SEMICONDUCTOR MATERIALS PDF
covalently bonded (mainly). There are also semiconducting organic, magnetic and ferroelectric materials. Some high–Tc superconductors are semiconducting in. Semiconductor like properties are also found in "organic compounds" like Strong magnetooptical effect allows the material to be used in optical modulators. Semiconductor materials are found in column IV and neighboring column of periodic table. ○ The column IV semiconductor are called elemental semiconductor.
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semiconductor material to be successfully commercialized, its volume of use was and additional materials called II-VI compound semiconductors are also. Define conductor, insulator and semiconductor, and state the resistance or conductance of each. – Name at least three semiconductor materials and state the. CHAPTER 1: Semiconductor Materials & Physics. In this chapter, the basic properties of semiconductors and microelectronic devices are discussed.
A semiconductor material has an electrical conductivity value falling between that of a metal , like copper, gold, etc. Their resistance decreases as their temperature increases, which is behaviour opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities " doping " into the crystal structure. Where two differently-doped regions exist in the same crystal, a semiconductor junction is created. The behavior of charge carriers which include electrons , ions and electron holes at these junctions is the basis of diodes , transistors and all modern electronics.
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The materials segments that SEMI follows include: Reclaim is defined as the removal of several microns of the silicon wafer and subsequent re-polishing of the wafer surface.
Market estimates for reclaim wafers include semiconductor applications including equipment and IC manufacturing markets. Tools Request permission Export citation Add to favorites Track citation.
Semiconductor - Wikipedia
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Crystalline faults are a major cause of defective semiconductor devices. The larger the crystal, the more difficult it is to achieve the necessary perfection. There is a combination of processes that is used to prepare semiconducting materials for ICs.
One process is called thermal oxidation , which forms silicon dioxide on the surface of the silicon. This is used as a gate insulator and field oxide. Other processes are called photomasks and photolithography. This process is what creates the patterns on the circuity in the integrated circuit.
Ultraviolet light is used along with a photoresist layer to create a chemical change that generates the patterns for the circuit. Etching is the next process that is required. The part of the silicon that was not covered by the photoresist layer from the previous step can now be etched.
The main process typically used today is called plasma etching. Plasma etching usually involves an etch gas pumped in a low-pressure chamber to create plasma.
A common etch gas is chlorofluorocarbon , or more commonly known Freon. A high radio-frequency voltage between the cathode and anode is what creates the plasma in the chamber. The silicon wafer is located on the cathode, which causes it to be hit by the positively charged ions that are released from the plasma.
The end result is silicon that is etched anisotropically. The last process is called diffusion.
This is the process that gives the semiconducting material its desired semiconducting properties. It is also known as doping. The process introduces an impure atom to the system, which creates the p-n junction. In order to get the impure atoms embedded in the silicon wafer, the wafer is first put in a 1, degree Celsius chamber.
The atoms are injected in and eventually diffuse with the silicon. After the process is completed and the silicon has reached room temperature, the doping process is done and the semiconducting material is ready to be used in an integrated circuit.
Semiconductors are defined by their unique electric conductive behavior, somewhere between that of a conductor and an insulator. These states are associated with the electronic band structure of the material. Electrical conductivity arises due to the presence of electrons in states that are delocalized extending through the material , however in order to transport electrons a state must be partially filled , containing an electron only part of the time.
The energies of these quantum states are critical, since a state is partially filled only if its energy is near the Fermi level see Fermi—Dirac statistics. High conductivity in a material comes from it having many partially filled states and much state delocalization.
Metals are good electrical conductors and have many partially filled states with energies near their Fermi level. Insulators , by contrast, have few partially filled states, their Fermi levels sit within band gaps with few energy states to occupy. Importantly, an insulator can be made to conduct by increasing its temperature: An intrinsic semiconductor has a band gap that is smaller than that of an insulator and at room temperature significant numbers of electrons can be excited to cross the band gap.
A pure semiconductor, however, is not very useful, as it is neither a very good insulator nor a very good conductor. However, one important feature of semiconductors and some insulators, known as semi-insulators is that their conductivity can be increased and controlled by doping with impurities and gating with electric fields.
Doping and gating move either the conduction or valence band much closer to the Fermi level, and greatly increase the number of partially filled states. Some wider-band gap semiconductor materials are sometimes referred to as semi-insulators.
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When undoped, these have electrical conductivity nearer to that of electrical insulators, however they can be doped making them as useful as semiconductors. Semi-insulators find niche applications in micro-electronics, such as substrates for HEMT. An example of a common semi-insulator is gallium arsenide.
The partial filling of the states at the bottom of the conduction band can be understood as adding electrons to that band. The electrons do not stay indefinitely due to the natural thermal recombination but they can move around for some time.
The actual concentration of electrons is typically very dilute, and so unlike in metals it is possible to think of the electrons in the conduction band of a semiconductor as a sort of classical ideal gas , where the electrons fly around freely without being subject to the Pauli exclusion principle. In most semiconductors the conduction bands have a parabolic dispersion relation , and so these electrons respond to forces electric field, magnetic field, etc.
For partial filling at the top of the valence band, it is helpful to introduce the concept of an electron hole. Although the electrons in the valence band are always moving around, a completely full valence band is inert, not conducting any current.
If an electron is taken out of the valence band, then the trajectory that the electron would normally have taken is now missing its charge. For the purposes of electric current, this combination of the full valence band, minus the electron, can be converted into a picture of a completely empty band containing a positively charged particle that moves in the same way as the electron. Combined with the negative effective mass of the electrons at the top of the valence band, we arrive at a picture of a positively charged particle that responds to electric and magnetic fields just as a normal positively charged particle would do in vacuum, again with some positive effective mass.
When ionizing radiation strikes a semiconductor, it may excite an electron out of its energy level and consequently leave a hole. This process is known as electron—hole pair generation. Electron-hole pairs are constantly generated from thermal energy as well, in the absence of any external energy source.
Electron-hole pairs are also apt to recombine. Conservation of energy demands that these recombination events, in which an electron loses an amount of energy larger than the band gap , be accompanied by the emission of thermal energy in the form of phonons or radiation in the form of photons.
In some states, the generation and recombination of electron—hole pairs are in equipoise. The number of electron-hole pairs in the steady state at a given temperature is determined by quantum statistical mechanics. The precise quantum mechanical mechanisms of generation and recombination are governed by conservation of energy and conservation of momentum.
As the probability that electrons and holes meet together is proportional to the product of their numbers, the product is in steady state nearly constant at a given temperature, providing that there is no significant electric field which might "flush" carriers of both types, or move them from neighbour regions containing more of them to meet together or externally driven pair generation.
The probability of meeting is increased by carrier traps—impurities or dislocations which can trap an electron or hole and hold it until a pair is completed. Such carrier traps are sometimes purposely added to reduce the time needed to reach the steady state. The conductivity of semiconductors may easily be modified by introducing impurities into their crystal lattice. The process of adding controlled impurities to a semiconductor is known as doping. The amount of impurity, or dopant, added to an intrinsic pure semiconductor varies its level of conductivity.
Doped semiconductors are referred to as extrinsic. By adding impurity to the pure semiconductors, the electrical conductivity may be varied by factors of thousands or millions. The addition of 0. The materials chosen as suitable dopants depend on the atomic properties of both the dopant and the material to be doped.
In general, dopants that produce the desired controlled changes are classified as either electron acceptors or donors. Semiconductors doped with donor impurities are called n-type , while those doped with acceptor impurities are known as p-type. The n and p type designations indicate which charge carrier acts as the material's majority carrier.
The opposite carrier is called the minority carrier , which exists due to thermal excitation at a much lower concentration compared to the majority carrier.
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