 Hello and welcome to a lecture on Avalanche photodiode, learning outcomes of this session. By the end of this session, student will be able to explain the operation of semiconductor photodiodes with internal gain. Before going to Avalanche photodiode, we have seen in the previous lectures two photodiodes that the first one is p-injection photodiode and second one is pin photodiode. Those both photodiodes were photodiodes without internal gain and today the photodiode which we are going to discuss is Avalanche photodiode and it is having its internal gain. Internal gain means it generates more number of electron hole pair per incident photon. The previously discussed pin photodiode and p-injection photodiode were without internal gains means they just generate or produce a single electron hole pair per incident photon. So let's discuss the Avalanche photodiode which has internal gain. For many years, for optical amplification, photo multiplier tube was a well known solution. Photo multiplier tube has high gain but it has some drawbacks or setbacks. The first is photo multiplier tube is a bulky vacuum tube and it generates heat. Along with generating heat, it provides limited linearity as compared to Avalanche photodiodes. It provides narrow spectral response as well as it has low quantum efficiency of about 25%. So alternative to photo multiplier tube is Avalanche photodiode which is a solid state efficient and effective alternative with internal gain is used. We will see the construction of Avalanche photodiode. It has four layers. One layer is n-type material and the remaining three layers are of p-type material but having different doping levels. The higher heavily doped region is this p-plus region. This p-region is lightly doped while this intrinsic region is also p-layer but it is just like or almost it is intrinsic. Along with this diagram, you can see the electric field distribution across the Avalanche photodiode. The electric field is high at the junction between this n-layer and p-layer. It goes on decreasing in the intrinsic region and n nearly at the end of intrinsic region. The electric field is nearly uniform across this intrinsic region. Here you can see the impact ionization process. So we will first discuss what is impact ionization process. Avalanche photodiodes are always operated under reverse bias. This reverse bias creates a high electric field distribution near this junction between the n-layer and p-layer which goes on decreasing to the p-plus layer. Then the photon with energy higher than band gap energy incident on this n-layer a single electron hole pair is generated. This electron hole pair generated acquires sufficient or much kinetic energy due to this high electric field and it collides with the bound electrons in the covalence band. So creates a secondary electron hole pair. So this creation of secondary electron hole pair is nothing but ionization, impact ionization process which is shown in this diagram. Here you can see the primary generated electron collides with the electron in the valence band and generates a newly electron hole pair. This impact ionization process goes on repeating and create an avalanche process or avalanche effects generating more number of electrons and hole pairs which results in multiplication of photocurrent. When a photon incident on an avalanche photodiode it generates an electron hole pair in this intrinsic region. The electron get drifted towards this n region while holes get drifted towards this p region due to the presence of this uniform electric field across this intrinsic region. When the electron get drifted towards n and it reaches in this p-layer it experiences a high electric field which is shown here and it acquires a kinetic energy and it collides with the bound electron in the valence band to produce another electron hole pair. So this impact ionization process goes on multiplicating and produce a larger photocurrent. This multiplication of carrier generation process is known as gain process. So this high electric field region is named as gain region while this intrinsic region is named as absorption region. Whenever due to this application of high reverse field the depletion layer or the depletion region at this junction widens. Hence this avalanche photodiode is also referred as reach through avalanche photodiode multiplication factor. Let us see the multiplication factor. The multiplication factor M is a major of internal gain provided by the avalanche photodiode. It is defined as the ratio of multiplied output photocurrent to the initial or primary unmultiplied photocurrent which is given by I output by I photocurrent. It is a unit less quantity. You may pause here the video and try to answer this question. Why the speed of avalanche photodiode is less than pin photodiode? The obvious answer to this question is the speed of avalanche photodiode is slower than pin photodiode because of multiplicative process of generation of electron hole pairs which is due to impact ionization process which contributes to larger photocurrent making the quantum efficiency high. But there is a tradeoff between the quantum efficiency and the speed or the response time of the avalanche photodiode. We will see which factors affect the speed of avalanche photodiode. The first factor which affects the avalanche photodiode is the time it takes for photogenerated electrons to cross the absorption region to the multiplication region. The second factor which affects the speed of avalanche photodiode is the time it takes for the avalanche process through impact ionization process to build up in the p region and generate the electron hole pairs. The last factor which affect is the time it takes for the last hole released in the avalanche process to transit through the intrinsic region. We will see the drawbacks of avalanche photodiode. The fabrication cost is high due to the complex structure at the time of manufacturing. The avalanche multiplication process is a statistical process and hence it leads to the carrier generation fluctuations which leads to the excess noise in the photocurrent which is undesired. Avalanche photodiode are operated under high reverse bias voltage of around 50 to 400 volts. The gain or the multiplication factor is a temperature dependent and hence temperature compensation is necessary for the stable operation of the devices and these are the sum of the drawbacks of avalanche photodiode. These are the references used. Thank you.