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Electronics
Electronics comprises the physics, engineering, technology and applications that deals with the emission, flow and control of electrons in vacuum and matter.
In other words, it is the branch of physics and technology concerned with the design of circuits using transistors and microchips, and with the behaviour and movement of electrons in a semiconductor, conductor, vacuum, or gas.
Branches of Electronics
Digital electronics
Analogue electronics
Microelectronics
Circuit design
Integrated circuits
Power electronics
Optoelectronics
Semiconductor devices
Embedded systems
Audio electronics
Telecommunication
Conductors
Conductors are substances which have the property to pass different types of energy. They are materials through which current flows freely. Most familiar conductors are metallic. Copper is the most common material used for electrical wiring. Silver is the best conductor but it is expensive. Gold is used for high-quality surface-to-surface contacts because it does not corrode. However, there are also many non-metallic conductors, including graphite, solutions of salts and all plasmas. There are even conductive polymers.
Band Theory – The band structure of solid describes those ranges of entry called energy bands, that an electron within the solid may have (allowed bands) and ranges of energy called band gaps (forbidden bands), which it may not have. Band theory models the behaviour of electrons in solids by postulating the existence of energy bands. It successfully uses a material’s band structure to explain many physical properties of solids. Bands may also be viewed as the large-scale limit of molecular orbital theory.
In the case of conductors, the last occupied band of energy levels is only partially filled. The electrons of the valence band (V), which is the lower completely filled band, move freely in partially filled conduction band. The highest energy level occupied at absolute zero by electrons in partially filled conduction band is called Fermi level, and the corresponding energy is called Fermy energy. There is no gap band between the valence and conduction bands in conductors since they overlap. This means that electrons move freely between the valence and conduction band, and this allows conduction.
In addition to metals, salts can also conduct electricity. There are no free electrons, so the conductivity depends on the ions which can be dissolved when salt is melting or dissolving so that the ions are free to move.
Insulators
Insulators are non-conductive. In other words, they are materials through which current does not flow easily such as diamond, wood, oil, plastic, paper, etc.
The atomic bond is based on shared electron pairs of non-metals. The elements which behave like non-metals have the desire to attract electrons. Thus, there are no free electrons.
In solid-state, ions are arranged in a grid network by electrical forces, the particles are held together. There are no free charge carriers to enable a current flow. Thus, substances composed of ions can be both conductor and insulator.
In insulators, the band gap between the valence band and the conduction band is so large that electrons cannot make the energy jump from the valence band to the conduction band. The valence band is full as no electrons can move up the conduction band. As a result, the conduction band is empty. Only the electrons in a conduction band can move easily, so since there are no electrons in an insulator’s conduction band, the material cannot conduct. The band that separates two bands (conduction and valence band) is called forbidden band (F).
Semiconductors
Semiconductors are materials that have properties in between those of normal conductors and insulators. This means that they do not allow the free flow of electron like conductors, and they do not block the flow like insulators. A semiconductor has an electrical conductivity value falling between that of a conductor such as metallic copper, and an insulator such as glass. Its resistance falls as its temperature rises; metals are the opposite. Its conducting properties may be altered in useful ways by adding impurities (doping) into the crystal structure. When two differently-doped regions exist in the same crystal, a semiconductor junction is created.
The behaviour of electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semiconductors are silicon, germanium, gallium arsenide, and elements near the “metalloid staircase” on the periodic table. After silicon, gallium arsenide is the second most common semiconductor and is used in laser diodes, solar cells, microwave-frequency integrated circuits and others. Silicon is a critical element for fabricating most electronic circuits.
Semiconductors have a small energy gap between the valence band and the conduction band. Electrons can make the jump to the conduction band but not with the same ease as they do in conductors. At room temperature, there is sufficient energy available to move some electrons from the valence band into the conduction band. This allows some conduction to take place. An increase in temperature increases the conductivity of a semiconductor because more electrons will have enough energy to move into the conduction band.
Intrinsic Semiconductors
The two types of semiconductors are intrinsic and extrinsic semiconductors.
Intrinsic semiconductor can be defined as chemically pure material without any impurity added to it. The most commonly known intrinsic or pure semiconductors available are Silicon (Si) and Germanium (Ge). The behaviour of the semiconductor on applying a certain voltage is dependent on its atomic structure. The outermost shell of both Silicon and Germanium has four electrons each. To stabilize each other nearby atoms form covalent bonds based on the sharing of valence electrons. This bonding in the crystal lattice structure of Silicon is illustrated in the figure below. Here it can be seen that the valence electrons of two silicon atom pair together to form a Covalent Bond.
In intrinsic semiconductors, current flows due to the motion of free electrons as well as holes. The total current is the sum of the electron current Ie due to thermally generated electrons and the hole current Ih.
Total current (I) = Ie + Ih
For an intrinsic semiconductor, at finite temperature, the probability of electrons to exist in conduction band decreases exponentially with increasing band gap. When a semiconductor material is subjected to hear or applied voltage, few of the covalent bonds break, which generates free electrons as shown above. These electrons get excited and gain energy to overcome the forbidden gap and enter the conduction band from the valence band. As the electron leaves the valence band, it leaves behind a hole in the valence band. An equal number of electrons and holes will always be created, hence, it exhibits electrical neutrality. Both the electrons and holes are responsible for conduction of current in the intrinsic semiconductor.
Extrinsic Semiconductors
The extrinsic semiconductor is defined as the material with an added impurity. Doping is the process of deliberately adding impurities to increase the number of carriers. The impurities used are termed as dopants. As the number of electrons and holes is greater in extrinsic conductor it exhibits greater conductivity than intrinsic semiconductors. The purpose of adding impurities or dopants in the semiconductor crystal is to increase the number of free electrons or holes to make it conductive.
If a trivalent dopant having three valence electrons is added, a large number of free electrons will exist. If a pentavalent impurity having five valence electrons is added to a pure semiconductor, a large number of free electrons will exist.
Depending on the type of impurity added, the extrinsic semiconductor may be classified as N-type semiconductor and P-type semiconductor
N-type Semiconductor
The N-type semiconductors are doped with pentavalent impurities. The pentavalent elements are called so as they have five electrons in their valence shell. The examples of pentavalent impurity are Phosphorus (P), Arsenic (As) and Antimony (Sb).
As depicted in the figure below, the dopant atom establishes covalent bonds by sharing four of its valence electrons with four neighbouring silicon atoms and the fifth electron remains loosely bound to the nucleus of the dopant atom. Very less ionization energy is required to set free the fifth electron so that it leaves the valence band and enters the conduction band. The pentavalent impurity imparts one extra electron to the lattice structure and hence it is called as the donor impurity. The figure below is an N-type semiconductor with phosphorus as the impurity.
P-type Semiconductor
P-type semiconductors are doped with the trivalent semiconductor. The trivalent impurities have 3 electrons in their valence shell. Examples of trivalent impurities include Boron (B), Gallium (G), Indium (In), Aluminium (Al).
As depicted in the figure below, the dopant atom establishes covalent bonds with only three neighbouring silicon atoms and a hole or vacancy is generated in the bond with the fourth silicon atom. The hole acts as a positive carrier or space for the electron to occupy. Thus the trivalent impurity has imparted a positive vacancy or hole which can readily accept electrons and hence it is called an acceptor impurity. The figure below shows a P-type semiconductor with boron as the acceptor impurity.
Anomalous Expansion of Water
The way substances generally react to heat is that they expand when heated as the density decreases and vice versa takes place then they are cooled. This is not the case with water. The general tendency of cold water remains unchanged until 4oC. The density of water gradually increases as you cool it. When you reach 4oC, its density reaches a maximum. When you cool water further to make ice (i.e. 0oC), it expands with a further drop in temperature; meaning the density of water decrease when you cool it form 4oC to 0oC.
The effect of this expansion of water is that the coldest water is always present on the surface which is why ice is seen on the surface of water. Since water at 4oC is the heavest, this water settles on the bottom of the water body and the lightest i.e. the coldest layer accumulates on the top layer. This is why the top of the water is always the first to freeze. Since ice and water are both bad conductors of heat, this top layer of ice insulates the rest of the water body from the cold thereby protecting all the life in the water body.
Furthermore, when water is cooled and freezes, the frozen water (ice) expands becoming less dense causing problems with roads, buildings, etc. This property is very unusual and explains why ice floats.
Diodes
A diode is a 2-lead semiconductor that acts as a one-way gate to electron flow. It allows current to pass in only one direction. A PN-junction diode is formed by joining together n-type and p-type silicon.
In practice, as the n-type Si crystal is being grown, the process is abruptly altered to grow p-type Si crystal. Finally, a glass or plastic coating is placed around the joined crystal. The p-side is called the anode and the n-side is called the cathode. When the anode and cathode of a PN-junction diode are connected to an external voltage such that the potential at the anode is higher than the potential at the cathode, the diode is said to be forward-biased. In a forward biased diode current is allowed to flow through the device. When the potential at the anode is smaller than the potential at the cathode, the diode is said to be reverse biased. In a reverse-biased diode current is blocked.
