Lecture 1 OUTLINE Semiconductors Junction Diode characteristics Bipolar

Lecture 1 • • • OUTLINE Semiconductors, Junction, Diode characteristics, Bipolar Transistors: characteristics, small signal low frequency h-parameter model, Hybrid-pi model,

Course Outline Amplifiers voltage and current amplifiers, Oscillators, Differentials Amplifiers, Operational Amplifiers, Linear application of OP-AMPs, Gain input and Output Impedance.

What is a Semiconductor? A semiconductor is a substance which has resistivity (10 E-4 to 0. 5 Ohm-m) in between conductors and insulators e. g. germanium, silicon, selenium, carbon etc. • Low resistivity => “conductor” • High resistivity => “insulator” • Intermediate resistivity => “semiconductor” – conductivity lies between that of conductors and insulators – generally crystalline in structure for IC devices • In recent years, however, non-crystalline semiconductors have become commercially very important. polycrystalline amorphous crystalline

Properties of semiconductor • The resistivity of a semiconductor is less than an insulator but more than a conductor. • Semiconductors have negative temperature co-efficient of resistance i. e. the resistance • of a semiconductor decreases with the increase in temperature and vice-versa. For example, germanium • is actually an insulator at low temperatures but it becomes a good conductor at high temperatures. • When a suitable metallic impurity (e. g. arsenic, gallium etc. ) is added to a semiconductor its current conducting properties changes appreciably.

Semiconductor Materials Phosphorus (P) Gallium (Ga)

Silicon • Atomic density: 5 x 1022 atoms/cm 3 • Si has four valence electrons. Therefore, it can form covalent bonds with four of its nearest neighbors. • When temperature goes up, electrons can become free to move about the Si lattice.

Electronic Properties of Si Silicon is a semiconductor material. – Pure Si has a relatively high electrical resistivity at room temperature. There are 2 types of mobile charge-carriers in Si: – – Conduction electrons are negatively charged; Holes are positively charged. The concentration (#/cm 3) of conduction electrons & holes in a semiconductor can be modulated in several ways: 1. 2. 3. 4. by adding special impurity atoms ( dopants ) by applying an electric field by changing the temperature by irradiation

Electron-Hole Pair Generation • When a conduction electron is thermally generated, a “hole” is also generated. • A hole is associated with a positive charge, and is free to move about the Si lattice as well.

Effects of Temperature on Semiconductor (i) At absolute zero: At absolute zero temperature, all the electrons are tightly held by the semiconductor atoms. The inner orbit electrons are bound whereas the valence electrons are engaged in co-valent bonding. At this temperature, the covalent bonds are very strong and there are no free electrons. Therefore, the semiconductor crystal behaves as a perfect insulator. • In terms of energy band description, the valence band is filled and there is a large energy gap between valence band conduction band. Therefore, no valence electron can reach the conduction band to become free electron. It is due to the non-availability of free electrons that a semiconductor behaves as an insulator.

Effects of Temperature on Semiconductor • Above absolute zero. When the temperature is raised, some of the covalent bonds in the semiconductor break due to thermal energy supplied. The breaking of bonds sets those electrons free which are engaged in the formation of these bonds. The result is that a few free electrons exist in the semiconductor. These free electrons can constitute a tiny electric current if potential difference is applied across the semiconductor crystal.

Classification of Semiconductor • Intrinsic Semiconductor: A semiconductor in an extremely pure form is known as an intrinsic semiconductor. • Extrinsic Semiconductor: The intrinsic semiconductor has little current conduction capability at room temperature. To be useful in electronic devices, the pure semiconductor must be altered so as to significantly increase its conducting properties. This is achieved by adding a small amount of suitable impurity to a semiconductor. It is then called impurity or extrinsic semiconductor.

Extrinsic Semiconductor • n-type : • p-type:

Carrier Concentrations in Intrinsic Si • The “band-gap energy” Eg is the amount of energy needed to remove an electron from a covalent bond. • The concentration of conduction electrons in intrinsic silicon, ni, depends exponentially on Eg and the absolute temperature (T):

Doping (N type) • Si can be “doped” with other elements to change its electrical properties. • For example, if Si is doped with phosphorus (P), each P atom can contribute a conduction electron, so that the Si lattice has more electrons than holes, i. e. it becomes “N type”: Notation: n = conduction electron concentration

Doping (P type) • If Si is doped with Boron (B), each B atom can contribute a hole, so that the Si lattice has more holes than electrons, i. e. it becomes “P type”: Notation: p = hole concentration

Summary of Charge Carriers

Electron and Hole Concentrations • Under thermal equilibrium conditions, the product of the conduction-electron density and the hole density is ALWAYS equal to the square of ni: N-type material P-type material

Terminology donor: impurity atom that increases n acceptor: impurity atom that increases p N-type material: contains more electrons than holes P-type material: contains more holes than electrons majority carrier: the most abundant carrier minority carrier: the least abundant carrier intrinsic semiconductor: n = p = ni extrinsic semiconductor: doped semiconductor

Summary • The band gap energy is the energy required to free an electron from a covalent bond. – Eg for Si at 300 K = 1. 12 e. V • In a pure Si crystal, conduction electrons and holes are formed in pairs. – Holes can be considered as positively charged mobile particles which exist inside a semiconductor. – Both holes and electrons can conduct current. • Substitutional dopants in Si: – Group-V elements (donors) contribute conduction electrons – Group-III elements (acceptors) contribute holes – Very low ionization energies (<50 me. V)