chap2 - Solid-State Electronic Materials Electronic...

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1 Solid-State Electronic Materials • Electronic materials fall into three categories: – Insulators Resistivity ( ρ )> ±10 5 -cm – Semiconductors 10 -3 < ρ < 10 5 -cm – Conductors ρ < 10 -3 -cm • Elemental semiconductors are formed from a single type of atom. • Compound semiconductors are formed from combinations of column III and V elements or columns II and VI. • Germanium was used in many early devices. • Silicon quickly replaced germanium due to its higher bandgap energy, lower cost, and easy oxidation to form silicon-dioxide insulating layers. Semiconductor Materials (cont.) 1.70 Cadmium selenide 3.26 Silicon carbide 7.50 Boron nitride 1.35 Indium phosphide 3.49 Gallium nitride 1.42 Gallium arsenide 0.082 Tin 0.66 Germanium 1.12 Silicon 5.47 Carbon (diamond) Bandgap Energy E G (eV) Semiconductor Silicon Covalent Bond Model Near absolute zero, all bonds are complete. Each Si atom contributes one electron to each of the four bond pairs. Increasing temperature adds energy to the system and breaks bonds in the lattice, generating electron-hole pairs. Intrinsic Carrier Concentration The density of carriers in a semiconductor as a function of temperature and material properties is: E G = semiconductor bandgap energy in eV (electron volts) k = Boltzmann’s constant, 8.62 x 10 -5 eV/K T = absolute temperature, K B = material-dependent parameter, 1.08 x 10 31 K -3 cm -6 for Si Bandgap energy is the minimum energy needed to free an electron by breaking a covalent bond in the semiconductor crystal. n i 2 = BT 3 exp E G kT cm -6 Intrinsic Carrier Concentration (cont.) Electron density is n (electrons/cm 3 ) and n = n i for intrinsic material. Hole density is p (holes/cm 3 ) and p = n i for intrinsic material. Intrinsic refers to properties of pure materials (no dopants). n i 10 10 cm -3 for Si at room temperature (300 K). Electron-hole concentrations • A vacancy is left when a covalent bond is broken. • The vacancy is called a hole. • A hole moves when the vacancy is filled by an electron from a nearby bond (hole current). • For intrinsic silicon , n = n i = p . • The product of electron and hole concentrations is pn = n i 2 . •T h e pn product above holds when a semiconductor is in
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2 Drift Current Electrical resistivity ρ and its reciprocal, conductivity σ , characterize current flow in a material when an electric field is applied. Charged particles move or
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This note was uploaded on 10/11/2011 for the course ECE 322 taught by Professor Staff during the Spring '08 term at Boise State.

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chap2 - Solid-State Electronic Materials Electronic...

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