BIRLA VISHWAKARMA MAHAVIDHYALAYA Sub : Electronics devices & circuits Topic : Special purpose diode Name Enrollment Rathod Pragnesh 140070110056

Introduction: Describe the characteristics of a zener diode and analyze its operation Explain how a zener is used in voltage regulation and limiting Describe the varactor diode and it’s variable capacitance characteristics Discuss the operation and characteristics of LEDs and photodiodes Discuss the basic characteristics of the current regulator diode, the pin diode, the step-recovery diode, the tunnel diode, and the laserdiode.

Zener Diodes The basic function of zener diode is to maintain a specific voltage across it’s terminals within given limits of line or load change. Typically it is used for providing a stable reference voltage for use in power supplies and other equipment The zener diode – silicon pn-junction device-designed for operate in the reverse-biased region Schematic diagram shown that this particular zener circuit will work to maintain 10 V across the load Zener diode symbol

Zener Diodes Breakdown voltage – set by controlling the doping level during manufacture When diode reached reverse breakdown – voltage remains constant- current change drastically If zener diode is FB – operates the same as a rectifier diode A zener diode is much like a normal diode – but if it is placed in the circuit in reverse bias and operates in reverse breakdown. Note that it’s forward characteristics are just like a normal diode. 1.8V – 200V

Zener Diodes Two type of reverse breakdown: avalanche breakdown & zener breakdown i) avalanche breakdown - occurs in both rectifier & zener diodes at high reverse voltage - breakdown voltages greater than approximately 5V ii) zener breakdown - occurs at relatively low reverse voltage - n-type & p-type of materials of a zener diode are heavily doped - very thin depletion region – can break down at lower reverse voltage (VR) - as a result – intense electric field exits within the depletion region - the field have enough energy to pull electrons from their valence bands and create current. Note: both type called zener diode (breakdown voltages of 1.8V – 200 V)

Breakdown Characteristics The reverse voltage (VR) is increased – the reverse current (IR) remains extremely small up to the “knee”of the curve Reverse current – called the zener current, IZ At the bottom of the knee- the zener breakdown voltage (VZ) remains constant although it increase slightly as the zener current, IZ increase. IZK – min. current required to maintain voltage regulation IZM – max. amount of current the diode can handle without being damage/destroyed IZT – the current level at which the VZ rating of diode is measured (specified on a data sheet) The zener diode maintains a constant voltage for value of reverse current rating from IZK to IZM

Zener Equivalent Circuit Since the actual voltage is not ideally vertical, the change in zener current produces a small change in zener voltage By ohm’s law: Normaly -Zz is specified at IZT Zener impedance

Zener Diodes Applications Zener diode can be used as Voltage regulator for providing stable reference voltages 2. Simple limiters or clippers

Zener Regulation with a Varying Input Voltage - as i/p voltage varies (within limits) – zener diode maintains a constant o/p voltage but as VIN changes, IZ will change, so i/p voltage variations are set by the min. & max. current value (IZK & IZM) with which the zener can operate -Resistor, R –current limiting resistor

Zener Regulation with a Variable Load In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage. Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation. Note: The zener diode maintains a nearly constant voltage across RL as long as the zener current is greater than IZK and less than IZM

Zener Limiting Zener diode also can be used in ac applications to limit voltage swings to desired level To limit the +ve peak of a signal voltage to the selected zener voltage - During –ve alternation, zener arts as FB diode & limits the –ve voltage to -0.7V b) Zener diode is turn around -The –ve peak is by zener action & +ve voltage is limited to +0.7V c) Two back-to-back zeners limit both peaks to the zener voltage ±7V -During the +ve alternation, D2 is functioning as the zener limiter – D1 is functioning as a FB diode. -During the –ve alternation-the roles are reversed

Varactor Diodes A varactor diode is best explained as a variable capacitor – the junction capacitance varies with the amount of reverse-bias voltage. Think of the depletion region a variable dielectric. The diode is placed in reverse bias- the dielectric is “adjusted” by bias changes. Operates in reverse-bias and is doped to maximize the inherent capacitance of depletion region. The depletion region-widened by the reverse bias – acts as a capacitor dielectric because of its nonconductive characteristic. The p & n regions are conductive – acts as the capacitor plates. The reverse-biased varactor diode acts as a variable capacitor Varactor diode symbol

LED Basic operation: When the devices is FB – e- cross the pn junction from the n-type material & recombine with holes in p-type material. Recombining release energy in the form of heat and light. A large exposed surface area on one layer of the semiconductive material permits the photons to be emitted as visible light - electroluminescence Various impurities are added during the doping process to establish the wavelength of the emitted light – determine the color of the light and if it is visible or infrared (IR) Symbol of LED Electroluminescence in a FB LED

LED LED Semiconductor Materials: Early LEDs - GaAs 1st visible red LEDs – gallium arsenide phosphide (GaAsP) on a GaAs subtrate GaP substrate – brighter red LEDs & orange LEDs GaAs LEDs – emit infrared (IR) readiation – invisible Others LEDs – refer textbook pg. 131. LED Biasing: The forward voltage across an LED is considerably greater than for a silicon diode The max. VF for LEDs is 1.2V – 3.2V Reverse breakdown << for a silicon rectifier diode (typically 3V – 10V) The amount of power o/p translated into light is directly proportional to the forward current LEDs emits light in response to a sufficient forward current

LED Applications: Used for indicator lamps & readout displays on a wide variety of instruments, ranging from consumer appliances to scientific apparatus. Common type of display device using LEDs – 7-segment display. by forward-biasing selected recombinations of segments-decimal digit/point can be form IR light-emitting diodes – used in optical coupling applications – often in conjunction with fiber optics. Area of application include – industrial processing & control, position encoder, bar Graph readers and optical switching.

Other Types of Diodes Current Regulator Diode Schottky Diode PIN Diode Step-recovery Diode Tunnel Diode Laser Diode

The Schottky Diode The Schottky diode’s significant characteristic is its fast switching speed – also known as hot-carrier diodes This is useful for high frequencies and digital applications. It is not a typical diode in that it does not have a pn junction. Form by joining a doped n-material with a metal such as gold,silver or platinium Forward voltage drop is around 0.3 V Operate only with majority carrier – no minority carrier/ no reverse leakage current When forward biased – the higher energy e- in the n region are injected into the metal region where they give up their excess energy very rapidly Schottky diode symbol Basic internal construction

Summary The zener diode operates in reverse breakdown. A zener diode maintains a nearly constant voltage across its terminals over a specified range of currents. Line regulation is the maintenance of a specific voltage with changing input voltages. Load regulation is the maintenance of a specific voltage for different loads. There are other diode types used for specific RF purposes such as varactor diodes (variable capacitance), Schottky diodes (high speed switching), and PIN diodes (microwave attenuation and switching).

Summary Light emitting diodes (LED) emit either infrared or visible light when forward-biased. Photodiodes exhibit an increase in reverse current with light intensity. The laser diode emits a monochromatic light

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