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SPECIAL DIODES. P. SYAM SUNDAR ASSOCIATE PROFESSOR DEPT. OF ECE K L UNIVERSITY. SPECIAL DIODES. ZENER DIODE SCHOTTKY – BARRIER DIODE (SBD) VARACTOR DIODE PHOTO DIODE LIGHT EMITTING DIODE (LED). ZENER DIODE. i. -V Z0. -V Z. -V ZK. 0. SYMBOL. V. -I ZK. -. +. Q. Slope = 1/ r Z.
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SPECIAL DIODES P. SYAM SUNDAR ASSOCIATE PROFESSOR DEPT. OF ECE K L UNIVERSITY
SPECIAL DIODES • ZENER DIODE • SCHOTTKY – BARRIER DIODE (SBD) • VARACTOR DIODE • PHOTO DIODE • LIGHT EMITTING DIODE (LED)
ZENER DIODE i -VZ0 -VZ -VZK 0 SYMBOL V -IZK - + Q Slope = 1/rZ VZ -IZT IZ IZK – Knee Current (from data sheets) IZT – Test Current VZ – Zener Voltage rZ – Incremental (Dynamic) Resistance ΔV ΔI ΔV = ΔI rZ
Used in designing Voltage Regulators • Operate in Break down region • Current flows in to cathode • Cathode positive w.r.t. anode • Can operate safely up to 70 mA • VZO and VZK are approximately equal • The dependence of Zener Voltage on temperature is given by Temperature coefficient TC or temco. VZ = VZO + rZIZ For IZ > IZK and VZ > VZO
SCHOTTKY BARRIER DIODE (SBD) • It is a metal-semiconductor (MS) diode. (These are the oldest diodes). • Metal contact with moderately doped n type material. • The general shape of the Schottky diode and I-V characteristics are similar to PN junction diodes, but the details of current flow are different. • In a PN junction diodes, current is due to • Recombination in the depletion layer under small forward bias. • Hole injection from p+ side under larger forward bias. • In a Schottky diodes current is due to • Electron injection from the semiconductor to the metal.
V – I cHARECTERISTICS where B is Schottky barrier height, VAis applied voltage, Ais area, A* is Richardson’s constant.
V – I cHARECTERISTICS • Current is conducted by majority carrier (electrons). • Switching speed of the SBD is much higher. • The forward voltage of SBD is lower than that of PN junction • diode.
VARACTOR DIODE • VariableCapacitors • Transition capacitance under reverse bias • Diffusion capacitance under forward bias • Used in automatic tuning of radio receivers
PHOTO DIODE • Used to convert light to electric signal • Reverse biased PN diode is exposed to light • Photons liberated causes breakage of covalent bonds • Liberation of electron – hole pairs • Results in flow of reverse current across the junction called photo current • Photo current is proportional to intensity of light
LIGHT EMITTING DIODE (LED) • The operation is inverse to that of a photo diode • It converts forward current in to light • Minority carriers are injected across the junction and diffuse • in to P & N regions • Minority carriers recombine with majority carriers emitting photons • Use direct band gap materials like Gallium Arsenide • Light emitted proportional to the no. of re-combinations • Wide range of applications in different types of displays
LEDapplications • Display instruments like DVMs • Colourful lights • Produce coherent light with narrow band width (Laser Diode – used in CD Players & Optical communications) • Opto-isolator – combination of LED and Photo diode used to reduce electrical interference on signal transmission in a system and used in digital system design and design of medical instruments to reduce risk of electric shock to patients
Direct band gap semiconductors used for LEDs: Galium Arsenide (Ga As) • Gallium Antimony (GaSb) • Arsenic, Antimony, Phosphorous • Impurities added: Group – II materials like Zinc (Zn), Magnesium (Mg), Cadmium (Cd) • Donors: Group – VI materials like Tellicum (Te), Sulphur (S) etc… • Impurity Concentration: 1017– 1018 /cm3 for donor atoms and • 1017 – 1019/cm3 for Acceptor atoms • Colours:Gallium Phosphide – Zinc Oxide RED • Gallium Phosphide – N GREEN • Silicon Carbide – SiCYELLOW • Gallium Phosphide, P, N AMBER