Introduction to Semiconductors CSE251

Atomic Theory Consists of Electron, proton, neutron Electron revolve around nucleus in specific orbitals/shells

Material Classification(in terms of conductivity)  Conductor: High level of conductivity due to loosely bound electron. Ex: Metals (Cu, Al, Ag), nonmetals (C)  Insulator :Low level of conductivity (high resistivity)due to tightly bound electron. Ex: Non Metals ( marble, fused quartz, p.v.c. plastics, rubber )  Semiconductor :conductivity in between the two. Not good conductor & not good insulator (that’s why the name SEMI). Have very few free electrons.Ex : Si, Ge, GaAs

Energy Bands and the Band Gap 4 In metals, bands either overlap or are partially filled, which allows free movement of electrons. E g ≈ 0 Conduction and Valence Bands of an Insulator, Semiconductor, and Conductor.

Silicon : the most common semiconductor Electronic configuration :

Silicon : the most common semiconductor Covalent bonding : covalent bond Si

Temperature Effect in Si At T=0k, no free electron For T>0 k, thermal energy causes bond break Electron leaves parent atom So, WHAT HAPPENS THEN ?? - - = free electron +

Holes :Visualize it as an empty state but its not empty at all!! It is a missing negative charge / missing of electron positive charge equals the charge of electron, Q= * c Electron hole pair (EHP) generation Concentration(electron )=concentration(holes)=intrinsic carrier concentration More T increase : more EHP generation

Current Flow in Semiconductor Two types of carrier : Electron: negative charged Hole: positive charged Current flows due to electron motion, hole transfer When biasing applied generated electron or holes move in desired directions.

10 Electron-hole pairs in pure Si at temperature T > 0 K. Intrinsic Semiconductor and the Electron-Hole Pair  An intrinsic semiconductor is a pure semiconductor.  At T = 0 K, there are no free electrons in a pure semiconductor.  At T>0 K, some electrons in the valence band receives enough energy and move into the conduction band.  The excited electrons leave behind empty or unoccupied states, called Holes. Thus electron-hole pairs (EHP) are created.  Since the electrons and holes are created in pairs, the concentration of electrons (n) in the conduction band is equal to concentration of holes (p) in the valence band. Electrons (negatively charged) Holes (positively charged) Conduction band Valence band A pure semiconductor is known as intrinsic semiconductor. n i is called the intrinsic carrier concentration.

Semiconductor Materials Semiconductors: Group of materials having conductivities between those of metals and insulators. 11 MaterialConductivity (S.cm -1 ) Resistivity ( Ω -cm) Metal63.01×10 6 (Silver) 1.587×10 -8 Semiconductor (variable) 300 (pure Silicon) 3.33×10 -3 Insulator1× Important Features of Semiconductors: - Bonding - Band Gap Energy - Carrier Concentration

Bonding Forces in Solids Types of Bonding: Ionic, Metallic and Covalent Ionic Bonding 12  Electrostatic attractive force btwn. Na + and Cl - creates the Ionic Bond.  The outer shells of both Na and Cl are full. All the electrons are tightly bound and hence no electrons are available for current flow. NaCl is thus a good insulator. W hen metallic elements (example: Na ) donate an electron to the more electronegative element (example Cl), it creates an Na + and Cl - ion pair.

Bonding Forces: Ionic Bonding 13  The outer shells of both Ca and Cl are full. All the electrons are tightly bound and hence no electrons are available for current flow. CaCl 2 is thus a good insulator.

Bonding Forces: Metallic Bonding 14 Electron shells for Sodium Aluminum Potassium atom In metals, the metal atoms lose their outer electrons to form metal cations. (Example of metals: Na, K, Al, Fe, Cu, Ni, Au, Sn)

15  These electrostatic forces are called metallic bonds, and these are what hold the particles together in metals.  As metals have a huge number of electrons free to move about, they are characterized by very high conductivity. Bonding Forces: Metallic Bonding  The electrons from all the metal atoms form a "sea" of electrons that are "not fixed in one place" and "free to move“.  As the metal cations and the electrons are oppositely charged, they will be attracted to each other, and also to other metal cations.

Bonding Forces: Covalent Bonding 16 Group IV elements of the periodic table (e.g. Ge, Si, and C) has 4 electrons in the outer shell. Electronic structure of Si (Z=14): [1s 2 2s 2 2p 6 ]3s 2 3p 2 Ge (Z=32): [1s 2 2s 2 2p 6 3s 2 3p 6 3d 10 ]4s 2 4p 2 Two dimensional representation of covalent bond in silicon. Each atom shares its valence electrons with its four neighbors.

Bonding Forces: Covalent Bonding 17 As in the case of ionic bonding, no free electrons are available at 0 K. As such Ge and Si are perfect insulator at 0 k. However, at higher temperature some of the electrons will gain enough energy to break away from the covalent bond. The free electrons are available for conduction, giving rise to semiconducting nature of Si and Ge at room temperature. Compound semiconductors such as GaAs have an ionic component to a dominantly covalent bond because of the differing electronegativity of the two elements. Semiconductor%ionic Si0 Ge0 GaAs32 InP44

Elemental & Compound Semiconductor 18 Group II III IV V VI VII

Intrinsic Semiconductor pure semiconductor For a given semiconductor material at a constant temperature, the value of the intrinsic carrier concentration ni is a constant

Extrinsic Semiconductors 20 Extrinsic Semiconductor and Doping:  One way to increase carrier concentration in semiconductors is introduce impurities or foreign elements. The process is called Doping and the doped semiconductors are known as Extrinsic.  By doping, a semiconductor can have predominance of either electrons or holes. So there are two type of extrinsic semiconductors: n-type (mostly electrons) and p-type (mostly holes)  Group V elements have 5 valence electrons. 4 of them form covalent bond with 4 silicon atoms, leaving the 5th loosely bound to the phosphorous atom.  The 5 th electron is referred to as the donor electron.  If a small amount of energy, such as, thermal energy will enable the donor electron, to overcome its coulombic binding to the impurity atom and be donated to the lattice.

Extrinsic Semiconductor Doping :Introduction of impurity in Si in a controlled way Carriers of one kind predominate Doped Si is extrinsic Si Impurities : 1. Trivalent 2. Pentavalent: Group 5 elements

Pentavalent Impurity : DONOR Group 5 elements have 5 valence electrons. 4 of them form covalent bond with 4 silicon atoms, leaving the 5th loosely bound to the pentavalent atom. Since each such impurity atom donates an electron they are called DONOR Si P -

N Type Semiconductor A small amount of energy, such as, thermal energy will enable the donor electron, to overcome its coulombic binding to the impurity atom and be donated to the lattice. As almost all the donor atoms are ionized at room temperature, concentration of electrons >> concentration of holes. The resulting material is referred to as an n-type semiconductor. The number of electrons is almost equal to the number of donor atoms: n ≈ N d >> p In such a case, electrons are called the majority carriers and holes are the minority carriers.

Trivalent Impurity : Acceptor Group III elements has three valence electrons. When added to the silicon lattice, it can form covalent bond with only three neighboring silicon atoms, leaving the fourth one incomplete. A valence band electron may gain sufficient energy to momentarily occupy the site leaving behind a vacancy in the valence band, that is a hole. This impurity atom is called an acceptor impurity atom. Si B B hole

P Type Semiconductor The hole can move through the crystal generating a current Hole concentration >> electron concentration the resulting material is a p-type semiconductor The number of holes is almost equal to the number of acceptor atoms: p ≈ N a >> n In such a case, holes are called the majority carriers and electrons are the minority carriers.

P Type Semiconductor For a p-type material at thermal equilibrium, hole concentration is much much larger than the electron concentration, i.e., p o >> n o, for p-type In such a case, holes are called the majority carriers and electrons are the minority carriers.