 Hello everyone, welcome to this video lecture, myself Deepali Vardhakar working as assistant professor at WIT, Sallapur. In this video lecture, we will study photodiode. At the end of this video lecture, student will be able to examine working of photodiode. Student will be able to draw the characteristic of photodiode and student will be able to state applications of photodiode. The photodiode it is the special form of p-n junction diode that produces current when junction is exposed to the radiation. This is the symbolic representation of photodiode. Here inverted arrow represent its exposure to light. This is the real appearance of photodiode. Here the p-n junction is placed inside the glass material. This is done in order to allow the light falls on this p-n junction. Because only p-n junction is exposed to the light, so other portion of this glass material is painted with the black color or it is metallized. Again here two electrodes anode and cathode. Types of photodiode, basically there are four types of photodiode, p-n photodiode, short key photodiode, pin photodiode and avalanche photodiode. The materials which are used for the manufacturing of photodiode and their respective wavelengths in electromagnetic spectrum, silicon, germanium, indium gallium arsenide and lead sulphide. So these four materials basically used for a manufacturing of photodiode. The photodiode consists basically three layers. One is a p-region, n-type semiconductor region and in between that the p-n junction. So this p-n junction is exposed to the radiation. Now photodiode basically offered into two modes. One is a photovoltaic mode. In the photovoltaic mode, the photodiode is unbiased that is no any external connection or external supply is applied to the photodiode. So here dark current which is flows to the photodiode that current is very very small. So in the next slide we will see what is that dark current. Next mode of operation of this photodiode that is the photoconductive mode. In photoconductive mode, external supply is applied to the photodiode. So photodiode works in a reverse biasing mode that is the positive terminal of the external DC source is connected to the cathode and negative terminal of external DC source is connected to the anode. So basically photodiode works in a photoconductive mode. Now next the working principle of photodiode works in a reverse biasing mode that is the negative terminal of external DC source is connected to the p region or p type semiconductor and positive terminal of external DC source is connected to the n type semiconductor. So when battery connected in this way and when the certain reverse biasing is applied to this photodiode, due to this negative terminal or negative supply, the minority charge carrier that is the electron which are present here p side they experience a repulsive force and these minority charge carriers repels towards the junction. Similarly the minority charge carrier holes which are present in a n side they also experience a repulsive force due to this positive and they repels towards the junction. Due to the movement of these electrons and holes minority charge carriers very small current flows through the device that current is called as a dark current. Now as these minority charge carriers flows through the towards the junction, here the holes electron recombination takes place and they generate a deflation region. So once the deflation region generated here after that the current flowing through the device restricted. When light inserted on the deflation region or p-n junction the temperature across the deflation region increases and the positive and negative ions in that whatever the valence electrons present they gains the energy from this incident photons and they separate out from their parents atom. So here the electron and holes separation takes place. So as these electrons are negatively charged these electrons attract towards the positive terminal of the battery and as these holes are a positive charge they will attract towards the negative terminal of the battery. So at light intensity increases more electrons and hole separation takes place and large amount of electrons flows through the device. So due to this reverse current whatever the generated that current is used to drive the number of circuit. So this reverse current is depend upon the intensity of light. As light intensity increases the reverse current is also increases. So this is the basic principle of photodiode. Now photoelectric effect, photodiode is works on this photoelectric effect. What is that photoelectric effect is emission of electrons or other free carriers when light heats the material. So whatever the electrons emitted due to the insertion of light these electrons are called as a photoelectrons and the current which flows due to this movement of this photoelectron that current is called as a photocurrent. So how this emission of electrons takes place the actually electrons which are at the depletion region they are present in a balanced band. So whenever the photons are incident on this depletion region these electrons gains energy from this photon and they get excited and their movement takes place at the conduction band. So the vacancy of this electron that is nothing but hole also generated here. So in this way for each insertion of photon the hole and electron pair generated at the depletion region. So this effect is called as a photoelectric effect. Now the VI characteristic of photodiode here on the x axis there is a reverse bias voltage and on y axis reverse current. So at the initially a very small current flows through the device due to the minority charge carrier that current is called as a dark current. As the reverse voltage increases and intensity of light increases the equally spaced of the reverse current also increases. So after this reverse current is completely independent on the reverse bias voltage and only depends on the intensity of light. So as the intensity increases reverse current is also increases. Now question what is the basic difference between the LED and photodiode? Pause the video for a while and think. One of the function of LED that is it emits the photon due to the electron hole recombination and the function of photodiode it provides the energy to electron and holes by exposing itself towards the light radiation. It's operating principle for LED, electro luminance and for this photo conduction. The biasing which is used for LED it operates in forward biasing mode. For photodiode it operates in reverse biasing mode. For LED converts the electrical energy into light energy while the photodiode converts light energy into electrical energy. Now the application of photodiode the one of the most important application of photodiode that is in object counting. So this application mostly used in the industry. So what is that? Here the object which are placed on a conveyor belt that object are get counted by using this photodiode. Here the LED light source is used to continuously focus light on a photodiode. Whenever the object passes through this light beam it will interrupt this light beam and at that time the current flowing through the photodiode becomes zero. And this object get counted. So in this way the photodiode is used in a industry for a counting object. Now next application of photodiode that is in security system or in alarm system. Here the light source is placed on the doorway. The detector is placed on the another side of the doorway. Current flowing through reverse current flowing through the photodiode is maximum till the light source focus the light on the photodiode. So whenever person passes through this doorway the light beam get interrupted or get broken and due to this the current flowing through the photodiode becomes zero. And it will make alarm on. So in this way the photodiode is used in a security system or in alarm system. Some more applications of photodiode it is used in compact disc players. It is used for smoke detectors applications. It is also used in a space applications, medical applications such as computer tomography. It is used to analyze the samples using optical communications also. It is used to measure the extremely low light intensity. The advantages of photodiode its sensitivity is high and its gain is large. Disadvantages of photodiode it generates high level of noise than PN photodiode. These are the references. Thank you.