13 Special Diodes

VALILA, MARY GRACE CATHERINE I. BS ECE 3B

SEPTEMBER 7, 2015 ENGR. SEVILLA TAUZON

13 SPECIAL DIODES 1. ZENER DIODE A. OVERVIEW The zener diode is a silicon pn junction devices that differs from rectifier diodes because it is designed for operation in the reverse-breakdown region. The breakdown voltage of a zener diode is set by carefully controlling the level during manufacture. 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. A zener diode is much like a normal diode. The exception being is that it is placed in the circuit in reverse bias and operates in reverse breakdown. This typical characteristic curve illustrates the operating range for a zener. Note that it’s forward characteristics are just like a normal diode. Zener breakdown effect: Zener breakdown effect is the one from which the diode gains its popular name. It is the quantum mechanical effect tunnelling effect, but when applied to the voltage reference diode, it retains the Zener name after the man who discovered it. TYPE: WORKING PRINCIPLE: INVENTOR:

PASSIVE ZENER BREAKDOWN CLARENCE MELVIN ZENER

B. CONSRTRUCTION DIAGRAM

C. SCHEMATIC DIAGRAM

D. I-V CHARACTERISTIC CURVE A zener diode is heavily doped to reduce the reverse breakdown voltage. This causes a very thin depletion layer. As a result, a zener diode has a sharp reverse breakdown voltage VZ. This is clear from the reverse characteristic of zener diode shown in Fig. 7.1. Note that the reverse characteristic drops in an almost vertical manner at reverse voltage VZ. As the curve reveals, two things happen when VZ is reached : (i) The diode current increases rapidly. (ii) The reverse voltage VZ across the diode remains almost constant. In other words, the zener diode operated in this region will have a relatively constant voltage across it, regardless of the value of current through the device. This permits the zener diode to be used as a voltage regulator.

VALILA, MARY GRACE CATHERINE I. BS ECE 3B

SEPTEMBER 7, 2015 ENGR. SEVILLA TAUZON

E. OPERATION OF THE DEVICE The reverse conduction effects, in common with many other aspects of semiconductor technology are subject to temperature variations. It is found that the impact ionisation and Zener effects have temperature coefficient in opposite directions. The Zener effect which predominates below 5.5 volts exhibits a negative temperature coefficient. However the avalanche effect which is the major effect above 5.5 volts has a positive temperature coefficient. A positive Temperature Coefficient means that the zener voltage increases with an increase in temperature or decreases with a decrease in temperature. A negative Temperature Coefficient means that the zener voltage decreases with an increase in temperature or increases with adecrease in temperature. As a result Zener diodes or voltage reference diodes with reverse voltages of around 5.5 volts where the two effects occur almost equally have the most stable overall temperature coefficient as they tend to balance each other out for the optimum performance. F. APPLICATION Zener diodes find numerous applications in transistor circuitry. Some of their common uses are : As voltage regulators As a fixed reference voltage in a network for biasing and comparison purposes and for calibrating voltmeters. As peak clippers or voltage limiter. For metre protection against damage from accidental application of excessive voltage. For reshaping waveform.

VALILA, MARY GRACE CATHERINE I. BS ECE 3B

SEPTEMBER 7, 2015 ENGR. SEVILLA TAUZON

2. LIGHT – EMITTING DIODE A. OVERVIEW A light-emitting diode (LED) is a diode that gives off visible light when forward biased. Light-emitting diodes are not made from silicon or germanium but are made by using elements like gallium, phosphorus and arsenic. By varying the quantities of these elements, it is possible to produce light of different wavelengths with colours that include red, green, yellow and blue. For example, when a LED is manufactured using gallium arsenide, it will produce a red light. If the LED is made with gallium phosphide, it will produce a green light. The forward voltage ratings of most LEDs is from 1V to 3V and forward current ratings range from 20 mA to 100 mA. In order that current through the LED does not exceed the safe value, a resistor RS is connected in series with it. The input voltage is VS and the voltage across LED is VD.

General light output versus forward current.

TYPE: WORKING PRINCIPLE: INVENTOR:

B. CONSRTRUCTION DIAGRAM

PASSIVE, OPTOELECTRONIC ELECTROLUMINESCENCE OLEG LOSEV (1927), JAMES BIARD (1961)

VALILA, MARY GRACE CATHERINE I. BS ECE 3B

SEPTEMBER 7, 2015 ENGR. SEVILLA TAUZON

C. SCHEMATIC DIAGRAM

D. IV CHARACTERISTIC CURVE

E. OPERATION OF THE DEVICE When the device is forward-biased, electrons cross the pn junction from the n-type 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. The difference in energy between the electrons and the holes corresponds to the energy of visible light. When recombination takes place, the recombining electrons release energy in the form of photons. The emitted light tends to be monochromatic (one color) that depends on the band gap (and other factors). 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. F. APPLICATION A. As an indicator or sign The low energy consumption, low maintenance and small size of LEDs has led to uses as status indicators and displays on a variety of equipment and installations. Large-area LED displays are used as stadium displays and as dynamic decorative displays. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries. B. LIGHTING LEDs are used as street lights and in other architectural lighting where color changing is used. The mechanical robustness and long lifetime is used in automotive lighting on cars, motorcycles, and bicycle lights.LEDs are used for infrared illumination in night vision uses including security cameras. A ring of LEDs around a video camera, aimed forward into a retroreflective background, allows chroma keying in video productions. LEDs are used in mining operations, as cap lamps to provide light for miners. Research has been done to improve LEDs for mining, to reduce glare and to increase illumination, reducing risk of injury for the miners. C. DATA COMMUNICATION Light can be used to transmit data and analog signals. For example, lighting white LEDs can be used in systems assisting people to navigate in closed spaces while searching necessary rooms or objects.

VALILA, MARY GRACE CATHERINE I. BS ECE 3B

SEPTEMBER 7, 2015 ENGR. SEVILLA TAUZON

3. PHOTO – DIDOE A. OVERVIEW A photo-diode is a reverse-biased silicon or germanium pn junction in which reverse current increases when the junction is exposed to light. The reverse current in a photo-diode is directly proportional to the intensity of light falling on its pn junction. This means that greater the intensity of light falling on the pn junction of photo-diode, the greater will be the reverse current. When a rectifier diode is reverse biased, it has a very small reverse leakage current. The same is true for a photo-diode. The reverse current is produced by thermally generated electron hole pairs which are swept across the junction by the electric field created by the reverse voltage. In a rectifier diode, the reverse current increases with temperature due to an increase in the number of electron-hole pairs. A photo-diode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the increase in light intensity and vice-versa. This is explained as follows. When light (photons) falls on the pn junction, the energy is imparted by the photons to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current. As the intensity of light incident on the pn junction increases, the reverse current also increases. In other words, as the incident light intensity increases, the resistance of the device (photo-diode) decreases. MATERIAL Silicon Germanium Indium gallium arsenide Lead (II) sulfide Mercury cadmium telluride TYPE: WORKING PRINCIPLE: PIN CONFIGURATION: B. CONSRTRUCTION DIAGRAM

ELECTROMAGNECTIC SPECTRUM WAVELENGTH RANGE (nm) 190-1100 400-1700 800-2600