 Welcome to a lecture on photodiodes learning outcomes. By the end of this session student will be able to explain the operation of a semiconductor photodiodes without internal gains. A semiconductor photodiod without internal gain generates a single electron hole pair per absorbed photon. We are going to discuss the photodiodes without internal gains in this video. So we will be considering only, we will discuss only pn photodiodes and pin photodiodes because these two photodiodes generate a single electron hole pair per absorbed incident photon. Here in this figure you can see that we have one pn junction photodiodes where I have numbered the different regions as 1, 2 and 3. The first region this middle one is nothing but a drift region. This number 2 is a diffusion region. Here we have not applied any bias. Usually the pn photodiodes are operated under reverse bias. The first region here the electrons after the absorption of the photons electron hole pairs are generated and these generated electrons will drift towards the n region while the holes will drift towards the p region creating a charge separation. So we get an open circuit voltage across this photodiodes. The electrons and the holes in the diffusion region move randomly unless and until they come under the influence of the built-in electric field of the pn junction photodiodes. Here we have applied an electric field, a reverse bias electric field is applied to a photodiodes. Here both drift and diffusion regions are shown similarly like the previous image. The drift region is formed by the immobile positively charged donor items in the n-type semiconducting material and immobile negatively charged acceptor atoms in the p-type material. The width of the drift region is therefore dependent upon the doping concentration of the p-type and n-type materials. Photons may be absorbed in both drift and diffusion regions. The absorption region's position and the width depends upon the energy of the incident photons and on the material from which the photodiod is fabricated. Thus in case of weak absorption of photons, the absorption region may extend completely throughout the device. Carbon hole pairs are therefore generated in both drift and diffusion regions as shown over here. This is a drift region, this is a diffusion region. As we apply the electric field, the drift region gets widened. In the drift region, carrier pairs are separated and under the influence of electric field, the charge carriers get drifted to the battery terminals, whereas outside this drift region means in this region or here. Outside this region, the holes diffuse towards the drift region in order to be collected. The diffusion process is very slow as compared with drift and thus this limits the response of a photodiode. So we have to make sure that we do not have a dominant diffusion current component. It is therefore important that the photons are absorbed in the drift region only. Thus it is made as wide as possible by decreasing the doping levels of n-type material. The drift region width in a p-n junction photodiode is normally 1 micrometer to 3 micrometer. The detection wavelength range of silicon photodiode is 400 to 700 nanometers. Here you may pause the video and try to find out the answer to this question. The question is what can be done to reduce the diffusion current component of a p-n photodiode? Previously we have discussed that as the diffusion process is slow, it limits the response or speed of a photodiode. So we have to reduce the diffusion current component of a p-n photodiode and this can be done by increasing the drift region or the width of the drift region so that more and more number of incident photons get absorbed in the drift region only. Here this figure shows the structure of a p-n photodiode. Here in contrast to p-n photodiode, the middle region intrinsic region i is different or additional. In order to allow the operation at the longer wavelengths where the light penetrates more deeply into the semiconductor material, a wider drift region is necessary. A p-n photodiode is made of p-region and n-region separated by a highly resistive intrinsic layer in order to increase the width of the drift region which allows operation at longer wavelengths. Since the middle layer consists of a nearly intrinsic material, such a structure is referred to as a p-n photodiode. In the figure, electric field distribution is also shown for a p-n photodiode. The middle layer offers high resistance and most of the voltage drop occurs across it. Here electric field distribution is shown and most of the voltage drop occurs across the middle region that is intrinsic region only. The main difference from the p-n photodiode is that the drift component of the photocurrent dominates over the diffusion component simply because most of the incident power is absorbed in the i-region of the p-n photodiode. So the optimum width of the drift region depends on a compromise between the speed and the sensitivity of the p-n photodiode. The p-type and n-type semiconductors are heavily doped, therefore the p-region and the n-region of the p-n photodiode has a large number of charge carriers to carry electric current. However, these charge carriers will not carry electric current under the reverse bias condition. On the other hand, intrinsic semiconductor is an undoped semiconductor material, therefore the intrinsic region does not have charge carriers to conduct the electric current. Under reverse bias conditions, the majority charge carriers in n-region and the p-region moves away from the junction as a result the width of the depletion region becomes very wide. The holes and electron move away from the junction making the width of the depletion region wide. Therefore majority charge carriers will not carry electric current under reverse bias conditions. However, minority carriers will carry electric current because they experience repulsive forces from the external electric field. In p-n photodiode, charge carriers generated in the depletion region carry most of the electric current. The charge carriers generated in p-region or n-region carry only a small electric current. When photon energy is applied or light energy is applied to p-n photodiode, most part of the energy is absorbed by the intrinsic region because of the wide depletion region. As a result large number of electron hole pairs are generated. Free electrons generated in the intrinsic region move towards the n-side whereas holes generated in the intrinsic region move towards the p-side. The free electrons and the holes moved from one region to another region carry electric current. When free electrons and holes reach n-region and p-region respectively, they are attracted towards the positive and negative terminal of the battery and constitute a photocurrent. These are the references. Thank you.