Semiconductor Physics Session 6 (Zener diode, Light Emitting Diode, Solar Cell).pdf

https://www.youtube.com/c/EngineeringPhysicsbySanjiv SEMICONDUCTOR PHYSICS https://youtu.be/85KspLpuoXU CHAPTER – 3 SEMICONDUCTOR PHYSICS Session 6 : Light Emitting Diode, Zene...

https://www.youtube.com/c/EngineeringPhysicsbySanjiv SEMICONDUCTOR PHYSICS https://youtu.be/85KspLpuoXU CHAPTER – 3 SEMICONDUCTOR PHYSICS Session 6 : Light Emitting Diode, Zener diode, Solar cell LIGHT EMITTING DIODE (LED): LED is a diode that gives visible light when it is forward biased. The basic principle involved in working of LED is recombination. Recombination is the phenomenon in which electrons from conduction band recombine with holes in valence band releasing energy- EC c h  h  E g  Ec  Ev  Ev In silicon and Germanium Diodes, energy is given out in the form of heat as values of E g are 1.1eV and 0.7eV respectively and the corresponding wavelength does not lie in the visible region. For LEDs, direct gap, compound semiconductors such as GaAs, GaP, GaAsP are used. Sym bol for LED EC EC EC h  E c  E v EF EC EF Ev Ev Ev p-side Ev p-side n-side n-side Energy Bands aligned when Energy Bands of unbiased LED forward biased As shown in the energy band diagram of unbiased LED, conduction band on n-side has many electrons but there are not many holes in valence band. Also, valence band on p-side has many Prepared by – Sanjiv Badhe Page 1 of 5 https://www.youtube.com/c/EngineeringPhysicsbySanjiv SEMICONDUCTOR PHYSICS https://youtu.be/85KspLpuoXU holes in valence band but there are not many electrons in conduction band. So, recombination occurs at very small scale due to minority carriers. When LED is forward biased and forward bias voltage is greater than barrier voltage, energy bands on n-side and p-side get aligned. Now, electrons transfer from n-side conduction band to p- side conduction band and holes transfer from p-side valence band to n-side valence band. This gives rise to large number of electrons in conduction band and large number of holes in valence band in same region and recombination rate suddenly increases. Due to large recombination, we get considerable radiation emitted which is seen in the form of light if wavelength corresponding to energy gap falls in the visible region. Applications of LED : i) LEDs are widely used in digital displays. ii) LEDs are used as optical source in fibre optic communication. iii) Infrared LEDs are used in remote control devices. iv) LEDs are widely used in automobile industry. ZENER DIODE Zener Diode or “Breakdown Diode” as they are sometimes called, are basically the same as the standard PN junction diode but are specially designed to have a low pre-determined Reverse breakdown Voltage that takes advantage of this high reverse voltage. The zener diode is the simplest types of voltage regulator and the point at which a zener diode breaks down or conducts is called the “Zener Voltage” ( Vz ). The Zener diode is like a general-purpose signal diode consisting of a silicon PN junction. When biased in the forward direction it behaves just like a normal signal diode passing the rated current, but as soon as a reverse voltage applied across the zener diode exceeds the rated voltage of the device, the diodes breakdown voltage is reached at which point a process called Avalanche Breakdown occurs in the semiconductor depletion layer and a current starts to flow through the diode to limit this increase in voltage. The current now flowing through the zener diode increases dramatically to the maximum circuit value (which is usually limited by a series resistor) and once achieved this reverse saturation current remains fairly constant over a wide range of applied voltages. The voltage point at which the voltage across the zener diode becomes stable is called the “zener voltage” for zener diodes this voltage can range from less than one volt to hundreds of volts. Prepared by – Sanjiv Badhe Page 2 of 5 https://www.youtube.com/c/EngineeringPhysicsbySanjiv SEMICONDUCTOR PHYSICS https://youtu.be/85KspLpuoXU Zener Diode I-V Characteristics The Zener Diode is used in its “reverse bias” or reverse breakdown mode, i.e. the diodes anode connects to the negative supply. From the I-V characteristics curve above, we can see that the zener diode has a region in its reverse bias characteristics of almost a constant negative voltage regardless of the value of the current flowing through the diode and remains nearly constant even with large changes in current as long as the zener diodes current remains between the breakdown current IZ(min) and the maximum current rating IZ(max). This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations. The fact that the voltage across the diode in the breakdown region is almost constant turns out to be an important application of the Zener diode as a voltage regulator. The function of a regulator is to provide a constant output voltage to a load connected in parallel with it in spite of the ripples in the supply voltage or the variation in the load current and the zener diode will continue to regulate the voltage until the diodes current falls below the minimum IZ(min) value in the reverse breakdown region. AVALANCHE BREAKDOWN AND ZENER BREAKDOWN Avalanche breakdown happens due to the avalanche effect. If the reverse bias voltage is very high, then the depletion region widens, and the electric field is very strong. The minority charge carriers get accelerated in this depletion region, and gain kinetic energy. Once the field is strong enough, they can knock off, other electrons from the negative ions in the depletion layer. These new electrons can knock off more electron from the negative ions in the depletion layer and the number of electrons in the depletion layer suddenly gets multiplied. The increase in number of electrons is exponential, just like an Avalanche. And current suddenly rises due to these electrons. Prepared by – Sanjiv Badhe Page 3 of 5 https://www.youtube.com/c/EngineeringPhysicsbySanjiv SEMICONDUCTOR PHYSICS https://youtu.be/85KspLpuoXU The problem is, due to this 'impact' (this process is called Impact Ionization), heat is generated, and this heat could destroy the diode. Zener breakdown, only happens, when the doping concentration is extremely high. So high, that the depletion region becomes few atoms wide. The electric field becomes extremely strong, but at the same time, it is extremely narrow, hence many charge carriers can't get accelerated. Instead a quantum mechanical effect takes place. This is called as quantum tunneling. So there is no impact ionization, the electrons just tunnel through. (like digging a hole, instead of overcoming the mountain). So most electrons tunnel through, and some of-course will give impact ionization. Photovoltaic Solar Cell Solar cell can be used as an alternative source of energy which converts solar rnergy into electrical energy. It is a p-n junction with a large surface area as compared to normal p-n junction diodes and very thin p-layer so that most of the light energy reaches the junction area. EC Light EF EC EF Ev Light p-side Ev n-side Electron hole pair generation When light falls on the p-n junction, electron-hole pairs are generated as many electrons transfer from valence band to conduction band due to absorption of light energy. Due to this large number of electrons in conduction band on p-side near the junction are available which can easily climb down the energy hill and give rise to current. Also, large number of holes appear in valence band on n-side which can easily climb up the energy hill and give rise to current. Light A + p-side RL V n-side - Prepared by – Sanjiv Badhe Page 4 of 5 https://www.youtube.com/c/EngineeringPhysicsbySanjiv SEMICONDUCTOR PHYSICS https://youtu.be/85KspLpuoXU If an external load is connected across the solar cell, current flows through it and power is delivered. Applications of Solar cell : i) Used in satellites, space vehicles and in remote places as a source of energy. ii) Used in battery charging system, outdoor lighting system. iii) Solar cells are widely used in electronic equipment’s with low power consumption like calculator. iv) Solar cells are pollution free source of energy. v) Solar cells are maintenance free and cost effective in long run. However initial cost is high and their function is affected in rainy season when sufficient sunlight is not available. It requires large space for high power operation. Prepared by – Sanjiv Badhe Page 5 of 5