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INTRINSIC SEMICONDUCTOR A pure semiconductor. Its conductivity is low. It has thermally generated current carries. Examples of pure or intrinsic semiconductor used frequently are germanium and silicon. At 0 K, all the covalent bonds is complete . Therefore, no free electron is available in the crystal for the conduction of current . Hence, silicon crystal behaves as an insulator at 0 K. At room temperature, a covalent bond breaks, an electron becomes free. The electron which leave the bonds is called free electron and the vacancy created in the covalent bond due to the release of electron is called a hole. If the potential difference is applied across an intrinsic semiconductor, electrons will moves towards the positive terminal, while the holes will drift toward the negative terminal. Drift Mobility(cm2 V-1s-1) 2000 1000 Holes Electrons 100 50 1015 1016 1017 1018 1019 1020 Dopant Concentration, cm-3 The variation of the drift mobility with dopant concentration in Si for electrons and holes Fig 5.19 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Ne= Nh = Ni Ne- Number of free electrons per unit volume Nh- Number of holes per unit volume Ni – Number density of intrinsic carries Total current inside the semiconductor = currents due to free electron + currents due to holes The process of adding suitable impurities in the intrinsic is called doping. The impurities added in the intrinsic semiconductor to increased its conductivity are known as dopant. A semiconductor obtained after adding impurities atoms in the intrinsic semiconductor is called extrinsic or doped semiconductor. 600oC 400oC L L 200oC 27oC 0oC L L Intrinsic Concentration (cm-3) 1018 2.41013 cm-3 1015 Ge 1012 1.451010 cm-3 109 Si 106 2.1106 cm-3 GaAs 103 2.5 3.5 3 4 1000/T (1/K) The temperature dependence of the intrinsic concentration. Fig 5.16 1 1.5 2 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) INTRINSIC R esistivity LO G A R ITH M IC SC A LE log( ) Semiconductor Metal log(n) T EXTRINSIC Lattice IONIZATION scattering T log( ) -3/2 T 3/2 Impurity scattering 1/T High Temperature Low Temperature Temperature dependence of electrical conductivity for a doped (ntype) semiconductor. Fig 5.20 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) EXTRINSIC SEMICONDUCTOR • Pentavalent Impurities - The elements whose each atom has five valence electrons. For example Arsenic, Antimony, Phosphorus etc. • Trivalent Impurities – The elements whose each atom has three valence electorns. For example , Indium, Gallium , Aluminium etc. When Trivalent Impurity is added to pure germanium or silicon crystal , we get extrinsic semiconductor known as p-type semiconductor. Majority charge carries in p-type semiconductor are holes and minority charge carries are electrons which are thermally generated. Since each trivalent impurities atom accepts one electron from the neighboring silicon atom, so it is known as acceptor impurities. E Impurities forming a band g(E) CB CB EF n Ec Ec Ev EFp Ev VB (a) (b) (a) Degenerate n-type semiconductor. Large number of donors form a band that overlaps the CB. (b) Degenerate p-type semiconductor. Fig 5.21 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) o When pentavalent impurities is added to the pure germanium or silicon crystal, we get an extrinsic semiconductor known as n-type semiconductor. o Majority charge carries in n-type semiconductor are electrons and minority charge carries are holes which are thermally generated. Since each pentavalent impurity atom dontes one electron to the crystal, so it is known as donor impurities. Ne = Nh = Ni Nh > Ne . In p-type semiconductor Ne > Nh . In n-type semiconductor THE DIFFERENCE OF INTRINSIC SEMICONDUCTOR EXTRINSIC SEMICONDUCTOR It is pure elements like Ge and Silicon. It is impure elements. N e = Nh High conductivity Low conductivity Conductivity depends on the temperature as Conductivity mainly depend on their temperature. Ne ≠ Nh well as the amount of impurity added in them. Light D L W V Iph A semiconductor slab of length L, width W and depth D is illuminated with light of wavelength . Iph is the steady state photocurrent. Fig 5.28 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Semitransparent electrode n-Type Semiconductor Electron Diffusion Light Electron Drift x Hole Diffusion Hole Drift Ex When there is an electric field and also a concentration gradient, charge carriers move both by diffusion and drift. (Ex is the electric field.) Fig 5.31 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) Exposed As+ Donor n2 Vo Ex n1 Diffusion Flux Drift Net current = 0 Non-uniform doping profile results in electron diffusion towards the less concentrated regions. This exposes positively charged donors and sets up a built-in field Ex . In the steady state, the diffusion of electrons towards the right is balanced by their drift towards the left. Fig 5.32 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005) h >Eg Vo+Vr E >> Eo iphoto Metal W Vr n-Si Sampling Resistor, R Reverse biased Schottky photodiodes are frequently used as fast photodetectors. Fig 5.42 From Principles of Electronic Materials and Devices, Third Edition, S.O. Kasap (© McGraw-Hill, 2005)
