1 LECTURE 2 CONTENTS MAXWELL BOLTZMANN STATISTICS FERMI- DIRAC STATISTICS & ITS DISTRIBUTIONSEMICONDUCTORS AND ITS CLASSIFICATION andFERMI ENERGY LEVEL DISTRIBUTION IN INTRINSIC SEMICONDUCTORS.
2 Maxwell Boltzmann Statistics (Classical law) This law states that, the total fixed amount of energy is distributed among the various members of an assembly of identical particles in the most proable distribution.The Maxwell Boltzmann law isWhere ni ─ number of particles having energy Ei.gi ─ number of energy states.and(Here k ─ Boltzmann constant; T ─ Absolute temperature of the gas, EF ─ Fermi energy)
3 Therefore,Particles are distinguishable.Classical particles can have any spin.Particles do not obey Pauli’s exclusion principle.Any number of particles may have identical energies.
4 Fermi-Dirac Statistics (Quantum law) This statistics applicable to the identical, indistinguishable particles of half spin.These particles obey Pauli’s exclusion principle and are called fermions (e.g.) Electrons, protons, neutrons …,In such system of particles, not more than one particle can be in one quantum state.Fermi Dirac Distribution Law isor ni =
5 Possible distribution in various energy level ExampleLet us consider two particles a and a. Let if, these two particles occupy the three energy levels (1,2,3). The number of ways of arranging the particles 31=3 (not more than one particle can be in any one state)Energy levelPossible distribution in various energy level1aA-23
6 Fermi Energy (EF) and Fermi-Dirac Distribution Function f(E) Fermi Energy is the energy of the state at which the probability of electron occupation is ½ at any temperature above 0 K.It is also the maximum kinetic energy that a free electron can have at 0 K.The energy of the highest occupied level at absolute zero temperature is called the Fermi Energy or Fermi Level.
7 The Fermi energy at 0 K for metals is given by When temperature increases, the Fermi level or Fermi energy also slightly decreases.The Fermi energy at non–zero temperatures,Here the subscript ‘0’ refers to the quantities at zero kelvin.
8 Fermi-Dirac Distribution Function f(E) The free electron gas in a solid obeys Fermi-Dirac statistics.Suppose in an assemblage of fermions, there are M(E) allowed quantum states in an energy range between E and E+dE and N(E) is the number of particles in the same range. Then,The Fermi-Dirac distribution function is defined as,N(E) / M(E) is the fraction of the possible quantum states which are occupied.
9 The distribution of electrons among the levels is described by function f (E), probability of an electron occupying an energy level ‘E’.If the level is certainly empty, then f(E) = 0.Generally the f(E) has a value in between zero and unity.When E< EF (i.e.,) for energy levels lying below EF,(E –EF) is a negative quantity and hence,That means all the levels below EF are occupied by the electrons.
10 Fermi Dirac distribution function at different temperatures
11 When E > EF (i.e.) for energy levels lying above EF, (E – EF) is a positive quantityThis equation indicates all the levels above EF are vacant.At absolute zero, all levels below EF are completely filledand all levels above EF are completely empty.This level, which divides the filled and vacant states, is known as the Fermi energy level.
12 When E = EF ,, at all temperaturesThe probability of finding an electron with energy equal to the Fermi energy in a metal is ½ at any temperature.At T = 0 K all the energy level upto EF are occupied and all the energy levels above EF are empty .When T > 0 K, some levels above EF are partially filled while some levels below EF are partially empty.
13 SemiconductorsIntroductionThe materials are classified on the basis of conductivity and resistivity.Semiconductors are the materials which has conductivity, resistivity value inbetween conductor and insulator . The resistivity of semiconductor is in the order of 10−4 to 0.5 Ohm-metre.It is not that, the resistivity alone decides whether a substance is a semiconductor (or) not , because some alloys have resistivity which are in the range of semiconductor’s resistivity. Hence there are some properties like band gap which distinguishes the materials as conductors, semiconductors and insulators.
14 semi-conductor is a solid which has the energy band similar to that of an insulator. It acts as an insulator at absolute zero and as a conductor at high temperatures and in the presence of impurities.Semiconductors are materials whose electronic properties are intermediate between those of metals and insulators.These intermediate properties are determined by the crystal structure, bonding characteristics and electronic energy bands.They are a group of materials having conductivities between those of metals and insulators.
15 Classification of Semiconductors According to their StructureAmorphous semiconductors-have poor electrical characteristics.Polycrystalline semiconductors – have better electricalcharacteristics and lower conductivity.Single crystal semiconductor – have superior electricalcharacteristics and higher conductivity. The majority of the semiconductor devices, single-crystal materials are used.
16 Classification of Semiconductor According to the nature of the current carriersIonic semi conductor, in which conduction takes place through the movement of ions and.Electronic semiconductor, in which conduction takes place through the movement of electrons and no mass transport, is involved
17 Classification of semiconductors According to the constituent atomsElemental semiconductor: All the constituent atoms are of the same kind (i.e) composed of single species of atoms. (eg) germanium and silicon.Compound semiconductor: They are composed of two or more different elements (eg) GaAS, AlAs etc.,
18 Crystal structure of silicon and germanium The structure of Si and Ge, which are having covalent bonding. Covalent bondings are stereo specific; i.e. each bond is between a specific pair of atoms.The pair of atoms share a pair of electrons (of opposite magnetic spins).
19 Three dimensional representation of the structures Si, and Ge, with the bonds shown in below figure, the region of high electron probability (shaded).(b)(a)Structure of (a) silicon and (b) germanium crystals
20 Each atom of a material is coordinated with its neighbours. All atoms have coordination number 4; each material has an average of 4 valence electrons per atom, and two electrons per bond.Each atom of a material is coordinated with its neighbours.(a)(b)Structure of (a) silicon and (b) germanium crystals
21 The thermal vibrations on one atom influence the adjacent atoms; the displacement of one atom by mechanical forces, or by an electric field, leads to adjustments of the neighbouring atoms.The number of coordinating neighbours that each atom has is important. Covalent bonds are very strong.(b)(a)Structure of (a) silicon and (b) germanium crystals
22 Some important properties of elemental semiconductor PropertySilicon (Si)Germanium (Ge)Atomic number1432Atom/m35.02 10284.42 1028Electronic shell configuration1s22s22p63s23p21s22s22p63s23p63d104s24p2Atomic weight28.0972.6Crystal structureDiamondBreakdown field (V/m)~ 3.0107~ 107Density (gm/m3)2.329106at 298K5.3234106 at 298 KEnergy gap (eV)1.12 at 300 K1.17 at 77K0.664 at 291 K0.741 at 4.2KDielectric constant11.7 at 300K16.2 at 300KIntrinsic carrier concentration (m3) at 300 K1.02 10162.33 1019Lattice constant (Å)at 298.3Kat KMelting point (C)1412937.4Thermal conductivity [Wm1(C1)]131 at 300K60 at 300KMobility of electrons (m2V1s1)0.135 at 300K0.39 at 300KMobility of holes (m2V1s1)0.048 at 300K0.19 at 300K
23 Intrinsic Semiconductors In semiconductors and insulators, when an external electric field is applied the conduction is not possible as there is a forbidden gap, which is absent in metals.In order to conduct, the electrons from the top of the full valence band have to move into the conduction band, by crossing the forbidden gap.The field that needs to be applied to do this work will be extremely large.
24 Eg: Silicon where the forbidden gap is about 1 eV. The distance between these two locations is about 1 Å (1010 m).A field gradient of approximately 1V/ (1010 m) = 1010Vm1 is necessary to move an electron from the top of the valence band to the bottom of the conduction band.
25 The other possibility by which this transition can be brought about is by thermal excitation. At room temperature, the thermal energy that is available can excite a limited number of electrons across the energy gap. This limited number accounts for semi-conduction.When the energy gap is large as in diamond, the number of electrons that can be excited across the gap is extremely small.
26 In intrinsic semiconductors, the conduction is due to the intrinsic processes (without the influence of impurities).A pure crystal of silicon or germanium is an intrinsic semiconductor. The electrons that are excited from the top of the valence band to the bottom of the conduction band by thermal energy are responsible for conduction.The number of electrons excited across the gap can be calculated from the Fermi-Dirac probability distribution.
27 f(E) =The Fermi level EF for an intrinsic semiconductor lies midway in the forbidden gap.The probability of finding an electron here is 50%, even though energy levels at this point are forbidden.Then (EEF) is equal to Eg /2,where Eg is the magnitude of the energy gap.
28 For a typical semiconductor like silicon, Eg = 1 For a typical semiconductor like silicon, Eg = 1.1 eV, so that (EEF) is 0.55 eV, which is more than twenty times larger than the thermal energy kBT at room temperature (=0.026 eV).The Fermi level in an intrinsic semiconductor lies in the middle of the energy gap.
29 The probability f(E)of an electron occupying energy level E becomes f(E) = exp(Eg / 2kBT ).The fraction of electrons at energy E is equal to the probability f(E). The number n of electrons promoted across the gap,n = N exp(Eg / 2kBT)where N is the number of electrons available for excitation from the top of the valence band.
30 The promotion of some of the electrons across the gap leaves some vacant electron sites in the valence band. These are called holes.An intrinsic semiconductor contains an equal number of holes in the valence band and electrons in the conduction band, that is ne = nh.Under an externally applied field, the electrons, which are excited into the conduction band by thermal means, can accelerate using the vacant states available in the conduction band.
31 At the same time, the holes in the valence band also move, but in a direction opposite to that of electrons.The conductivity of the intrinsic semiconductor depends on the concentration of these charge carriers, ne and nh.In the case of metals, the drift velocity acquired by the free electrons in an applied field.The mobility of conduction electrons and holes, e and h, as the drift velocity acquired by them under unit field gradient.
32 The conductivity of an intrinsic semiconductor as i = ne e e + nh e hwhere e is the electronic charge, ne and nh are concentrations of electrons and holes per unit volume.
33 Fermi levelThe number of free electrons per unit volume in an intrinsic semiconductor isThe number of holes per unit volume in an intrinsic semiconductor isp =Since n = p in intrinsic semiconductors.
34 Taking log on both sides, orTaking log on both sides,or Ef =
35 If we assume that,[ since loge1 = 0]Thus, the Fermi level is located half way between the valence and conduction band and its position is independent of temperature. Since mh* is greater than me*, EF is just above the middle, and rises slightly with increase in temperature
36 Position of Fermi level in an intrinsic semiconductor at various temperatures (a) at T = 0 K, the Fermi level in the middle of the forbidden gap(b) as temperature increases, EF shifts upwards