Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Lecture 2 Semiconductor Basic

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 What is a semiconductor ? Semiconductors are materials with electrical conductivity between conductors and insulators. The most commonly used semiconductor materials are silicon, Si and germanium, Ge. Some compounds, such as gallium arsenate (GaAs), Silicon carbide (SiC) and silicon germanium (SiGe). Most important property is its conductivity can be controlled by adding certain impurities in the process called doping.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 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 WAFER FABRICATION Hasnizah Aris, 2008 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 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.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Resistivity Resistivity is the capability of a material resisting electric current. A good conductor has a very low resistivity and a good insulator has a very high resistivity. Unit: Ohm.cm

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 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 conduction band has a lot of e. For insulators, the band gap is so large that electrons cannot jump across it.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 DOPING SEMICONDUCTOR

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Two types of Semiconductor Materials 1.Intrinsic Semiconductor 2.Extrinsic Semiconductor

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 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. As the temperature increase above 0 K, a few valence bond electrons may gain enough thermal energy to break the bond 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.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 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 Electron concentration = hole concentration, n i = p i and n i p i = n i 2 (MASS ACTION LAW) –the product of n p is always a constant for a given semiconductor material at given temperature

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Extrinsic Semiconductor Extrinsic s/c is defined as a semiconductor in which controlled amounts of specific dopant or impurity atoms have been added so that the thermal equilibrium electron and hole concentration are different from the intrinsic carrier concentration.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Doping of Semiconductors The purpose of doping is to alter the conductivity of semiconductor materials. Two types of dopant; p-type (B), n- type (P, As) P-type dopants provide a hole in s/c materials, hence called acceptor. N-type dopants provide an electron in s/c materials, hence called donors.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 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 WAFER FABRICATION Hasnizah Aris, 2008 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 WAFER FABRICATION Hasnizah Aris, 2008 Dopant Concentration and Resistivity - Pls verify this graph

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Basic Semiconductor Devices Resistor Capacitor Diode Bipolar transistor MOS transistor

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Resistor The simplest electronic device. In the IC fabrication, patterned doped silicon normally used to make resistors with resistance determined by the length, line width, junction depth and dopant concentration. Poly silicon also used a resistor.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Example 1 Many people use polysilicon to form gates and local interconnect. Resistivity of polysilicon is determined by dopant concentration, about cm -3, and ρ= 200 μΩ.cm. Assume polysilicon gate and local interconnect line width, height, and length are 1μm, 1μm and 100μm respectively. Calculate the resistance.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Solution 1 R = ρl / wh = 200 μΩ.cm x (100 x ) cm / [(1x10 -4 cm) x 1X10 -4 cm) = 2 x 10 8 μΩ = 200 Ω

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Capacitor One of the most important IC components When two conducting materials are separated by a dielectric, a capacitor is formed. Charge storage device Memory devices esp. DRAM Challenge: Reduce capacitor size while keeping the capacitance

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 ε 0 -Absolute permittivity of vacuum(8.85 x F/m κ-Dielectric constant

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Example 2 Calculate the capacitance for a capacitor shown with h = l = 10 μm. Assume the dielectric between the 2 conducting plates is silicon dioxide, with k=3.9 and d=1000 Å.

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 Solution 2

Taklimat UniMAP Universiti Malaysia Perlis WAFER FABRICATION Hasnizah Aris, 2008 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 WAFER FABRICATION Hasnizah Aris, 2008