CHAP 3 zener diode

Chapter ELECTRONIC DEVICES EMT 116  Describe the characteristic of zener diode and analyze its operation  Explain how a zener diode is used in voltage reference, regulation and limiting applications  Troubleshoot zener diode regulators    Zener diode – silicon pn-junction device that is designed for operation in the reverse-breakdown region The basic function of zener diode is to maintain a nearly constant dc voltage under proper operating condition Typically it is used to provide a stable reference voltage for use in power supplies, voltmeter and other equipment Zener diode symbol  Breakdown voltage – set by controlling the doping level during manufacture  When diode reached reverse breakdown – voltage remains constant even though 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 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 General diode V-I characteristic  Two types of reverse breakdown: avalanche breakdown & zener breakdown  i) avalanche breakdown - occurs in both rectifier & zener diodes at high reverse voltage - breakdown voltage greater than approximately 5V  ii) zener breakdown - occurs at relatively low reverse voltage - a zener diode is heavily doped to reduce breakdown voltage - this causes very thin depletion region - as a result, intense electric field exists within the depletion region - Near zener breakdown voltage (Vz), 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)         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 that point, breakdown effect begin where zener impedance (Zz) begin to decrease as IZ increases rapidly At the bottom of the knee- the zener breakdown voltage (VZ) remains constant although it increase slightly as the zener current, IZ increase IZK – 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 nearly constant voltage for value of reverse current rating from IZK to IZM Zener impedance Zener diode equivalent circuit model and the Characteristic curve illustrating Zz     Since the actual voltage is not ideally vertical, the change in zener current produces a small change in zener voltage By ohm’s law: ∆VZ ZZ = ∆I Z Normally -Zz is specified at IZT In most cases, assume Zz is constant over full range of zener current values and is purely resistive Fig.3-13: LED that are produced in an array of shapes and sizes LED characteristics: characteristic curves are very similar to those for p-n junction diodes higher forward voltage (VF) lower reverse breakdown voltage (VBR) The basic operation of LED is as illustrated in Fig 3-14: “When the device is forward-biased, electrons cross the p-n junction from the ntype material and recombine with holes in the p-type material These free electrons are in the conduction band and at a higher energy than the holes in the valence band When the recombination process takes place, the recombining electrons release energy in the form photons A large exposed surface area on one layer of the semiconductive material permits the photons to be emitted as visible light.” This process is called electroluminescence Fig.3–14: Electroluminescence in a forward-biased LED Various impurities are added during the doping process to establish the wavelength of the emitted light The wavelength determines the color of visible light The color emitted by a given LED depends on the combination of elements used to produce the component Some common element combinations are identified in Table 3-1 TABLE 3-1: Common LEDs Compound VF is measured at IF = 20 mA in each case Forward Voltage (VF) Color Emitted GaAs 1.5 V Infrared (invisible) AlGaAs 1.8 V Red GaP 2.4 V Green GaAsP 2.0 V Orange GaN 4.1 V White AlGaInP 2.0 V Amber (yellow) AlGaInN 3.6 V Blue When used in most practical applications, LED require the use of a series currentlimiting resistor, as shown in Fig 3-15 (a) The resistor ensures that the maximum current rating of the LED can not be exceeded by the circuit current The amount of power output translated into light is directly proportional to the forward current, as indicated in Fig 3-15 (b) Fig.3-15: Basic operation of a LED The limiting resistor (RLIMIT) is determined using the following question: RLIMIT VBias − VF = IF The seven segment display is an example of LEDs use for display of decimal digits Fig.3-16: The 7-segment LED display Photodiode is a p-n junction that can convert light energy into electrical energy A light detector It operates in reverse bias voltage (VR), as shown in Fig 3-17, where Iλ is the reverse light current It has a small transparent window that allows light to strike the p-n junction The resistance of a photodiode is calculated by the formula as follows: RR VR = Iλ Iλ Fig.3-17: Photodiode When its p-n junction is exposed to light, the reverse current increases with the light intensity as shown by the graph in Fig 3-18 expressed as irradiance (mW/cm2) When there is no incident light, the reverse current is almost negligible and is called the dark current Fig.3-18: Typical photodiode characteristics Fig 3-19 illustrates that the photodiode is placed in the circuit in reverse bias As with most diodes when in reverse bias, no current flows when in reverse bias, but when light strikes the exposed junction through a tiny window, reverse current increases proportional to light intensity Fig.3-19: Operation of photodiode 3.5.1 The Schottky Diode A Schottky diode symbol is shown in Fig 3-20(a) The Schottky diode’s significant characteristic is its fast switching speed This is useful for high frequencies and digital applications It is not a typical diode in that it does not have a p-n junction Instead, it consists of a doped semiconductor (usually n-type) and metal bound together, as shown in Fig 3-20(b) Fig.3-20: (a) Schottky diode symbol and (b) basic internal construction of a Schottky diode The laser diode (light amplification by stimulated emission of radiation) produces a monochromatic (single color) light Laser diodes in conjunction with photodiodes are used to retrieve data from compact discs Fig.3-21: Basic laser diode construction and operation The pin diode is also used in mostly microwave frequency applications Its variable forward series resistance characteristic is used for attenuation, modulation, and switching In reverse bias it exhibits a nearly constant capacitance Fig.3-22: PIN diode Current regulator diodes keeps a constant current value over a specified range of forward voltages ranging from about 1.5 V to V Fig.3-23: Symbol for a current regulator diode The Step-Recovery Diode The step-recovery diode is also used for fast switching applications This is achieved by reduced doping at the junction The Tunnel Diode The tunnel diode has negative resistance It will actually conduct well with low forward bias With further increases in bias it reaches the negative resistance range where current will actually go down This is achieved by heavily-doped p and n materials that creates a very thin depletion region Fig.3-24: Tunnel diode symbol and characteristic curve ... Datasheets for zener Diodes VZ: zener voltage IZT: zener test current ZZT: zener Impedance IZK: zener knee current IZM: maximum zener current Ex 3- 2 The zener voltage of a zener diode is very... of zener diode and analyze its operation  Explain how a zener diode is used in voltage reference, regulation and limiting applications  Troubleshoot zener diode regulators    Zener diode. .. loads        Optical Diode Current Regulator Diode Schottky Diode PIN Diode Step-recovery Diode Tunnel Diode Laser Diode Varactor is a type of p-n junction diode that operates in reverse