The Hall effect

Theory of Relativity – A Brief History The Theory of Relativity, proposed by the Jewish physicist Albert Einstein (1879-1955) in the early part of the 20th century, is one of the most significant scientific advances of our time. Although the concept of relativity was not introduced by Einstein, his major contribution was the recognition that the speed of light in a vacuum is constant and an absolute physical boundary for motion. This does not have a major impact on a person's day-to-day life since we travel at speeds much slower than light speed. For objects travelling near light speed, however, the theory of relativity states that objects will move slower and shorten in length from the point of view of an observer on Earth. Einstein also derived the famous equation, E = mc2,which reveals the equivalence of mass and energy.  When Einstein applied his theory to gravitational fields, he derived the "curved space-time continuum" which depicts the dimensions of space ...

Diffraction, the spreading of waves around obstacles. Diffraction takes place with sound; with electromagnetic radiation, such as light, X-rays, and gamma rays; and with very small moving particles such as atoms, neutrons, and electrons, which show wavelike properties. One consequence of diffraction is that sharp shadows are not produced. The phenomenon is the result of interference (i.e., when waves are superimposed, they may reinforce or cancel each other out) and is most pronounced when the wavelength of the radiation is comparable to the linear dimensions of the obstacle. When sound of various wavelengths or frequencies is emitted from a loudspeaker, the loudspeaker itself acts as an obstacle and casts a shadow to its rear so that only the longer bass notes are diffracted there. When a beam of light falls on the edge of an object, it will not continue in a straight line but will be slightly bent by the contact, causing a blur at the edg...

Quantum mechanics, sciencedealing with the behaviour of matterand light on the atomic and subatomic scale. It attempts to describe and account for the properties of molecules and atoms and their constituents—electrons, protons, neutrons, and other more esoteric particles such as quarks and gluons. These properties include the interactions of the particles with one another and with electromagnetic radiation (i.e., light, X-rays, and gamma rays). The behaviour of matter and radiation on the atomic scale often seems peculiar, and the consequences of quantum theory are accordingly difficult to understand and to believe. Its concepts frequently conflict with common-sense notions derived from observations of the everyday world. There is no reason, however, why the behaviour of the atomic world should conform to that of the familiar, large-scale world. It is important to realize that quantummechanics is a bra...

The Big Bang Theory is the leading explanation about how the universe began. At its simplest, it says the universe as we know it started with a small singularity, then inflated over the next 13.8 billion years to the cosmos that we know today. Because current instruments don't allow astronomers to peer back at the universe's birth, much of what we understand about the Big Bang Theory comes from mathematical formulas and models. Astronomers can, however, see the "echo" of the expansion through a phenomenon known as the cosmic microwave background. While the majority of the astronomical community accepts the theory, there are some theorists who have alternative explanations besides the Big Bang — such as eternal inflation or an oscillating universe. The phrase "Big Bang Theory" has been popular among astrophysicists for decades, but it hit the mainstream in 2007 when a comedy show with the same name premiered on CBS. The show follows the home and...

Zener breakdown: A strong reverse-bias applied to a p-n junction leads to high reverse current. When the increasing reverse bias attains a certain value, the junction breaks down and reverse current rises sharply. This specific value of the reverse bias voltage is called breakdown voltage (v z ). The breakdown voltage depends upon the width of depletion layer, which in turn depends upon the doping concentration. Zener Breakdown: Zener breakdown occurs in heavily doped p-n junction diodes which have strong electric field across a thin depletion region. This field breaks the covalent bonds and generates electrons which are pulled into the conduction band and become available for conduction. Minority carrier thus generated quantum mechanically tunnel through the thin depletion layer. As the number of such free electrons becomes large, the reverse current through the Zener diode is b...

Fabrication of p-n junction: Thermal Oxidation: Thermal oxidation is a technique that uses high temperature (700- 1300 o C) for growing the layer of silicon dioxide (SiO 2 ) on the substrate silicon. In thermal oxidation, silicon substrate is exposed to an oxidizing environment of O 2 or H 2 O at elevated temperature, producing oxide films whose thicknesses range from 60 to 10000 A o . Silicon has a natural inclination to form a stable oxide even at room temperature, as long as an oxidizing ambient is present. The elevated temperature used in thermal oxidation therefore serves primarily as an accelerator of the oxidation process, resulting in thicker oxide layers per unit of time. Thermal oxidation is accomplished using an oxidation furnace or diffusion furnace, since oxidation is basically a diffusion process involving oxidant species. A furnace typically consists of: 1) a cabinet; 2) a heating system; 3) a temperature measurement and control system;...

Molecular Beam Epitaxy: MBE is non-CVD epitaxial process using evaporation at very low pressures. Using it we can grow high-purity epitaxial layers of compound semiconductors. MBE operates at lower temp than CVD, it minimizes auto-doping, facilitates abrupt interfaces and good control of thickness, doping, and composition. It allows complicated doping profiles that have applications in microwave and photonic devices, e.g. linear doping profile for linear C-V of Varactor diodes, which are used as FM modulators. In a highly evacuated chamber, heat a base material (substrate) such as silicon, germanium, or gallium arsenide to 500 – 600°C. Shoot precise beams of atoms or molecules in gas form (suitably heated) at the substrate from guns called effusion cells. A separate effusion cell is needed for each different beam, shooting a different kind of molecule at the substrate, depending on the nature of the crystal t...

Doping Intrinsic semiconductor is pure, has no impurity atoms and is called i-type semiconductor. It is un-doped semiconductor and its conductivity is negligible so it cannot make a useful device. Its number density of electrons is equal to the number density of holes. [ n e = n h ]. Its valence band is filled and the conduction band is empty. If its temperature is raised, some of the valence electrons may get lifted to conduction band leaving behind holes in the valence band. These electrons and holes can move randomly. With increasing temperature, the resistivity of the material decreases and the conductivity increases. If we intentionally modify an intrinsic semiconductor by adding external impurities, it gets c onverted to extrinsic semiconductor. Such modification – i.e. the process of adding impurities to intrinsic semiconductors to change their electrical pro...

Growth of semiconductor Crystals Electronic grade silicon is made by purifying 95-97% polysilicon, which is prepared by heating silica and carbon in an ambient that is ideal for the reduction of former. The resulting material is converted to SiHCl 3 , thermally decomposed in H 2 chamber at 1100 o C. Precise amount of dopant (not in the form of Phosphorous or Boron elements) but as powder of doped semiconductor may be mixed in the melt to get semiconductor crystals of desired doping. Main impurities in this poly-silicon are O 2 and Carbon, which get further reduced when it is melted and re-crystallized into a single crystal by Czochralski (CZ) technique. This technique is used to grow single crystals of semiconductors (Si, Ge, GaAs), metals (Ag, Au, Pt, Sn, Bi), bimetals like AlSb, InSb, GaSb, CsJ, and other compounds such as KBr, CaF 2 , BaF 2 , NaCl, LiN, Al-Pd-Mn etc. Large cylindrical ingots or boules of single crystal silicon are grown by us...

Atomic spectra When atoms are excited they emit light of certain wavelengths which correspond to different colors. The emitted light can be observed as a series of colored lines with dark spaces in between; this series of colored lines is called a line or atomic spectra. Each element produces a unique set of spectral lines. Since no two elements emit the same spectral lines, elements can be identified by their line spectrum. The examples are neon signs, sodium and mercury vapour lamps. Electromagnetic Radiation and the Wave Particle Duality Electromagnetic radiation (energy) can travel through vacuum or through materials as a transverse wave. The spectrum of electromagnetic radiation has all possible wavelengths and frequencies including visible light. According to the wave particle duality concept, the electromagnetic radiation is a wave, and it also behaves like a ...

Photoelectric effect  When the atoms of a metal absorb electromagnetic radiation, they eject electrons causing  a flow of current in a circuit. This phenomenon is called photoelectric effect. It was observed that blue light incident on Potassium could eject photoelectrons but red light failed to eject any electron from the same metal. The energy of the ejected electrons did not depend on the intensity of the incoming light, but it depended upon its frequency. When low-frequency light of weak intensity was incident upon a metal, a few low energy electrons were ejected. Increasing the intensity of light, increased the number of ejected electrons but these electrons had the same low energy. In order to eject high energy electrons, one must illuminate the metal with high-frequency light.  Einstein used Planck's energy quanta (which determines the energy of the photons based upon its frequency) to explain the photoelectric effect. Thes...