SPECIAL DIODES P. SYAM SUNDAR ASSOCIATE PROFESSOR DEPT. OF ECE K L UNIVERSITY

ZENER DIODE SCHOTTKY – BARRIER DIODE (SBD) VARACTOR DIODE PHOTO DIODE LIGHT EMITTING DIODE (LED) SPECIAL DIODES

ZENER DIODE +- VZVZ IZIZ SYMBOL -V ZK -V Z0 -V Z -I ZK -I ZT ΔIΔI ΔVΔV Slope = 1/r Z 0 ΔV = ΔI r Z V i Q I ZK – Knee Current (from data sheets) I ZT – Test Current V Z – Zener Voltage r Z – Incremental (Dynamic) Resistance

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 V ZO and V ZK are approximately equal The dependence of Zener Voltage on temperature is given by Temperature coefficient TC or temco. V Z = V ZO + r Z I Z For I Z > I ZK and V Z > V ZO

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.

SCHOTTKY BARRIER DIODE (SBD)

where  B is Schottky barrier height, V A is applied voltage, A is 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. SBD Forward Voltage Drop PN diode Forward Voltage Drop Silicon0.3V – 0.5V0.6V – 0.8V V – I cHARECTERISTICS

VARACTOR DIODE Variable Capacitors Transition capacitance under reverse bias Diffusion capacitance under forward bias Used in automatic tuning of radio receivers

VARACTOR DIODE

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

PHOTO DIODE

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

LED applications  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

LIGHT EMITTING DIODE (LED)

 Direct band gap semiconductors used for LEDs: Galium Arsenide (Ga As) Gallium Antimony (Ga Sb) 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: – /cm 3 for donor atoms and – /cm 3 for Acceptor atoms  Colours: Gallium Phosphide – Zinc Oxide RED Gallium Phosphide – N GREEN Silicon Carbide – SiC YELLOW Gallium Phosphide, P, N AMBER

LIGHT EMITTING DIODE (LED)

SEMICONDUCTORS SYMBOLS