Introduction to Semiconductor Devices and Circuit Model Reading Chapter 2 of Howe and Sodini Electrical Resistance I V W t homogeneous sample L Resistance R V L I Wt where is the resistivity EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu Units Units cm 2 What is a Semiconductor Low resistivity conductor High resistivity insulator Intermediate resistivity semiconductor Generally the semiconductor material used in integrated circuit devices is crystalline In recent years however non crystalline semiconductors have become commercially very important polycrystalline EE40 Summer 2005 Lecture 10 amorphous crystalline Instructor Octavian Florescu 3 Semiconductor Materials Elemental Compound EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 4 The Silicon Atom 14 electrons occupying the 1st 3 energy levels 1s 2s 2p orbitals filled by 10 electrons 3s 3p orbitals filled by 4 electrons To minimize the overall energy the 3s and 3p orbitals hybridize to form 4 tetrahedral 3sp orbitals Each has one electron and is capable of forming a bond with a neighboring atom EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 5 The Si Crystal diamond cubic lattice EE40 Summer 2005 Lecture 10 Each Si atom has 4 nearest neighbors lattice constant 5 431 Instructor Octavian Florescu 6 Compound Semiconductors Ga As zinc blende structure III V compound semiconductors GaAs GaP GaN etc important for optoelectronics and high speed ICs EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 7 Electronic Properties of Si Silicon is a semiconductor material Pure Si has relatively high resistivity at room temperature There are 2 types of mobile charge carriers in Si Conduction electrons are negatively charged Holes are positively charged They are an absence of electrons The concentration of conduction electrons holes in a semiconductor can be affected in several ways 1 2 3 4 by adding special impurity atoms dopants by applying an electric field by changing the temperature by irradiation EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 8 Conduction Electrons and Holes 2 D representation When an electron breaks loose and becomes a conduction electron a hole is also created Si Si Si Si Si Si Si Si Si Note A hole along with its associated positive charge is mobile EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 9 Definition of Parameters n number of mobile electrons per cm3 p number of holes per cm3 ni intrinsic carrier concentration cm3 In a pure semiconductor n p ni EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 10 Generation We have seen that conduction mobile electrons and holes can be created in pure intrinsic silicon by thermal generation Thermal generation rate increases exponentially with temperature T Another type of generation process which can occur is optical generation The energy absorbed from a photon frees an electron from covalent bond In Si the minimum energy required is 1 1eV which corresponds to 1 m wavelength infrared region 1 eV energy gained by an electron falling through 1 V potential qeV 1 6 x 10 19 C x 1 V 1 6 x 10 19 J Note that conduction electrons and holes are continuously generated if T 0 EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 11 Recombination When a conduction electron and hole meet each one is eliminated a process called recombination The energy lost by the conduction electron when it falls back into the covalent bond can be released in two ways 1 to the semiconductor lattice vibrations thermal recombination semiconductor is heated 2 to photon emission optical recombination light is emitted Optical recombination is negligible in Si It is significant in compound semiconductor materials and is the basis for light emitting diodes and laser diodes EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 12 Pure Si conduction ni 1010 cm 3 at room temperature EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 13 Doping By substituting a Si atom with a special impurity atom Column V or Column III element a conduction electron or hole is created Donors P As Sb Acceptors B Al Ga In Dopant concentrations typically range from 1014 cm 3 to 1020 cm 3 EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 14 Charge Carrier Concentrations ND ionized donor concentration cm 3 NA ionized acceptor concentration cm 3 Charge neutrality condition ND p NA n At thermal equilibrium np ni2 Law of Mass Action Note Carrier concentrations depend on net dopant concentration ND NA EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 15 N type and P type Material If ND NA so that ND NA ni 2 n ND N A and ni p ND N A n p material is n type If NA ND so that NA ND ni 2 p N A ND and ni n N A ND p n material is p type EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 16 Terminology intrinsic semiconductor undoped semiconductor electrical properties are native to the material extrinsic semiconductor doped semiconductor electrical properties are controlled by the added impurity atoms donor impurity atom that increases the electron concentration group V elements P As acceptor impurity atom that increases the hole concentration group III elements B In n type material semiconductor containing more electrons than holes p type material semiconductor containing more holes than electrons majority carrier the most abundant carrier in a semiconductor sample minority carrier the least abundant carrier in a semiconductor sample EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 17 Carrier Scattering Mobile electrons and atoms in the Si lattice are always in random thermal motion Average velocity of thermal motion for electrons in Si 107 cm s 300K Electrons make frequent collisions with the vibrating atoms Other scattering mechanisms lattice scattering or phonon scattering deflection by ionized impurity atoms deflection due to Coulombic force between carriers The average current in any direction is zero if no electric field is applied 2 3 1 electron 4 5 EE40 Summer 2005 Lecture 10 Instructor Octavian Florescu 18 Carrier Drift When an electric field e g due to an externally applied voltage is applied to a semiconductor mobile charge carriers will be accelerated by the electrostatic force This force superimposes on the random motion of electrons 2 3 1 electron 4 5 E Electrons drift in the direction opposite to the E field Current flows Because of scattering electrons in a semiconductor do not achieve constant acceleration However they can be viewed as
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