Physics Project

Project Prepared By:

Sushruta Dey  XII A Roll Number :46 Board’s Roll Number:

Kendriya Vidyalaya Fort William

AIM

To study and understand the working of a semiconductor.

CERTIFICATE It is hereby to certify that, the original and genuine project work has been carried out to study about the subject matter and the related data collection and investigation has been completed solely, sincerely and satisfactorily by Sushruta Dey of  CLASS XII A, Kendriya Vidyalaya Fort William , regarding his project titled “N type ,P Type and working theory of semiconductors ”.

Teacher’s Signature

Acknowledgement It would be my utmost pleasure to express my sincere thanks to My Physics Teacher Mrs. J Sahoo and our dearest Mr. A.K Das Sir in providing a helping hand in this project. Their valuable guidance, support  and supervision all through this project titled “N

type ,P Type and working theory of semiconductors ”. are responsible for attaining its present form.

Sushruta Dey  XII – A

PURPOSE In recent days, Semiconductors are the most used things which are used in electronics, so I wanted to know its working.  Another fact which inspired me to do this project is that I am in touch with qualitative analysis whose knowledge with other factors helped me to do so.

CONTENTS

Introduction  Theory and Definition  Effect of temperature on conductivity of Semiconductor  Intrinsic Semiconductors  N-type Semiconductor  P-type Semiconductor Resistivity of  Electrical Semiconductors 

INTRODUCTION Most of the solids can be placed in one of the two classes: Metals and insulators. Metals are those through which electric charge can easily flow, while insulators are those through which electric charge is difficult to flow. This distinction between the metals and the insulators can be explained on the basis of the number of free electrons in them. Metals have a large number of free electrons which act as charge carriers, while insulators have practically no free electrons. There are however, certain solids whose electrical conductivity is

intermediate insulators.

between metals and They are called

„Semiconductors‟.

Carbon, silicon and

 germanium are examples of semiconductors. In semiconductors the outer most electrons are neither so rigidly bound with the atom as in an insulator, nor so loosely bound as in metal. At absolute zero a semiconductor becomes an ideal insulator.

Theory and Definition Semiconductors are the materials whose electrical conductivity lies in between metals and insulator. The energy band structure of the semiconductors is similar to the insulators but in their case, the size of the forbidden energy gap is much smaller than that of the insulator. In this class of crystals, the forbidden gap is of the order of about 1ev, and the two energy bands are distinctly separate with no overlapping. At absolute o0, no electron has any energy even to jump the forbidden gap and reach the conduction band. Therefore the substance is an insulator. But when we

heat the crystal and thus provide some energy to the atoms and their electrons, it becomes an easy matter for some electrons to jump the small (» 1 ev) energy gap and go to conduction band. Thus at higher temperatures, the crystal becomes a conductors. This is the specific property of the crystal which is known as a semiconductor.

Effect of temperature on conductivity of Semiconductor  At 0K, all semiconductors are insulators. The valence band at absolute  zero is completely filled and there are no free electrons in conduction band. At room temperature the electrons jump to the conduction band due to the thermal energy. When the temperature increases, a large number of electrons cross over the forbidden gap and jump from valence to conduction band. Hence conductivity of semiconductor increases with temperature.

INTRINSIC SEMICONDUCTORS

Pure semiconductors are called intrinsic semi-conductors. In a pure semiconductor, each atom behaves as if there are 8 electrons in its valence shell and therefore the entire material behaves as an insulator at low temperatures.  A semiconductor atom needs energy of the order of 1.1ev to shake off the valence electron. This energy becomes available to it even at room temperature. Due to thermal agitation of crystal structure, electrons from a few covalent bonds come out. The bond from which electron is freed, a vacancy

is created there. The vacancy in the covalent bond is called a hole. This hole can be filled by some other electron in a covalent bond. As an electron from covalent bond moves to fill the hole, the hole is created in the covalent bond from which the electron has moved. Since the direction of movement of the hole is opposite to that of the negative electron, a hole behaves as a positive charge carrier. Thus, at room temperature, a pure semiconductor will have electrons and holes wandering in random directions. These electrons and holes are called intrinsic carriers.

 As the crystal is neutral, the number of free electrons will be equal to the number of holes. In an intrinsic semiconductor, if ne denotes the electron number density in conduction band, nh the hole number density in valence band and ni the number density or concentration of charge carriers, then ne = nh = ni

Extrinsic semiconductors

 As the conductivity of intrinsic semiconductors is poor, so intrinsic semiconductors are of little practical importance. The conductivity of pure  semi-conductor can, however be enormously increased by addition of some pentavalent or a trivalent impurity in a very small amount (about 1 to parts of the semi-conductor). The process of adding an impurity to a pure  semiconductor so as to improve its conductivity is called doping. Such semiconductors are called extrinsic semiconductors. Extrinsic semiconductors are of two types : i) n-type semiconductor ii) p-type semiconductor

n-type semiconductor When an impurity atom belonging to  group V of the periodic table like Arsenic is added to the pure semi-conductor, then four of the five impurity electrons form covalent bonds by sharing one electron with each of the four nearest  silicon atoms, and fifth electron from each impurity atom is almost free to conduct electricity. As the pentavalent impurity increases the number of free electrons, it is called donor impurity. The electrons so set free in the silicon crystal are called extrinsic carriers and the ntype Si-crystal is called n-type extrinsic  semiconductor. Therefore n-type Sicrystal will have a large number of free

electrons (majority carriers) and have a  small number of holes (minority carriers). In terms of valence and conduction band one can think that all such electrons create a donor energy level just below the conduction band as shown in figure. As the energy gap between donor energy level and the conduction band is very  small, the electrons can easily raise themselves to conduction band even at room temperature. Hence, the conductivity of n-type extrinsic  semiconductor is markedly increased. In a doped or extrinsic semiconductor, the number density of the conduction band (ne) and the number density of

holes in the valence band (nh) differ from that in a pure semiconductor. If ni is the number density of electrons is conduction band, then it is proved that ne.nh =

p-type semiconductor If a trivalent impurity like indium is added in pure semi-conductor, the impurity atom can provide only three valence electrons for covalent bond formation. Thus a gap is left in one of the covalent bonds. The gap acts as a hole that tends to accept electrons. As the trivalent impurity atoms accept electrons from the silicon crystal, it is called acceptor impurity. The holes so created are extrinsic carriers and the ptype Si-crystal so obtained is called ptype extrinsic semiconductor. Again, as the pure Si-crystal also possesses a few electrons and holes, therefore, the p-type

 si-crystal will have a large number of holes (majority carriers) and a small number of electrons (minority carriers). It terms of valence and conduction band one can think that all such holes create an accepter energy level just above the top of the valance band as shown in figure. The electrons from valence band can raise themselves to the accepter energy level by absorbing thermal energy at room temperature and in turn create holes in the valence band. Number density of valence band holes (n ) in p-type semiconductor is h approximately equal to that of the a acceptor atoms (N ) and is very large as

compared to the number density of conduction band electrons (ne). Thus, nh>> Na > > ne

Electrical Resistivity of Semiconductors Consider a block of semiconductor of length l 1 area of cross-section A and having number density of electrons and holes as ne and nh respectively. Suppose that on applying a potential difference, say V, a current  I flows through it as shown in figure. The electron current (I c ) and the hole current (I h ) constitute the current I flowing through the semi conductor i.e. (i)  I=I e +I h  It ne is the number density of conduction band electrons in the semiconductor and ve , the drift velocity of electrons then  I e = ene Ave Similarly, the hole current, I h = enh Avh  From (i)  I = ene Ave + enh Avh  I = eA(neve + nhvh )  (ii)  If    is the resistivity of the material of the  semiconductor, then the resistance offered by the  semiconductor to the flow of current is given by : (iii)  R = l/A  Since V = RI, from equation (ii) and (iii) we have V = RI = l/A eA (neve + nh vh ) V=

le(neve +nhvh ) 

(iv)

 If E is the electric field set up across the semiconductor, then: (v)  E=V/l  from equation (iv) and (v), we have  E = e (neve + nhvh ) 1/  = e (ne ve /E + nh vh /E) On applying electric field, the drift velocity acquired by the electrons (or holes) per unit strength of electric field is called mobility of electrons (or holes). Therefore, mobility of electrons and holes is  given by : e = ve /E and h = vh /E  (vi) 1/  =e(ne e +nh h )   Also, = 1/  is called conductivity of the material of semiconductor  (vii)   =e(ne e +nh h )  The relation (vi) and (vii) show that the conductivity and resistivity of a semiconductor depend upon the electron and hole number densities and their mobilities. As ne and nh increases with rise in temperature, therefore, conductivity of semiconductor increases with rise in temperature and resistivity decreases with rise in temperature.