Chapter-1 1. ELECTRONIC DEVICES 1.1 Define Electronics & its application. The world's reliance on electronics is so great that commentators claim people live in an "electronic age." People are surrounded by electronics—televisions, radios, computers, mobiles, Laptop and DVD players, along with products with major electric components, such as microwave ovens, refrigerators, and other kitchen appliances, automatic vehicles, Robotics, as well as hearing aids and medical instruments and numerous applications in industry. Definition: The branch of engineering which deals with current conduction through a Vacuum or Gas or Semiconductor is known as Electronics. An electronic device is that in which current flows through a vacuum or gas or semiconductor. This control of electrons is accomplished by devices that resist, carry, select, steer, switch, store, manipulate, and exploit the electron. Or Electronics deals with electrical circuits that involve active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies. Commonly, electronic devices contain circuitry consisting primarily or exclusively of active semiconductors supplemented with passive elements; such a circuit is described as an electronic circuit. Pre Knowledge (Some of the basic definitions): Passive Components: Capable of operating without an external power source. Typical passive components are resistors, capacitors, inductors. Active components: Requiring a source of power to operate. Includes transistors (all types), integrated circuits (all types), TRIACs, SCRs, LEDs, etc. APPLICATIONS of Electronics: Electronic components: capacitor (C), cathode ray tube (CTR), diode (D), digital signal processor (DSP, field effect transistor (FET), integrated circuit (IC), junction gate field effect transistor (JFET), inductor (L), Liquid crystal display (LCD), light dependent resistor (LDR, light emitting diode (LED), Metal oxide semiconductor field effect transistor (MOSFET), transistor (Q), resistor (R), relay (RLA, RY), switch (SW), transformer (T), thermistor (TH), transistor (Tr), integrated circuit (U, IC), variable capacitor (VC), variable resistor (VR) and more. Consumer Electronics include products like – Audio Systems, Video Systems, TV (Television), Computer, Laptop, Digital Camera, DVD Players, Home and Kitchen Appliances, GPS, Mobiles Phones etc.
Communication. Electronic communication systems connect people around the world. Using telephones, Internet and computers, people in different countries communicate almost instantly. Radios transmit sounds and televisions transmit sounds and pictures great distances. Cellular telephones enable a person to call another person. Within seconds, fax machines send and receive copies of documents over telephone lines/Satellite. Information processing. Scientists, artists, students, government and business workers, and hobbyists at home all rely on computers, Internet to handle huge amounts of information quickly and accurately. Computers solve difficult mathematical problems, maintain vast amounts of data, create complex simulations, and perform a multitude of other tasks that help people in their everyday lives. Medicine and research. Include product like X-ray machines ECG (Electrocardiogram) use radiation to take images of bones and internal organs. Radiation therapy, or radiotherapy, uses X-rays and other forms of radiation to fight cancer. Many hearing-impaired people depend on hearing aids to electrically amplify sound waves. Computers and other electronic instruments provide scientists and other researchers with powerful tools to better understand their area of study. Computers, for example, help scientists design new drug molecules, track weather systems, and test theories about how galaxies and stars develop. Electron microscopes use electrons rather than visible light to magnify specimens 1 million times or more. Automation. Electronic components enable many common home appliances, such as refrigerators, washing machines, and toasters, to function smoothly and efficiently. People can electronically program coffeemakers, lawn sprinklers, and many other products to turn on and off automatically. Microwave ovens heat food quickly by penetrating it with short radio waves produced by a vacuum tube. Instrumentation. Measuring Instruments like CRO, Multimeter, ph-meter, strain gauge, VTVM, Frequency Counter are used in different Laboratory/organisations. Many automobiles have electronic controls in their engines and fuel systems. Electronic devices also control air bags, which inflate to protect a driver and passengers in a collision. 1.2 Define Electronic Emission & different types of Emission. The Electronics devices depends the movements of free Electrons in an evacuated space. The liberation of electrons from the surface of a metal is known as Electron Emission. For electron emission, metals are used because they have many free electrons. The electrons are free only to transfer from one atom to another within the metal but they cannot leave the metal surface to provide electron emission. Thus at the surface of the metal, a free electron encounters forces that prevent it to leave the metal. In other words, the metallic surface offer a barrier to free electrons, their kinetic energy increases and is known as surface barrier. However, if sufficient energy is given to the free electrons, their kinetic energy increases and thus the electrons will cross over the surface barrier to leave the metal. This additional energy required by an electron to overcome the surface barrier of the metal is called work function of the metal.
The metallic surface offers a barrier to free electrons and is known as surface barrier. Work function (W0): The amount of additional energy(such as heat energy, energy stored in electric field, light energy or kinetic energy of the electric charges bombarding the metal surface) required to emit an electron from a metallic surface is known as work function of that metal. The minimum energy required by an electron to just escape (i.e. with zero velocity) from metal's surface is called Work function (W0) of the metal. The work function of pure metals varies (roughly) from 2eV to 6eV. Its value depends upon the nature of the metal, its purity and the conditions of the surface. Different types of Emission: There are following four principal method of obtaining electron emission from the surface of a metal: 1. Thermionic Emission (Due to Thermal energy) 2. Field Emission (Due to application of strong electric field) 3. Secondary Emission – ( due to bombardment of high-speed electrons) 4. Photo Electric Emission – ( by the application of light) 1. Thermionic Emission The process of electron emission from a metal surface by supplying thermal energy to it is known as Thermionic emission. In this type of emission the electron emission is achieved by heating the electrode to a sufficient temperature (about 2500oC) to enable the free electrons to leave the metal surface. Due to heating the electrons get enough energy that they emit from the surface of that material heat energy is converted into kinetic energy, causing accelerated motion of free electrons and electrons acquire additional energy equal to the work function of the metal. An electron emitted from a hot cathode comes out with a velocity that presents different between the kinetic energy possessed by electron just before emission usually used in cathode of diode, triode, pentode, CRT and many other. The higher the temperature, the greater is the emission of electrons. The commonly used materials for electron emission are tungsten, thoriated tungsten and metallic oxides of barium and strontium. S.No. 1 2 3 Emitter Tungsten Thoriated tungsten Oxide-coated Work Function Operating Temperature 4.52 eV 2.63 eV 1.1 eV 2327ºC 1700ºC 750ºC 2. Field Emission The process of electron emission by the application of strong electric field at the surface of a metal is known as field emission. When metal surface is placed in an electric field, the electron rotating in their orbits experience a force due to electrostatic field. Hence the process of electron emission by application of strong electric field at the surface of a metal is called field emission. It is also called cold cathode emission or auto- electronic emission.
3. Secondary Emission Electron emission from a metallic surface by the bombardment of high-speed electrons or other particles is known as secondary emission. When high-speed electrons suddenly strike a metallic surface, they may give some or all of their kinetic energy to the free electrons in the metal. If the energy of the striking electrons is sufficient, it may cause free electrons to escape from the metal surface. This phenomenon is called secondary emission. The electrons that strike the metal are called primary electrons while the emitted electrons are known as secondary electrons. The intensity of secondary emission depends upon the emitter material, mass and energy of the bombarding particles. 4. Photo Electric Emission Electron emission from a metallic surface by the application of light is known as photo electric emission. When a beam of light strikes the surface of cathode normally made of potassium, Sodium the energy of photons of light is transfer to the free electrons of cathode. In this method, the energy of light falling upon the metal surface is transferred to the free electrons within the metal to enable them to leave the surface. The greater the intensity of light beam falling on the metal surface, the greater is the photoelectric emission. The emitted electrons are known as photo electrons and the phenomenon is known as photoelectric emission. Photo-electric emission is utilised in photo tubes which form the basis of television and sound films. 1.3 Classification of solid according to electrical conductivity (Conductor, Semiconductor & Insulator) with respect to energy band diagram only. Pre-Knowledge: (i) Valence band. The range of energies (i.e. band) possessed by valence electrons is known as valence band. The electrons in the outermost orbit of an atom are known as valence electrons. This band may be completely or partially filled. (ii) Conduction band. The range of energies (i.e. band) possessed by conduction band electrons is known as conduction band. Generally, insulators have empty conduction band. On the other hand, it is partially filled for conductors. The free electrons which are responsible for the conduction of current in a conductor are called conduction electrons. (iii) Forbidden energy gap. The separation between conduction band and valence band on the energy level diagram is known as forbidden energy gap.