Electrical Engineering material ioe notes very important questions and answers

 Electrical Engineering material(ii/i) ioe ALL SUBJECT NOTE  notes

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Extrinsic Semiconductor

The conductivity of semiconductor risen when some element belonging to either group iii or group v in Mendeleev periodic table are added on it. These elements are called impurities. The extrinsic semiconductors doped with these impurities is called extrinsic semiconductor. These conductor are of two type :

1. 1.       P- Type semiconductor
2. 2.       N- Type semiconductor

N –Type semiconductor:

When a pentavalent impurity like Arsenic, Potassium, and Antimony is added to a pure semiconductor a n-type semiconductor is formed. The four electrons out of five valance electron of these impurities make covalent bond with four valance electron of si so one electron remains free on every add of impurities. This extra electron continuously revolves around the impurities in core.

                                                                                In this type of semiconductor the concentration of electron increases with every add of impurities so it is called negative type(n-type) semiconductor since the pentavalent atom donate electrons for conduction so it is called donor. In n-type semiconductor e^- are majority charge carrier and holes are minty charge carrier.

                                                                If n and p represents extrinsic electron and hole concentration of semiconductor. The donor concentration then supposing all donor side are ionized.

The hole concentration in n-type semiconductor is less then the extrinsic hole concentration. This is because some electrons in conduction band recombines with holes in valance band so as to maintain n.p=ni². As we go an increasing concentration of donor atoms there by increasing majority carrier concentration. The munity carrier concentration will be decreased simultaneously this is called minty carriers suppressions.

The conductivity of semiconductor is given by:- 

So the concentration is mainly due to mobility of electrons in n-type semiconductor.

P-Type semiconductor

When a trivalent impurity likes Boron (B), Al, Ge, In is added to pure Si, a p-type semiconductor is formed. The valence electron of these impurities can form only three complete covalent bonds with silicon which has four valance electrons there is deficiency of one electron to four bonds. This give rise to holes.  In this type of semiconductor the concentration of hole increases with every add of impurities so is called positive type of semiconductor (P-type). Since trivalent atom creates a hole and this hole has tendency to accept an electrons so trivalent impurities is called as accepter.

Let n & p are electrons and hole in semiconductor Na is accepter concentration. The doping of trivalent impurities will increases the hole concentration in valance band but does not increases the electron concentration in conduction band so.

Therefore in p-type semiconductor hole concentration is nearly equal to accepter concentration

In trivalent doping of semiconductor the hole concentration increases and electron concentration decreases. This means the minority charger carrier are suppressed by majority charge carriers. This phenomenon is called minority carriers suppression.

Importance of Fermi –energy level:

• ð  For intrinsic semi conductor Fermi energy level.
• ð  It is important for4 understanding and determining electric and thermal properties of certain material.
• ð  They allows us to make calculation as to the density of electron and holes in material or relative amount of each depending on temperature.
• ð  This is crucial to our understanding of current flow throw semiconductor.
• ð  In metals the Fermi energy gives us information about velocity of electrons which participates in ordinary electron conduction.
• ð  For metals the density of conduction electrons can be implied from the Fermi energy
• ð  It is used to describe insulators metals and semiconductors
• ð  The Fermi level play an important role in bond theory of solids
• ð  In doped semiconductor p-type and n-type, the Fermi level is shifted by the impurities illustrated by their band gaps.

Compensation doping:-

It is a term used to describe the doping of semiconductor with both donors and accepters to control the properties. The effect of donors compensates for the effect of accepters and vice-versa.

For eg:-

                A  p-type semiconductor N_a accepter can be converted to an n-type semiconductor by simply adding donor until concentration N_d exceeds N_a . The holes due to accepter doping will recombine with electron due to donor doping.

Similarly a n-type semiconductor doped with Nd donor can be converted to P-type semiconductor by simply adding acceptor until concentration Na exceeds Nd. The electron due to donor doping will recombines with holes due to accepter doping so the hole concentration is 

Similarly a n-type semiconductor doped with Nd donor can be converted to P-type semiconductor by simply adding acceptor until concentration Na exceeds Nd. The electron due to donor doping will recombines with holes due to accepter doping so the hole concentration is
p=Na-Nd
From mass action law,
n.p=ni²

Therefore , the electron Concentration is given by  

Where ni=intrinsic concentration

Temperature dependence of carrier concentration:-

Intrinsic concentration of semi conductor is given by :- 

Equation (1) implies that electrons and holes concentration in intrinsic semiconductors are the functions of energy band gap and temperature of sample. Both Nc and Nv have T^(3/2) in their expression .So they are also the function of temperature. Since there is negative sign in exponential term it signifies that the increase in band gap will reduce intrinsic concentration and vice-versa. Both from the exponential term Nc and Nv   it can be observed that the increase in temperature will cause increase in concentration as well. The energy band gap is built in characteristics of all material which cannot be altered. Whereas the temperature of sample can be controlled to suit or purpose.

a.)    Low temperature range(T<T_s)

ð  As the temperature is increased starting from very low temperature. Some of donors becomes ionized and donates the electrons to conduction band as shown in figure-1 . The silicon-silicon bond breaking i.e. thermal excitation from valance band to conduction  is difficult because it takes too much energy. The donor ionizations continues until we reach a temperature called saturation temperature(T_s). where all donor atoms have been ionized. This temperature range up to T_s is called ionization range.