In the previous video, we saw some characteristics about BJT transistors, in this video, we are going to focus on the study of MOSFET transistors. Field effect transistors (FET) are the next generation of transistors, after BJT transistors. Within this group are N-channel and P-channel JFET transistors, as well as MOSFET transistors . The MOS prefix comes from the words metal-oxide-semiconductor, which is the type of material used in these particular transistors. MOSFET transistors can be incremental or N-channel and P-channel enrichment type, or they can also be N-channel and P-channel decremental or depletion type. BJT transistors are current-controlled devices , which is why they are used in amplifier circuits. While the field effect transistors within them are MOSFETs, they are voltage controlled devices and are used as switches due to their high speed. Here are the symbols of the different types of transistors, generally, they have three visible terminals. The gate that would be the equivalent to the base in a BJT, we also have the drain and the source that would be the equivalent to the collector and emitter respectively. The most used are the enrichment MOSFET transistors. Like BJT transistors , they use the same type of package and what changes is their internal configuration. In this example, it is a transistor with TO-220 encapsulation, of the IRF540N series, so it is an N-channel enrichment transistor. The most noticeable difference between the enrichment and depletion transistor symbols is the line that forms the channel. Since in enrichment transistors the line is discontinuous, and this is expressed like this, because it does not have a physical channel. While in that of impoverishment it is a continuous line , because it has a physical channel. Now, to identify the type of channel that the transistor has, we just have to look at the arrow that is in the symbol, since it represents a diode. For example, in this symbol, the diode would be in this way, where the arrow goes from positive to negative, and therefore the transistor would be channel N. In this other case, the diode is in the opposite direction and therefore so much the channel of the transistor remains of the type P. The same is for the depletion transistors. Let's detail a little more how this type of transistors works, for this, let's start by looking at the N-channel enrichment mosfet transistors . Let's take the IRF540N series transistor as an example , we can appreciate the type of encapsulation, its basic structure and its symbol. This type of transistor is basically composed of a metal gate, where the gate or gate terminal is connected , we also have the oxide, which behaves as an insulator, the metal substrate, in addition the drain and source terminals are connected to a positive n material . Do not confuse the order of the terminals of the internal structure with the terminals of the physical transistor. Since, for a better explanation, it has been arranged in that order. In most transistors, internally, the source terminal is connected to the metal substrate . Between the terminal G and S is the oxide that is an insulator, therefore, a capacitor or capacitor would be formed. Recall that this type of transistors are controlled by voltage but not by current as were the BJT transistors. Later I will explain everything about this. To better understand this, let's assume that the transistor is going to be connected to a resistor and a battery as shown. Then the S terminal would be negative polarity. Under these conditions, current does not flow through the transistor or through the resistance, since the G terminal or gate is not yet connected to a higher voltage level than the S terminal or source. If we send a signal through the G terminal, that is, a certain voltage level, then, this terminal will be more positive than the metal substrate, then it attracts the electrons from the substrate and accumulates them on the surface, under the oxide layer . If the VGS is greater than the VGSth or threshold voltage of the transistor, then a sufficient amount of electrons accumulates to form a virtual n channel and thus allow current to pass. The greater the voltage across gate G, the greater the electron channel and thus the current will flow from drain D to source S of the transistor with greater intensity. Note that, in the right circuit, I am assuming the conventional sense of current, but in reality, the electrons travel from the negative to the positive of the battery. If it stops sending a voltage signal through the gate, then the transistor will stop conducting. Like BJT transistors, MOSFET transistors feature 3 modes of operation. Cut region: V_GS≤ V_GSth, this can be seen in the lower part of the graph with a blue line. In this region, the transistor behaves like an open circuit between drain and source, so there is no current conduction. Saturation region: V_DS = V_GS-V_GSth, this region can be seen in the red graph. In this region, the transistor behaves like a short-circuit between drain and source, so the transistor conducts large amounts of current. Linear or Ohmic region: V_DS≤V_GS-V_GSth, this region can be seen in the green graph. In this region, the transistor behaves like a variable value resistor. So there will be more heat dissipation. For example, here we have the data sheet and specifications of this transistor, where we can find a lot of useful information and know if it is the type of transistor you want to use for our project. Here we have two examples of transistors, on the one hand, we have an npn type bjt transistor, and on the other a n-channel enrichment mosfet transistor . Both transistors have almost the same characteristics in terms of power, current and voltage. But let's see, what type of transistor should be used for certain applications. If we want to control the revolutions of a 12 V / 15 W motor with a potentiometer, the best option is to use a BJT transistor, since these can increase the current in the emitter if the current is increased through the base. If we want to control the same motor, but this time with a PWM signal, then the best option is to use a MOSFET transistor, since these can quickly switch and maintain the square signal at the output. The square signal can be obtained from a 555 chip or even a controller like Arduino can be used. A BJT transistor cannot maintain this square signal at its output and what it does is distort it, this happens because the PWM signal, its current is constant and what varies is the voltage, that is why MOSFET transistors are used since these , they can work with voltage. Hence, MOSFET transistors have certain advantages over BJT transistors, and they are: High switching speed, ideal for working with PWM signals. They can be voltage controlled with a very small gate current. They are more stable with temperature than BJTs. They withstand a very high current through the drain - source. But these mosfet transistors also have certain disadvantages, such as: They can be damaged due to static electricity. In general, they are less linear than BJTs. Well folks, up to this point, we've already learned more about mosfet transistors, especially N-channel enrichment transistors , which are the most widely used. The other types of mosfet transistors work in much the same way. I am going to make a video with the application, specifically about power with this type of transistors, although it will be seen in the other topics later, such as Buck and boost converters, inverters and others. If you want to go deeper into this topic, here are the names of the power electronics books, see you later.
