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WAFER FABRICATION Lecture 2 Taklimat UniMAP Semiconductor Basic Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION What is a semiconductor ? • Semiconductors are materials with electrical conductivity between conductors and insulators. • The most commonly used semiconductor Taklimat UniMAP materials are silicon, Si and germanium, Ge. • Some compounds, such as gallium arsenate (GaAs), Silicon carbideMalaysia (SiC) and silicon Universiti Perlis germanium (SiGe). • Most important property is its conductivity can be controlled by adding certain impurities in the process called doping. Hasnizah Aris, 2008 WAFER FABRICATION Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Band Gap • Atom is basic building block of all materials • Classical mechanics –every atom has it own orbit structure. • Electron orbits are called shells. • The outermost shell is called valence shell. • When electron leaves the valence shell, it becomes a free electron and can conduct electric current. Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION • When 2 or more identical atoms bond together to form solid materials, their orbit overlap and form so called energy bands. Can be represented by the energy band diagram. • The bottom of conduction band is called Ec, and the top of the valence band is called Ev. • Eg = Ec –Ev Taklimat UniMAP • Eg is defined as the energy required to break a bond in semiconductor to free an e to cond band and leave the hole in the valence band. • Electrons in conduction band are free to move and can conduct electric current. • Electrons in the valence band are bonded with nuclei and cannot move freely, therefore cannot conduct electric current. Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Resistivity • Resistivity is the capability of a material resisting electric current. Taklimathas UniMAP • A good conductor a very low resistivity and a good insulator has a very high resistivity. Universiti Malaysia Perlis • Unit: Ohm.cm Hasnizah Aris, 2008 WAFER FABRICATION Resistivity and Band Gap • For most metals, conduction and valence bands almost overlap or very small band gap. Electron can easily jump from valence to conduction band. Therefore the conductionTaklimat band has a lot of e. UniMAP • For insulators, the band gap is so large that electrons cannot jump across it. Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Taklimat UniMAP DOPING SEMICONDUCTOR Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Two types of Semiconductor Materials 1. Intrinsic Semiconductor Taklimat UniMAP 2. Extrinsic Semiconductor Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Intrinsic Semiconductor • Pure semiconductor materials with no impurity atoms and no lattice defect. • At T=0 K, all energy states in valence band are filled with electrons, states in conduction band are empty. Taklimat UniMAP • As the temperature increase above 0 K, a few valence bond electrons may gain enough thermal energy to break the bond Universiti Malaysia Perlis and jump into the conduction band. • As temperature increase further, more bonds broken, more electrons jump to the conduction band and more “empty states or holes” created in the valence band. Hasnizah Aris, 2008 WAFER FABRICATION • In intrinsic material, electrons and holes are created in pairs by thermal energy. So the number of electrons in conduction band is equal to the number of holes in the valence band Taklimat UniMAP • Electron concentration = hole concentration, ni= pi and nipi= ni2 (MASS ACTION LAW) –the product of n p Perlis is always Universiti Malaysia a constant for a given semiconductor material at given temperature Hasnizah Aris, 2008 WAFER FABRICATION Extrinsic Semiconductor • Extrinsic s/c is defined as a semiconductor in which controlled amounts of specific dopant or impurity Taklimat UniMAP atoms have been added so that the thermal equilibrium electron and hole Universiti Perlis concentration areMalaysia different from the intrinsic carrier concentration. Hasnizah Aris, 2008 WAFER FABRICATION Doping of Semiconductors • The purpose of doping is to alter the conductivity of semiconductor materials. Taklimat UniMAP • Two types of dopant; p-type (B), ntype (P, As) • P-type dopants provide a holePerlis in s/c Universiti Malaysia materials, hence called acceptor. • N-type dopants provide an electron in s/c materials, hence called donors. Hasnizah Aris, 2008 WAFER FABRICATION n-type Dopant • P and As have 5 electron valence • When doped into Si, 4 electrons used to form the covalence bond with Si. • 1 extra electron is left in the outermost shell and will occupy a new energy level called Donor Energy. • Energy required to elevate donor electron is less than that for electron involved in covalence bonding. • With small thermal energy, donor electron is elevated to the conduction band • This process add electron to the conduction band without creating holes in the valence band. • The resulting material is referred as n-type semiconductor. (draw the energy band and atom covalent bond) Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION p-type Dopant • B have 3 electron valence • When doped into Si, one empty state is created in the covalence bond • This empty state will occupy a new energy level called Acceptor Energy. • Some valence electron gain a small amount of energy to move around the crystal lattice. • This electron would occupy the “empty” position associated with B atom. • The vacated electron position is considered as holes. • This process generate holes in the valence band without creating electrons in the conduction band. • The resulting material is referred as p-type semiconductor Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Dopant Concentration and Resistivity - Pls verify this graph Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Basic Semiconductor Devices Resistor Taklimat UniMAP Capacitor Diode Universiti Malaysia Perlis Bipolar transistor MOS transistor Hasnizah Aris, 2008 WAFER FABRICATION Resistor • The simplest electronic device. • In the IC fabrication, patterned doped silicon normally used to make resistorsTaklimat with resistance UniMAP determined by the length, line width, junction depth and dopant Universiti concentration.Malaysia Perlis • Poly silicon also used a resistor. Hasnizah Aris, 2008 WAFER FABRICATION Example 1 • Many people use polysilicon to form gates and local interconnect. Resistivity of Taklimat UniMAP polysilicon is determined by dopant concentration, about 1022cm-3, and ρ= 200 μΩ.cm. Assume polysilicon gate and local Universiti Malaysia Perlis interconnect line width, height, and length are 1μm, 1μm and 100μm respectively. Calculate the resistance. Hasnizah Aris, 2008 WAFER FABRICATION Solution 1 •R = ρl / wh = 200 μΩ.cm x (100 x 10-4) cm / -4cm) [(1x10-4cm) x 1X10UniMAP Taklimat = 2 x 108μΩ = 200 Ω Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Capacitor • One of the most important IC components • When two conducting materials are UniMAP separatedTaklimat by a dielectric, a capacitor is formed. • Charge storage device Universiti Malaysia • Memory devices esp. DRAM Perlis • Challenge: Reduce capacitor size while keeping the capacitance Hasnizah Aris, 2008 WAFER FABRICATION Taklimat UniMAP • ε0-Absolute permittivity of vacuum(8.85 x 10-12F/m Universiti Malaysia Perlis • κ-Dielectric constant Hasnizah Aris, 2008 WAFER FABRICATION Example 2 • Calculate the capacitance for a capacitor shown with h = l = 10 μm. Assume the dielectric between the 2UniMAP conducting plates Taklimat is silicon dioxide, with k=3.9 and d=1000 Å. Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Solution 2 Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Diode • p-n junction • Allow electric conduction only in one way (positively biased) • When p-type and n-type semiconductors join together, they form a p-n junction diode. • Holes in p-type region will diffuse to the n-type region, and electrons in n-type region will diffuse to the p-type region (at thermal equilibrium, without applied bias). • The area dominated by minority carriers is called the transition region. • The voltage across the transition region given by; Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008 WAFER FABRICATION Taklimat UniMAP Universiti Malaysia Perlis Hasnizah Aris, 2008