is called Acceptor type semiconductor. An example of p-type material is Si doped with  Ga. Both N-type and P-type semiconductors are now called extrinsic semiconductors.

Thermal kinetic energy is not the only mechanism, we can also use different mechanism for providing external energy to release charge carriers in semiconductors.  Even we can also provide energy of Photons  which can be absorbed by a electron in an pure semiconductor materials , adds both itself and a hole as mobile carriers. In extrinsic semiconductors, photons of much lower energies can be used provided the material is cooled in order to reduce the population of thermally activated carriers. This nature of semiconductor materials is known as Photoconductivity.

All the semiconductors are Ohmic, with the conductivity constant at constant temperature. As we studied that when the temperature is increased, the conductivity increases rapidly. In semiconductor the conductivity is directly proportional to the heat absorbed by materials.

Dr  Subhash  Das

Professor & HOD, Department of Physics

Kalinga University, Raipur     

 Email  :dr.das@kalingauniversity.ac.in