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EEE1001 - ELECTRIC CIRCUITS AND SYSTEMS Module 3 Session 1 Semiconductor Devices • Introduction to semiconductors PN Junction Diode: Construction and operation with characteristics - Rectifiers: Half-wave and Full-wave. Zener Diode: Construction and operation with characteristics - Zener Regulator. BJT - Configuration – Characteristics – CEAmplifier, SCR & MOSFET. Introduction to semiconductors What is a Semiconductor? Microprocessors LED Transistors Capacitors Range of Conduciveness The semiconductors fall somewhere between conductors and insulators. midway Range of Conduciveness Semiconductors have special electronic properties which allow them to be insulating or conducting depending on their composition. Resistance (Ohms) 1833 Michael Faraday Temperature (ºC) Discovers that electrical resistively decreases as temperature increases in silver sulfide. This is the first investigation of a semiconductor. Lab: Metals vs. Semiconductors Data Chart Temperature 0ºC 25ºC 50ºC 75ºC 100ºC Copper 31Ω 33Ω 37Ω 41Ω 44Ω Germanium 5.2Ω 4.2Ω 1.2Ω 0.63Ω .029Ω 1874 Ferdinand Braun The first semiconductor device was born. Scientific Principle of Conduction Valence Band • The highest occupied energy band is called the valence band. • Most electrons remain bound to the atoms in this band. Conduction Band • The conduction band is the band of orbitals that are high in energy and are generally empty. • It is the band that accepts the electrons from the valence band. Energy Gap • The “leap” required for electrons from the Valence Band to enter the Conduction Band. Conduction Band Band Gap Valence Band Conductors In a conductor, electrons can move freely among these orbitals within an energy band as long as the orbitals are not completely occupied. Conductors • In conductors, the valence band is empty. Conductors Also in conductors, the energy gap is nonexistent or relatively small. Insulators In insulators, the valence band is full. Insulators Also in insulators, the energy gap is relatively large. Semiconductors In semiconductors, the valence band is full but the energy gap is intermediate. Semiconductors Only a small leap is required for an electron to enter the Conduction Band. Band Diagrams Semiconductor Materials Silicon is a very common element, the main element in sand & quartz. Silicon’s Arrangement Types of Semiconductor • Intrinsic semiconductor • Extrinsic semiconductor Intrinsic Silicon • At any temperature above absolute zero temperature, there is a finite probability that an electron in the lattice will be knocked loose from its position. Intrinsic Silicon • The electron in the lattice knocked loose from its position leaves behind an electron deficiency called a "hole". Current Flow • If a voltage is applied, then both the electron and the hole can contribute to a small current flow. Doping • Doping (adding an impurity) can produce 2 types of semi-conductors depending upon the element added. • P- Type • N- Type P-Type Doping In P-type doping, boron or gallium is the dopant. P-Type Doping • Boron and gallium each have only three outer electrons. • When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. P-Type Doping The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbor, moving the hole over a space. P-type silicon is a good conductor. N-Type • In N-type doping, phosphorus or arsenic is added to the silicon in small quantities. N-Type • Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. • The fifth electron has nothing to bond to, so it's free to move around. N-Type • It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. N-type silicon is a good conductor. • Electrons have a negative charge, hence the name N-type. P-N Junction • A p-n junction is created by joining together two pieces of semiconductor, one doped n-type, the other p-type. P-N Junction • In the n-type region there are extra electrons and in the ptype region, there are holes from the acceptor impurities . P-N Junction In the p-type region there are holes from the acceptor impurities and in the n-type region there are extra electrons. P-N Junction When a p-n junction is formed, some of the electrons from the n-region which have reached the conduction band are free to diffuse across the junction and combine with holes. P-N Junction • Filling a hole makes a negative ion and leaves behind a positive ion on the n-side. • A space charge builds up, creating a depletion region. P-N Junction This causes a depletion zone to form around the junction (the join) between the two materials. This zone controls the behavior of the diode. Forward Biasing Forward biasing the p-n junction drives holes to the junction from the p-type material and electrons to the junction from the n-type material. Forward Biasing At the junction the electrons and holes combine so that a continuous current can be maintained. Diode A diode is the simplest possible semiconductor device. One Way Electric “Turnstile” A diode allows current to flow in one direction but not the other. Jumping If you apply enough reverse voltage, the junction breaks down and lets current through. Reverse Biasing The application of a reverse voltage to the p-n junction will cause a transient current to flow as both electrons and holes are pulled away from the junction. Reverse Biasing When the potential formed by the widened depletion layer equals the applied voltage, the current will cease except for the small thermal current. When forward-biased, there is a small amount of voltage necessary to get the diode going. In silicon, this voltage is about 0.7 volts. This voltage is needed to start the hole-electron combination process at the junction. Diode Characteristic When reverse-biased, an ideal diode would block all current. A real diode lets perhaps 10 microamps through -- not a lot, but still not perfect. Zener diodes • A Zener diode is a silicon semiconductor device that permits current to flow in either a forward or reverse direction. • The diode consists of a special, heavily doped p-n junction, designed to conduct in the reverse direction when a certain specified voltage is reached. Working Principle • If reverse voltage is increased continuously, junction breaks down and suddenly a large reverse current flows. • This reverse current is controlled or limited by connecting a suitable series resistance so that excessive heat produced due to heavy current flow may not burn the diode. • Two ways breakdown occur • Zener breakdown • Avalanche break down Zener breakdown • If the depletion layer of a diode is narrow and a reverse voltage is applied, the voltage per unit of width of the depletion layer becomes high. • This establishes a strong electric field intensity which causes electrons to break away from their parent atoms. • Thus, a depletion layer which was insulating in nature, becomes a conducting path. • This kind of breakdown due to the creation of a strong electric field intensity, i.e., V/μm is called zener breakdown Avalanche breakdown • If the width of the depletion layer is wide for a zener breakdown, a sufficient reverse voltage may provide the free electrons (minority carriers causing saturation current) to gain sufficient energy to knockout electrons from the atoms of the semiconductor in the depletion region. • This is called ionization by collision. • The breakdown occurring this way is called avalanche breakdown. • The breakdown voltage of a diode can be accurately control at the time of the doping level. The breakdown voltage of a commonly available Zener diodes can vary from 1.2 V to 200 V. • Diodes which are lightly doped and the breakdown voltage is less than 5.6 V, is due to Zener effect. • Regulating the voltage of a circuit is its ability to keep the output voltage fixed regardless of the variation in input voltage or load current. • Here, RS is the current limiting resistor, VS is the voltage source and RL represents the load resistance. RS absorbs the voltage fluctuations so as to give a constant voltage at the output. Until load voltage is less than the breakdown voltage, the zener diode does not show conduction. • As the voltage at the load increases than breakdown voltage, the device now starts conduction in the breakdown region. Thus, at the breakdown region, a constant voltage is maintained Rectifiers • Rectifier is a device which convert AC voltage in to pulsating DC • A rectifier utilizes unidirectional conducting device Ex : P-N junction diodes Types • Depending up on the period of conduction Half wave rectifier Full wave rectifier • Depending up on the connection procedure Bridge rectifier Half wave rectifier • The ripple factor is quite high(1.21) • Rectifier efficiency is very low(40%) • Transformer Utilization Factor (TUF) is low(0.21) • The half wave rectifier circuit is normally not used as a power rectifier circuit Half wave Rectifiers As diodes conduct current in one direction and block in other. When connected with ac voltage, diode only allows half cycle passing through it and hence convert ac into dc. As the half of the wave get rectified, the process called half wave rectification. A diode is connected to an ac source and a load resistor forming a half wave rectifier. Positive half cycle causes current through diode, that causes voltage drop across resistor. • Advantages of Half Wave Rectifier • Affordable • Simple connections • Easy to use as the connections are simple • Number of components used are less • Disadvantages of Half Wave Rectifier • Ripple production is more • Harmonics are generated • Utilization of the transformer is very low • The efficiency of rectification is low • Applications of Half Wave Rectifier • Power rectification: Half wave rectifier is used along with a transformer for power rectification as powering equipment. • Signal demodulation: Half wave rectifiers are used for demodulating the AM signals. • Signal peak detector: Half wave rectifier is used for detecting the peak of the incoming waveform. Diode as Rectifiers Reversing diode. Average value of Half wave output voltage: VAVG = VP / pi VAVG is approx 31.8% of Vp PIV: Peak Inverse Voltage = Vp Full wave rectifier • Ripple factor is (0.48) • Rectifier efficiency is high(81.2%) • TUF is high(0.693) Full wave rectifiers A full wave rectifier allows unidirectional current through the load during the entire 360 degree of input cycle. Full Wave Rectifier The output voltage have twice the input frequency. VAVG = 2VP / pi VAVG is 63.7% of Vp 70 The Center-Tapped Full wave rectifiers • A center-tapped transformer is used with two diodes that conduct on alternating half-cycles. F + + – I Vin 0 D1 During the positive half-cycle, the upper diode is forwardbiased and the lower diode is reverse-biased. Vout – 0 + + RL – – – D2 + F – D1 + – During the negative half-cycle, the lower diode is forwardbiased and the upper diode is reverse-biased. Vin Vout + 0 0 – I + + D2 + RL – – 71