Engineering Module - Semester I - Electronic & Electrical Engineering PDF

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES Engineering Module - Semester I Electronic & Electrical Engineering Part 8 Diodes & LED’s Dr Nevan Bermingham Portions of Presentation are © SJTU Zhou...

INTERNATIONAL & ACCESS FOUNDATION PROGRAMMES Engineering Module - Semester I Electronic & Electrical Engineering Part 8 Diodes & LED’s Dr Nevan Bermingham Portions of Presentation are © SJTU Zhou Lingling Introduction to Diodes The diode is the simplest and most fundamental nonlinear circuit element. Just like resistor, it has two terminals. Unlike resistor, it has a nonlinear current- voltage characteristics. Its use in rectifiers is the most common application. 2 Diode Physical Structure The most important region, which is called pn junction, is the boundary between n-type and p-type semiconductor. Symbol and Characteristic for the Ideal Diode Characteristics The arrow-like circuit symbol shows the direction of conducting current. Diodes allow current (electricity) to flow in one way only: Forward biasing voltage makes it turn on. Reverse biasing voltage makes it turn off. Revision: Intrinsic Semiconductors A crystal of pure and regular lattice structure is called intrinsic semiconductor. Materials Silicon - today’s IC technology is based mostly on silicon Germanium was used previously Gallium arsenide also used for microwave circuits Intrinsic Semiconductor Two-dimensional representation of the silicon crystal. The circles represent the inner core of silicon atoms, its positive charge is neutralised by the charge of the four valence electrons. All bonds are intact and no free electrons are available for current conduction. Intrinsic Semiconductor Review Terms:  Valence electron: each silicon atom has four valence electrons  Covalent bond: two valence electrons from different two silicon atoms form the covalent bond  A free electron is negative charge and a hole is positive charge. Both of them can move in the crystal structure. They form the movement of electricity in the electric circuit.  Doped semiconductors are materials in which carriers of one kind predominate.  Two types of doped semiconductors are made with doping – P and N Types  Conductivity of doped semiconductor is greater Extrinsic carriers  Two types of dopants (donors or acceptors)  Donors bring electron (n-type) and become ive ions  Acceptors bring holes (p-type) and become ive ions N Type Semiconductor Doped silicon in which the majority of charge carriers are the negatively charged electrons is called n type semiconducto r. 10 P Type Semiconductor Doped silicon in which the majority of charge carriers are the positively charged holes is called p type semiconductor. Diode Physical Structure The most important region, which is called pn junction, is the boundary between n-type and p-type semiconductor. P Types & N Types The Diode B Al A SiO 2 p n Cross section of pn-junction in an IC process N-type region P-type region doped with doped with donor impurities acceptor (phosphorus, impurities arsenic) (boron) The Diode Simplified structure A Al A p n The pn B B region is One-dimensional assumed to representation diode symbol be thin The Diode Potential Barrier or Depletion Zone The P-N Junction Forward Bias  dc voltage on the positive terminal (p region) and negative to the negative (n region).  This is the condition that permits current through the pn-junction of a diode. 18 Reverse Bias dc voltage negative terminal connected to the p region and positive to the n region (reversed) Depletion region widens and no current flows 19 Diode Current Flow Characteristics 20 https://www.youtube.com/watch?v=JBtEckh3L9Q The Application of Diode Circuits Rectifier circuits Half-wave rectifier Full-wave rectifier Voltage regulator (not in the scope of this module) Limiter (not in the scope of this module) What is the output of this circuit? vs = Alternating Current Source Half-Wave Rectifier Half-wave rectifier. Black = Input (AC) Blue = Output Variable DC What about this one? vs = Alternating Current Source vs Transformer Full-Wave Rectifier Black = Input (AC) Blue = Output Variable DC Why is the Output slightly less tan the input? Output slightly less than input voltage? LED – Light emitting Diode A diode that produces light of one color when connected in forward biased mode Semiconductor Has polarity Inside a Light Emitting Diode Structure Structure High Power LED’s LED Lighting Efficiency LED Lighting Efficiency LED Lighting Efficiency LED: How It Works When current flows across a diode negative electrons move one way and positive holes move the other way LED: How It Works For each recombination of a negative and a positive charge, a quantum of electromagnetic energy is emitted in the form of a photon of light The frequency characteristic (colour of light) depends on the semi-conductor material (usually a combination of the https://www.youtube.com/watch?v=BH9LI973H8w Kinds of LEDs Numeric Displays 16-segment alphanumeric display Green, yellow, or red 18-pin package There is one LED inside for each segment Numeric Displays How to Connect a LED Usually requires 1.5~2.5V and 10 mA High power LED’s becoming more common To prevent overloading, use a resistor (approx. 470 Ω) Laser LED’s Light Amplification by Stimulated Emission of Radiation Laser LED’s – Relative Size http://faculty.uml.edu/carmiento/Special%20Lectures/Intro%20to%20EE%20Lecture.pdf Laser LED’s - Fibre Optics This is a standard copper cable used for telephone service. This carries about 300 phone calls. One of these fibers can carry up to 10 million telephone calls Tri Colour LED’s The most popular type of tri-color LED has a red, blue and a green LED combined in one package with three leads. They are called tri-color because they can create any colour of light. The diagram shows the organization of a tri-color LED. Note the different lengths of the leads. Any Questions?