BEE Module 2 Special Purpose Diode Notes PDF

Mod 5: Special Purpose diodes Zener diode A normal p-n junction diode allows electric current only in forward biased condition. When forward biased voltage is applied to the p-n junction diode, it allows large amount of electric current and blocks only a small amount of electric current. Hence, a f...

Mod 5: Special Purpose diodes Zener diode A normal p-n junction diode allows electric current only in forward biased condition. When forward biased voltage is applied to the p-n junction diode, it allows large amount of electric current and blocks only a small amount of electric current. Hence, a forward biased p-n junction diode offer only a small resistance to the electric current. When reverse biased voltage is applied to the p-n junction diode, it blocks large amount of electric current and allows only a small amount of electric current. Hence, a reverse biased p-n junction diode offer large resistance to the electric current. If reverse biased voltage applied to the p-n junction diode is highly increased, a sudden rise in current occurs. At this point, a small increase in voltage will rapidly increases the electric current. This sudden rise in electric current causes a junction breakdown called zener or avalanche breakdown. The voltage at which zener breakdown occurs is called zener voltage and the sudden increase in current is called zener current. A normal p-n junction diode does not operate in breakdown region because the excess current permanently damages the diode. Normal p-n junction diodes are not designed to operate in reverse breakdown region. Therefore, a normal p-n junction diode does not operate in reverse breakdown region. What is zener diode? A zener diode is a special type of device designed to operate in the zener breakdown region. Zener diodes acts like normal p-n junction diodes under forward biased condition. When forward biased voltage is applied to the zener diode it allows large amount of electric current and blocks only a small amount of electric current. Zener diode is heavily doped than the normal p-n junction diode. Hence, it has very thin depletion region. Therefore, zener diodes allow more electric current than the normal p-n junction diodes. Zener diode allows electric current in forward direction like a normal diode but also allows electric current in the reverse direction if the applied reverse voltage is greater than the zener voltage. Zener diode is always connected in reverse direction because it is specifically designed to work in reverse direction. Zener diode definition A zener diode is a p-n junction semiconductor device designed to operate in the reverse breakdown region. The breakdown voltage of a zener diode is carefully set by controlling the doping level during manufacture. The name zener diode was named after the American physicist Clarance Melvin Zener who discovered the zener effect. Zener diodes are the basic building blocks of electronic circuits. They are widely used in all kinds of electronic equipments. Zener diodes are mainly used to protect electronic circuits from over voltage. Breakdown in zener diode There are two types of reverse breakdown regions in a zener diode: avalanche breakdown and zener breakdown. Avalanche breakdown The avalanche breakdown occurs in both normal diodes and zener diodes at high reverse voltage. When high reverse voltage is applied to the p-n junction diode, the free electrons (minority carriers) gains large amount of energy and accelerated to greater velocities. The free electrons moving at high speed will collides with the atoms and knock off more electrons. These electrons are again accelerated and collide with other atoms. Because of this continuous collision with the atoms, a large number of free electrons are generated. As a result, electric current in the diode increases rapidly. This sudden increase in electric current may permanently destroys the normal diode. However, avalanche diodes may not be destroyed because they are carefully designed to operate in avalanche breakdown region. Avalanche breakdown occurs in zener diodes with zener voltage (Vz) greater than 6V. Zener breakdown The zener breakdown occurs in heavily doped p-n junction diodes because of their narrow depletion region. When reverse biased voltage applied to the diode is increased, the narrow depletion region generates strong electric field. When reverse biased voltage applied to the diode reaches close to zener voltage, the electric field in the depletion region is strong enough to pull electrons from their valence band. The valence electrons which gains sufficient energy from the strong electric field of depletion region will breaks bonding with the parent atom. The valance electrons which break bonding with parent atom will become free electrons. This free electrons carry electric current from one place to another place. At zener breakdown region, a small increase in voltage will rapidly increases the electric current. Zener breakdown occurs at low reverse voltage whereas avalanche breakdown occurs at high reverse voltage. Zener breakdown occurs in zener diodes because they have very thin depletion region. Breakdown region is the normal operating region for a zener diode. Zener breakdown occurs in zener diodes with zener voltage (Vz) less than 6V. Symbol of zener diode The symbol of zener diode is shown in below figure. Zener diode consists of two terminals: cathode and anode. In zener diode, electric current flows from both anode to cathode and cathode to anode. The symbol of zener diode is similar to the normal p-n junction diode, but with bend edges on the vertical bar. Avalanche Breakdown vs Zener Breakdown There is a clear difference between Avalanche Breakdown and Zener Breakdown which can easily be understood by the table discussed below,  Avalanche Breakdown Zener Breakdown Avalanche breakdown occurs when the high Zener breakdown happens when electrons from voltage increase the free electron in the the valance band gain energy and reaches the semiconductor and a sudden increase in current conduction band which then conducts electricity. is seen. Avalanche breakdown is seen in the diodes Zener breakdown is seen in the diodes having having breakdown voltage greater than 8 volts. breakdown voltage in the range of 5 to 8 volts. Avalanche breakdown is observed in diodes that Zener breakdown is observed in diodes that are are lightly doped. highly doped. In the Avalanche breakdown, the VI characteristics curve is not as sharp as the VI Zener Breakdown has a sharp VI characteristics characteristics curve in the Zener breakdown. curve. For Avalanche breakdown increase in For Zener breakdown increase in temperature temperature increases the breakdown voltage. decreases the breakdown voltage. VI Characteristics of Zener Diode When forward biased voltage is applied to the zener diode, it works like a normal diode. However, when reverse biased voltage is applied to the zener diode, it works in different manner. When reverse biased voltage is applied to a zener diode, it allows only a small amount of leakage current until the voltage is less than zener voltage. When reverse biased voltage applied to the zener diode reaches zener voltage, it starts allowing large amount of electric current. At this point, a small increase in reverse voltage will rapidly increases the electric current. Because of this sudden rise in electric current, breakdown occurs called zener breakdown. However, zener diode exhibits a controlled breakdown that does damage the device. The zener breakdown voltage of the zener diode is depends on the amount of doping applied. If the diode is heavily doped, zener breakdown occurs at low reverse voltages. On the other hand, if the diode is lightly doped, the zener breakdown occurs at high reverse voltages. Zener diodes are available with zener voltages in the range of 1.8V to 400V. The graph given underneath shows the V-I characteristics of the Zener diode. V-I characteristics of a Zener Diode can be studied under the following two headings, Forward Characteristics of Zener Diode Forward characteristics of the Zener Diode are similar to the forward characteristics of any normal diode. It is clearly evident from the above diagram in the first quadrant that the VI forward characteristics are similar to other P-N junction diodes. Reverse Characteristics of Zener Diode In reverse voltage conditions a small amount of current flows through the Zener diode. This current is because of the electrons which are thermally generated in the Zener diode. As we keep increasing the reverse voltage at any particular value of reverse voltage the reverse current increases suddenly at the breakdown point this voltage is called Zener Voltage and is represented as Vz. Applications of Zener Diode Zener diode is a very useful diode. Due to its ability to allow current to flow in reverse bias conditions, it is used widely for various purposes. Some of the common uses of Zener Diode are discussed below, Zener diode as Voltage Regulator Zener diode is utilized as a Shunt voltage controller for managing voltage across little loads. The breakdown voltage of Zener diodes will be steady for a wide scope of current. The Zener diode is associated with corresponding to the heap to make it switch predisposition and when the Zener diode surpasses knee voltage, the voltage across the heap will become consistent. Zener Diode in Over-Voltage Protection At the point when the info voltage is higher than the Zener breakage voltage, the voltage across the resistor drops bringing about a short-out. This can be kept away from by utilizing the Zener diode.  Zener Diode in Clipping Circuits Zener diode is utilized for adjusting AC waveform cutting circuits by restricting the pieces of it is possible that one or both the half patterns of an AC waveform. Advantages Voltage regulation: Zener diodes can provide a constant voltage output, even when there are changes in the supply voltage or load current. Low cost: Zener diodes are inexpensive, and can be integrated into ICs or used discretely. Fast response: Zener diodes can react quickly to transient loads, stabilizing output faster than other regulators. Low electromagnetic interference: Zener regulators produce little high frequency ripple, which can pollute sensitive analog systems. Operate in reverse bias: Zener diodes can operate in reverse bias without generating heat, unlike other semiconductor devices. Disadvantages Limited current handling: Zener diodes have limited capacity for handling current, and are mainly used in low-power applications. Power rating: Exceeding the power rating of a Zener diode can cause it to overheat and fail. Polarity: Zener diodes are polarized devices, so proper polarity must be maintained when connecting them. Efficiency:Zener diodes are less efficient than regular diodes for rectification purposes. Zener diodes are used for voltage regulation and reference purposes, as well as in switching operations, peak clipping, and meter protection applications. The Zener Diode as a Voltage Regulator Zener Diodes can be used to produce a stabilised voltage output with low ripple under varying load current conditions. By passing a small current through the diode from a voltage source, via a suitable current limiting resistor (RS), the zener diode will conduct sufficient current to maintain a voltage drop of Vout. We remember from the previous tutorials that the DC output voltage from the half or full-wave rectifiers contains ripple superimposed onto the DC voltage and that as the load value changes so to does the average output voltage. By connecting a simple zener stabiliser circuit as shown below across the output of the rectifier, a more stable output voltage can be produced. Zener Diode Regulator Resistor, RS is connected in series with the zener diode to limit the current flow through the diode with the voltage source, VS being connected across the combination. The stabilised output voltage Vout is taken from across the zener diode. The zener diode is connected with its cathode terminal connected to the positive rail of the DC supply so it is reverse biased and will be operating in its breakdown condition. Resistor RS is selected so to limit the maximum current flowing in the circuit. With no load connected to the circuit, the load current will be zero, ( IL = 0 ), and all the circuit current passes through the zener diode which in turn dissipates its maximum power. Also a small value of the series resistor RS will result in a greater diode current when the load resistance RL is connected and large as this will increase the power dissipation requirement of the diode so care must be taken when selecting the appropriate value of series resistance so that the zener’s maximum power rating is not exceeded under this no-load or high-impedance condition. The load is connected in parallel with the zener diode, so the voltage across RL is always the same as the zener voltage, ( VR = VZ ). There is a minimum zener current for which the stabilisation of the voltage is effective and the zener current must stay above this value operating under load within its breakdown region at all times. The upper limit of current is of course dependant upon the power rating of the device. The supply voltage VS must be greater than VZ. One small problem with zener diode stabiliser circuits is that the diode can sometimes generate electrical noise on top of the DC supply as it tries to stabilise the voltage. Normally this is not a problem for most applications but the addition of a large value decoupling capacitor across the zener’s output may be required to give additional smoothing. Then to summarise a little. A zener diode is always operated in its reverse biased condition. As such a simple voltage regulator circuit can be designed using a zener diode to maintain a constant DC output voltage across the load in spite of variations in the input voltage or changes in the load current. The zener voltage regulator consists of a current limiting resistor RS connected in series with the input voltage VS with the zener diode connected in parallel with the load RL in this reverse biased condition. The stabilised output voltage is always selected to be the same as the breakdown voltage VZ of the diode. -------------------------------------------------------------------------------------------------------------------- What is a Light Emitting Diode? The lighting emitting diode is a p-n junction diode. It is a specially doped diode and made up of a special type of semiconductors. When the light emits in the forward biased, then it is called a light- emitting diode. The LED symbol is similar to a diode symbol except for two small arrows that specify the emission of light, thus it is called LED (light-emitting diode). The LED includes two terminals namely anode (+) and the cathode (-). The LED symbol is shown below. Construction of LED The construction of LED is very simple because it is designed through the deposition of three semiconductor material layers over a substrate. These three layers are arranged one by one where the top region is a P-type region, the middle region is active and finally, the bottom region is N-type. The three regions of semiconductor material can be observed in the construction. In the construction, the P-type region includes the holes; the N-type region includes elections whereas the active region includes both holes and electrons. When the voltage is not applied to the LED, then there is no flow of electrons and holes so they are stable. Once the voltage is applied then the LED will forward biased, so the electrons in the N- region and holes from P-region will move to the active region. This region is also known as the depletion region. Because the charge carriers like holes include a positive charge whereas electrons have a negative charge so the light can be generated through the recombination of polarity charges. How does the Light Emitting Diode Work? The light-emitting diode simply, we know as a diode. When the diode is forward biased, then the electrons & holes are moving fast across the junction and they are combined constantly, removing one another out. Soon after the electrons are moving from the n-type to the p-type silicon, it combines with the holes, then it disappears. Hence it makes the complete atom & more stable and it gives the little burst of energy in the form of a tiny packet or photon of light. Working Principle of LED The working principle of the Light-emitting diode is based on the quantum theory. The quantum theory says that when the electron comes down from the higher energy level to the lower energy level then, the energy emits from the photon. The photon energy is equal to the energy gap between these two energy levels. If the PN-junction diode is in the forward biased, then the current flows through the diode. The flow of current in the semiconductors is caused by the flow of holes in the opposite direction of current and the flow of electrons in the direction of the current. Hence there will be recombination due to the flow of these charge carriers. The recombination indicates that the electrons in the conduction band jump down to the valence band. When the electrons jump from one band to another band the electrons will emit the electromagnetic energy in the form of photons and the photon energy is equal to the forbidden energy gap. For example, let us consider the quantum theory, the energy of the photon is the product of both the Planck constant and frequency of electromagnetic radiation. The mathematical equation is shown Eq = hf Where his known as a Planck constant, and the velocity of electromagnetic radiation is equal to the speed of light i.e c. The frequency radiation is related to the velocity of light as an f= c / λ. λ is denoted as a wavelength of electromagnetic radiation and the above equation will become as a Eq = he / λ From the above equation, we can say that the wavelength of electromagnetic radiation is inversely proportional to the forbidden gap. In general silicon, germanium semiconductors this forbidden energy gap is between the condition and valence bands are such that the total radiation of electromagnetic wave during recombination is in the form of infrared radiation. We can’t see the wavelength of infrared because they are out of our visible range. The infrared radiation is said to be as heat because the silicon and the germanium semiconductors are not direct gap semiconductors rather these are indirect gap semiconductors. But in the direct gap semiconductors, the maximum energy level of the valence band and minimum energy level of the conduction band does not occur at the same moment of electrons. Therefore, during the recombination of electrons and holes are migration of electrons from the conduction band to the valence band the momentum of the electron band will be changed. What is the Difference between a Diode and a LED? The main difference between a diode and a LED includes the following. Diode LED The semiconductor device like a diode The LED is one type of diode, used to generate conducts simply in one direction. light. The LED is designed with the gallium phosphide & The designing of the diode can be done with a gallium arsenide whose electrons can generate light semiconductor material & the flow of electrons while transmitting the energy. in this material can give their energy the heat form. The diode changes the AC into the DC The LED changes the voltage into light It has a high reverse breakdown voltage It has a low-reverse breakdown voltage. The on-state voltage of the diode is 0.7v for The on-state voltage of LED approximately ranges silicon whereas, for germanium, it is 0.3v from 1.2 to 2.0 V. The diode is used in voltage rectifiers, clipping & clamping circuits, voltage multipliers. The applications of LED are traffic signals, automotive headlamps, in medical devices, camera flashes, etc. I-V Characteristics of LED There are different types of light-emitting diodes are available in the market and there are different LED characteristics which include the color light, or wavelength radiation, light intensity. The important characteristic of the LED is color. In the starting use of LED, there is the only red color. As the use of LED is increased with the help of the semiconductor process and doing the research on the new metals for LED, the different colors were formed. The following graph shows the approximate curves between the forward voltage and the current. Each curve in the graph indicates a different color. Advantages and Disadvantages of LED’s The advantages of light-emitting diode include the following. The cost of LED’s is less and they are tiny. By using the LED’s electricity is controlled.  The intensity of the LED differs with the help of the microcontroller. Long Lifetime Energy efficient No warm-up period Rugged Doesn’t affect by cold temperatures Directional Color Rendering is Excellent Environmentally friendly Controllable The disadvantages of light-emitting diode include the following. Price Temperature sensitivity Temperature dependence Light quality Electrical polarity Voltage sensitivity Efficiency droop Impact on insects Applications of Light Emitting Diode Applications and uses of LEDs can be seen in:  TV Backlighting  Smartphone Backlighting  LED displays  Automotive Lighting  Dimming of lights  Picture phones and digital watches  Camera flashes and automottive heat lamps  aviation lighting  Digital computers & calculators   Traffic signals & Burglar alar systems  Microprocesor and multiplexers  Optical communication  Indicator lamps in electric equipment LEDs used for TV Backlighting A TV’s backlight is the major power-consuming source. Uses of LEDs can give an efficient power reduction. on the edges of the TV, using an LED will be a cost reduction choice. Using LEDs directly behind the display provides better contrast. When it comes to TV backlighting, LEDs have taken the place of CFLs and LCDs. It is used for Smartphone Backlighting The backlight design of the smartphone may be slimmer and less expensive thanks to the usage of LED. The price of LEDs may vary according to the size of the smartphone display. They provide greater battery life due to the lower output voltage. Uses of LED in Displays LED display boards are common these days and are used outdoors like storage signs, billboards, road signs, etc. In sign boards, which have multiple languages conveying signals, the use of more LEDs will be beneficial in terms of less power consumption. These are used in Automotives The Use of LEDs in the automotive industry is growing. With LEDs, energy is saved and there is clearer visibility. These are extensively used in the back and rear of an automobile for better accessibility. LED lighting can improve the safety of pedestrians and drivers as it enhances visibility when it is ON, OFF, and dimmed in any part of the journey. LEDs used in Dimming of Lights Few LED applications include dimming of lights which helps in reducing energy consumption. This dimming feature is also used in Appliances where it is of two types. Global Dimming where all LEDs are dimmed together. Local Dimming where LEDs are dimmed independently. *****