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Department of Chemistry School of Engineering Presidency University STUDY MATERIAL Module 1: Introduction to electronic materials Conductors, Semiconductors & Insulators: Definition of conductors, semiconductor and insulators based on band theory. Semiconductors: Introduction, production of electronic grade silicon from Czochralski process (CZ) and Float Zone (FZ) methods, Chemical and electronic properties & applications of inorganic semiconductors (SiGe, GaAs, InP). The electrons in the outermost orbitals of the atoms determine its electrical properties. The electron theory of solids aims to explain the electrical, thermal and magnetic properties of solids. This theory has been developed in three main stages.
- Classical free electron theory 2. Quantum free electron theory 3. Band theory of solids. Classical free electron theory: Drude and Lorentz developed this theory in 1900. According to this theory the metals containing free electrons obey the laws of Classical Mechanics (These laws describe how objects move under the influence of forces, and how energy, momentum, and angular momentum are conserved in isolated systems). Quantum free electron theory: Somerfield developed this theory during 1928. According to this theory free electrons obey the Quantum laws ( describe the behavior of matter and energy at the atomic and subatomic levels). Band theory of solids or Zone theory: Bloch stated this theory in 1928. According to this theory, the free electrons move in a periodic field provided by the lattice and the theory is also called Band theory of solids ( quantum mechanical model explaining how electrons are arranged in solids, forming energy bands and gaps that determine electrical conductivity (wave-particle duality, quantization, and uncertainty)) Origin of Energy band formation in Solids When we consider isolated atom, the electrons are tightly bound and have discrete, sharp energy levels.
When two identical atoms are brought closer the outer most orbits of these atoms overlap and interact.
- If more atoms are brought together more levels are formed and for a solid of N atoms, each of the energy levels of an atom splits into N levels of energy.
- The levels are so close together that they form an almost continuous band.
- The width of this band depends on the degree of overlap of electrons of adjacent atoms and is largest for outer most atomic electrons. Explanation According to the Bohr atomic model, in an isolated atom the energy of any of its electrons is decided by the orbit in which it revolves. But when the atoms come together to form a solid they are close to each other. So the outer orbits of electrons from neighbouring atoms would come very close or could even overlap. This would make the nature of electron motion in a solid very different from that in an isolated atom. Inside the crystal each electron has a unique position and no two electrons see exactly the same pattern of surrounding charges. Because of this, each electron will have a different energy level. These different energy levels with continuous energy variation form what are called energy bands. The energy band which
The lowest energy level in the conduction band is shown as E C and highest energy level in the valence band is shown as E V
. Above E C and below E V there are a large number of closely spaced energy levels, as shown in Fig. The gap between the top of the valence band and bottom of the conduction band is called the energy band gap (Energy gap Eg). It may be large, small, or zero, depending upon the material. These different situations and discussed below. Electrons in the valence band are tightly bound to their respective atoms. Electrons in the conduction band are free to move throughout the crystal. Case I – Metals: This refers to a situation. One can have a metal either when the conduction band is partially filled and the valence band is partially empty or when the conduction and valance bands overlap. When there is overlap, electrons from valence band can easily move into the conduction band. This situation makes a large number of electrons available for electrical conduction. When the valence band is partially empty, electrons from its lower
level can move to higher level making conduction possible. Therefore, the resistance of such materials is low or the conductivity is high. In metals the bands either overlap or are only partially filled. Thus electrons and empty states are mixed in the bands, so that electrons can move freely under the influence of electric of field. Eg.: Copper, Aluminium, graphite etc. Case II - Insulators : In this case, a large band gap Eg exists ( Eg > 3 eV). There are no electrons in the conduction band, and therefore no electrical conduction is possible. Note that the energy gap is so large that electrons cannot be excited from the valence band to the conduction band by thermal excitation. This is the case of insulators. An insulator has a large band gap, so that the thermally generated carriers are negligible at RT. The band gap of insulators are usually greater than 3 eV. Eg.: Paper, rubber, glass, plastic etc.
Semiconductor Semiconductors are materials which have a conductivity between conductors (generally metals) and insulators (such as most ceramics). Semiconductors can be pure elements, such as silicon or germanium, or compounds such as gallium arsenide or cadmium selenide. Forbidden energy gap very small Semiconductors have special properties that allow you to alter their conductivities from very low to very high values Charge transport in semiconductors can occur by two different kinds of particles – electrons and holes These will exhibit the properties of a conductor or insulator when voltage, current, light, heat, etc., are applied. Furthermore, by combining these, devices such as diodes, transistors, and ICs (integrated circuits) can be created. Examples: Silicon, Germanium, Arsenic etc., The ease with which electricity flows is related to the magnitude of the material's electrical resistance. If the electrical resistance is high, the current will not flow easily, and if the electrical resistance is low, the current will flow easily.
Semiconductor raw materials Silicon (Si) and germanium (Ge), which are the main raw materials for semiconductors, belong to Group IV. Adding trace amounts of impurities (diffusion or doping) significantly reduces the electrical resistance and makes them exhibit the properties of a conductor. Energy band diagram: Semiconductors
Intrinsic Semiconductor A pure semiconductor is called intrinsic semiconductor_._ At room temperature, some of the valence electron acquire sufficient (thermal) energy to jump from VB-CB becoming free electrons (also called conduction electrons). When an electron jumps to the CB, a vacancy left in the VB within the crystal. This is called a hole. For every electron raised to the CB by external energy, there is one hole left in VB creating an electron hole pair. An intrinsic (pure) semiconductor, also called an undoped semiconductor or i-type semiconductor, is a pure semiconductor without any dopant present. It has equal numbers of electrons and holes In intrinsic semiconductors, the number of free electrons, ne is equal to the number of holes, nh. That is ne = nh = ni where ni is called intrinsic carrier concentration. It may be noted that apart from the process of generation of conduction electrons and holes, a simultaneous process of recombination occurs in which Fig. Schematic two-dimensional representation of Si or Ge structure showing covalent bonds at
the electrons recombine with the holes. At equilibrium, the rate of generation is equal to the rate of recombination of charge carriers. The recombination occurs due to an electron colliding with a hole. An intrinsic semiconductor will behave like an insulator at T = 0 K as shown in Fig. (a). It is the thermal energy at higher temperatures (T > 0K), which excites some electrons from the valence band to the conduction band. These thermally excited electrons at T > 0 K, partially occupy the conduction band. Therefore, the energy-band diagram of an intrinsic semiconductor will be as shown in Fig. (b). Here, some electrons are shown in the conduction band. These have come from the valence band leaving equal number of holes there. Fig. (a) Schematic model of generation of hole at site 1 and conduction electron due to thermal energy at moderate temperatures. (b) Simplified representation of possible thermal motion of a hole. The electron from the lower left hand covalent bond (site 2) goes to the earlier hole site1, leaving a hole at its site indicating an apparent movement of the hole from site 1 to site 2. Fig. (a). Intrinsic semiconductor at T=0K behaves like insulator. (b). At T>0K, four thermally generated electron-hole pairs. (Filled circles represent holes, empty circles represent electrons) (a) (b)
Extrinsic Semiconductor Semiconductor materials do not conduct well and are of limited value in their intrinsic state. This is because of the limited number of free electrons in the CB and holes in the VB. Hence intrinsic silicon must be modified by increasing the no. of free electrons or holes to increase its conductivity. This is done by adding impurities to the intrinsic material. The conductivity of Extrinsic Semiconductor increased by the controlled addition of impurities to the intrinsic semiconductor material. This process is called doping. The purpose of doping is to increase the number of electron/holes to improve the conductivity of extrinsic sc. Doping: The process of addition of minute quantity of impurities in semiconductor to increase its conductivity is called doping. The impurity which is added is known as dopant. Carrier concentration: The number of charge carriers per unit volume is defined as carrier concentration. Unit: number of electrons/m^3 n-Type Solids containing electron donor impurities are called n-type semiconductors An n-type semiconductor is an intrinsic semiconductor doped with phosphorus (P), arsenic (As), or antimony (Sb) as an impurity. Silicon has four valence electrons and phosphorus has five valence electrons. If a small amount of phosphorus is added to a pure silicon crystal, one of the valence electrons of phosphorus becomes free to move around (free electron*) as a surplus electron. When this free electron is attracted to the “+” electrode and moves, current flows.
p-Type Semiconductors that contain acceptor impurities are called p-type semiconductors An p-type semiconductor is an intrinsic semiconductor doped with Aluminium (Al), Boron (B), Indium (In) or Gallium (Ga) as an impurity. Silicon has four valence electrons and acceptor has three valence electrons. Trivalent impurities have three valence electrons, while silicon has four. When the impurity is incorporated into the silicon lattice, it forms covalent bonds with three silicon atoms, but there's a missing electron, creating a "hole". When a voltage is applied, electrons from neighbouring atoms can jump into these holes, effectively causing the holes to move through the material. This movement of holes is what constitutes electrical current in a p-type semiconductor.
Production of electronic grade silicon from Czochralski process (CZ) Czochralski method is used to obtain single crystals of semiconductors (Si, Ge) metals (Pd, Pt, Au), salts and synthetic gemstones. This method was invented by Jan Czochralski in 1915. High purity, semiconductor grade Si is melted in a quartz crucible at 1425 C using RF coil. Dopant such as B or P can be added to the molten Si in precise amounts to obtain p-type or n-type Si. A monocrystalline seed crystal fixed to the puller rod is rotated slowly at the rate of 50 rpm and dipped into the melt. Then the rod is slowly pulled upwards at the rate of 1.5 to 5 cm per hour and rotated simultaneously. By precisely controlling the temperature , rate of pulling and speed of rotation , it is possible to extract a large, single-crystal of Si from the melt. This process is normally performed in an inert atmosphere, such as argon. Application
- The most important application of the Czochralski Process may be the growth of large cylindrical ingots, or boules, of single crystal silicon used in the electronics industry to make semiconductor devices like integrated circuits.
- Monocrystalline silicon (mono-Si) grown by the Czochralski method is also known as monocrystalline Czochralski silicon (Cz-Si). It is the basic material in the production of integrated circuits used in computers , TVs, mobile phones, electronic equipment and semiconductor devices.
- Monocrystalline silicon is used in Si solar cells. The Perfect crystal structure yields the highest light-to-electricity conversion efficiency for silicon. Advantages This method is used to grow large single crystals. Thus it is used extensively in the semiconductor industry. There is no direct contact between the crucible walls and the crystal which helps to produce unstressed single crystal The float zone method is based on the zone-melting principle and was invented by Theuerer in 1962. In this process high purity silicon crystal with extremely low concentrations of impurities, such as carbon and oxygen, can be obtained. It is carried out in an inert atmosphere. The process starts with a high purity polycrystalline rod fixed to the clamp and a monocrystalline seed crystal fixed to the seed holder. They are held face to face in a vertical position and rotated as shown in the figure.
Radio frequency (RF) and microwave circuits: SiGe devices are found in wireless communication (WLAN, WPAN), satellite communications, and radar systems, in integrated circuits for radio frequency (RF) signals. High-speed data converters: SiGe is utilized in analog-to-digital (A/D) and digital- to-analog (D/A) converters SiGe Properties Hood radiation tolerance, High temperature capability, Low power consumption High speed switching
Gallium Arsenide (GaAs)
Chemical Properties- GaAs is a compound semiconductor composed of gallium and arsenic, possessing a zinc-blende crystal structure. Electronic Properties- Gallium has superior electronic properties compared to silicon, such as direct bandgap higher electron mobility saturated electron velocity allowing for efficient light emission and absorption. This allows gallium arsenide to function at frequencies up to 250 GHz in transistors. It is less sensitive to high temperatures, generates less noise in electronic circuits, and provides efficient light absorption and emission due to its direct bandgap. Gallium arsenide's resistive nature and high dielectric constant make it an ideal substrate for integrated circuits. These properties make gallium arsenide a promising candidate for applications in satellites, mobile phones, communication technology, and microwave integrated circuit board. GaAs Applications It is a vital semiconductor and is commonly used to manufacture devices such as infrared emitting diodes, laser diodes, Optoelectronic devices: GaAs is used in infrared LEDs, laser diodes, and solar cells.
High-frequency and microwave circuits- GaAs-based devices are used in monolithic microwave integrated circuits (MMICs) and high-speed communication systems. Space applications- Its resistance to radiation damage and high efficiency make it suitable for space-based solar cells and other devices. Substrate material- GaAs is often used as a substrate for growing other III-V compound semiconductors, enabling the fabrication of complex heterostructures
Indium Phosphide (InP)
Chemical Properties- Indium phosphide (InP) is a binary semiconductor compound composed of indium (In) and phosphorus (P). It has a zinc blende structure, a type of cubic lattice. It can be can be toxic if inhaled, ingested, or if it comes into contact with the skin. Electronic Properties- InP has a direct bandgap and high electron mobility, making it suitable for high-frequency and high-speed applications. Applications Optical communication: InP-based devices, such as lasers and photodetectors, are crucial for high-speed optical fiber communication systems. High-speed transistors: InP HBTs are used in high-frequency circuits and systems, including millimeter-wave applications. Photovoltaic devices: InP is also use in solar cells. High-speed digital circuits
