字幕列表 影片播放 列印英文字幕 In 1949 it took ENIAC computer 70 hours to calculate the value Pi up to 2037 digits. Now the smartphone in your hand can do the same task in 0.5 seconds. This miraculous growth in speed was made possible by a tiny device inside the electronic gadget called a transistor. More specifically, a type of transistor called MOSFET. Let's get into a 3D animation, to learn the workings of a MOSFET. (soft music) - [Instructor] MOSFET is an electronically driven switch which allows and prevents a flow of current without any mechanical moving parts. Like any other conventional transistor, a MOSFET is also made from a semiconductor material such as Silicon. In its pure form, a semiconductor has very low electrical conductivity. However, when you introduce a controlled amount of impurities into the semiconductor material, its conductivity increases sharply. This procedure of adding impurities is called doping. To understand the physics of doping, let's first understand the internal structure of Silicon and also that of the impurity known as a dopant. Pure Silicon does not have any free electrons and because of this its conductivity is very low. However, when you inject an impurity which has extra electrons into the Silicon, the conductivity of the resultant material increases dramatically. This is known as n-type doping. We can also add impurities with fewer electrons, which will also increase the conductivity of pure Silicon. This is known as p-type doping. When the concentration of the impurity is lower, the doping is said to be low or light. On the other hand, if it is higher, the doping is referred to as high or heavy. Now let's get back to the workings of MOSFETs. If you dope a Silicon wafer in the following manner, you will get the basic structure of a MOSFET. It is interesting to note that even in the p region, there are very few free electrons that are capable of conducting electricity. We call them minority carriers. Later we will see why the minority carriers are significant in the MOSFET. Whenever a p-n junction is formed, the excess electrons in the end region have a tendency to occupy the holes in the p region. This means that the p-n junction boundary naturally becomes free of holes or free electrons. This region is called a depletion region. The same phenomenon also happens in the p-n junction of the MOSFET. Now let's connect a power cell across the MOSFET and observe what happens. On the right hand side p-n junction, the electrons are attracted to the positive side of the cell and the holes are moved away. In short, the depletion region width on the right hand side is increased due to the power source. This means that there won't be any electron flow through the MOSFET. In short, with this simple arrangement, the MOSFET will not work. Let's see how it is possible to have an electron flow in the MOSFET using a simple technique. To do this, we first need to understand the workings of the capacitor. Inside the capacitor, you can see two parallel metal plates separated by an insulator. When you apply a DC power source across these, the positive terminal of the cell attracts electrons in the metal plate and these electrons are accumulated on the other metal plate. This accumulation of charge creates an electric field between the plates. Let's replace one plate of the capacitor with the p-type substrate of the MOSFET. If you connect a power source across the MOSFET as shown, just as in a capacitor, the electrons will leave the metal plate. In a MOSFET, these electrons will be dispersed into the p substrate. The positive charge generated on the metal plate due to the electron displacement will generate an electric field as shown. Remember there are some free electrons even in the P type region. The electric field produced by the capacitive action will attract the electrons to the top. We will assume the electric field generated is quite strong and then observe the electron flow. To make things clear, let's rewind the animation. Some electrons were recombined with the holes. And you can see that the top region becomes overcrowded with electrons after all the holes there are filled. Just below this region, all the holes were filled, but there were no free electrons either. This region has become a new depletion region. You can see that this process essentially breaks the depletion region barrier and a channel for the flow of electrons is created. If we apply a power source as we did at the beginning of this video, the electrons easily flow as shown. This is the way a MOSFET turns to the on state. You can easily correlate the naming of the transistor terminals with the nature of the electron flow. If the applied voltage is not sufficient enough, the electric field will be weak and there won't be a channel formation and hence no electron flows. Thus just by controlling the gate voltage, we will be able to turn the MOSFET on and off. Now let's see a real life example where a MOSFET works as a switch. Consider this heat based fire alarm. The thermistor in the circuit decreases its resistance with an increase in temperature. Initially at room temperature, the voltage at the gate is low due to the high thermistor resistance. And that is not sufficient to turn on the MOSFET. If the temperature increases, the thermistor's resistance decreases. This will lead to a high gate voltage, which then turns on the MOSFET (alarm beeping) and the alarm. MOSFET has opened the door to digital memory and digital processing. Here you can see four MOSFETs combined together to form the basic memory element of a static Ram. At the lowest level, MOSFETs are interconnected to form logic Gates. At the next level, the Gates are combined to form processing units that perform thousands of logical and arithmetical operations. Unlike BJTs, MOSFETs have a scalable nature. So that millions of MOSFETs can be fabricated on a single wafer. A BJT wastes a small part of its main current when it's switched on. Such power wastage is not there in MOSFETS. The other advantage of a MOSFET is that it only operates with one type of charge carrier, be it a hole or an electron. So it is less noisy. These are the reasons why MOSFETs are the popular choice in digital electronics. We hope this video gave you a clear conceptual overview of the workings of MOSFETs, and please don't forget to support us on Patreon. Thank you.