1. What is a MOS tube?
The full name of MOS tube is metal-oxide-semiconductor field-effect transistor or metal-insulator-semiconductor field-effect transistor. The English name is metal oxide semiconductor, which belongs to the insulated gate type of field effect tube. Therefore, MOS tube is sometimes called insulated gate. Field effect tube.
2. The structure of MOS tube.
The MOS tube device has two electrodes, the drain D and the source S. Both the N-type in Figure 1 or the P-type in Figure 2 are on a P-type semiconductor silicon substrate with a lower doping concentration. The semiconductor photolithography and diffusion process make two N+/P+ regions with high doping concentration, and lead the drain D and source S with metal aluminum. Then the N/P type semiconductor surface between the drain and the source is covered with a very thin silicon dioxide (SiO2) insulating film, and an aluminum electrode is installed on this insulating film as the gate G. This constitutes an N/P channel (NPN type) enhanced MOS tube.
Third, the characteristics of MOS tube.
MOS tube has high input impedance, low noise, good thermal stability; simple manufacturing process and strong radiation, so it is usually used in amplifying circuit or switching circuit.
Figure 1 is the symbol of the N-channel MOS tube. In the figure, D is the drain, S is the source, and G is the gate. The arrow in the middle indicates the substrate. If the arrow points inward, it means an N-channel MOS tube. The outside represents a P-channel MOS tube.
In the actual MOS tube production process, the substrate is connected to the source before leaving the factory, so in the rules of the symbol; the arrow indicating the substrate must also be connected to the source to distinguish the drain from the source. Figure 3 is the symbol of the P-channel MOS tube.
The polarity of the applied voltage of the MOS tube is the same as our ordinary transistors. The N-channel is similar to the NPN transistor. The drain D is connected to the positive electrode, the source S is connected to the negative electrode, and the conductive channel is established when the gate G is positive. The MOS tube starts to work, as shown in Figure 2. The same P-channel similar PNP transistor, the drain D is connected to the negative electrode, the source S is connected to the positive electrode, and the gate G is negative voltage, the conductive channel is established, and the P-channel MOS transistor starts to work, as shown in Figure 4;
5. The working principle of MOS tube.
From Figure 1 above, it can be seen that there are two back-to-back PN junctions between the drain D and source S of the enhanced MOS tube.
When the gate-source voltage VGS=0, even if the drain-source voltage VDS is added, there is always a PN junction in the reverse bias state, and there is no conductive channel between the drain-source (no current flows), so the drain at this time Current ID=0.
At this time, if a forward voltage is applied between the gate and the source, as shown in Figure 2, that is, VGS>0, a gate is generated in the SiO2 insulating layer between the gate and the silicon substrate and points to the P-type silicon substrate Since the oxide layer is insulated, the voltage VGS applied to the gate cannot form a current. A capacitor is formed on both sides of the oxide layer. VGS is equivalent to charging the capacitor and forming an electric field. As VGS gradually Increased, attracted by the positive voltage of the gate, a large number of electrons are gathered on the other side of the capacitor and an N-type conductive channel from drain to source is formed. When VGS is greater than the turn-on voltage VT of the tube (generally about When it is 2V), the N-channel tube begins to conduct, forming a drain current ID. We call the gate-source voltage when the channel begins to form as the turn-on voltage, which is generally represented by VT. Controlling the magnitude of the gate voltage VGS changes the strength of the electric field, and the goal of controlling the magnitude of the drain current ID can be achieved. This is also an important feature of the MOS tube using electric field to control the current, so it is also called a field effect tube.
Six: The advantages of MOS:
1. The source S, the gate G, and the drain D of the field effect tube correspond to the emitter e, base b, and collector c of the transistor respectively. Their functions are similar. Figure 1 shows the N-channel MOS tube and NPN type transistor pin, Figure 2 shows the corresponding diagram of P-channel MOS transistor and PNP type transistor pin.
2. Field effect tube is a voltage control current device, ID is controlled by VGS, ordinary transistor is a current control current device, and IC is controlled by IB. The MOS pipe amplification factor is (transconductance gm) how many amperes can be caused when the gate voltage changes by one volt. The transistor is the current amplification factor (beta) when the base current changes by one milliampere, how much the collector current can change.
3. The grid of the field effect transistor and other electrodes are insulated, and no current is generated; while the transistor is working, the base current IB determines the collector current IC. Therefore, the input resistance of the FET is much higher than the input resistance of the triode.
4. Only majority carriers participate in the conduction of field effect transistors; the triode has two types of carriers: majority carriers and minority carriers participate in conduction. Because the concentration of minority carriers is greatly affected by factors such as temperature and radiation, the field The temperature stability of the effect tube is better than that of the triode.
5. When the source of the field effect transistor is not connected to the substrate, the source and drain can be used interchangeably, and the characteristics change little, while when the collector and emitter of the triode are used interchangeably, their characteristics are very different. Larger, the value of b will decrease a lot.
6. The noise coefficient of the field effect tube is very small. The field effect tube should be used in the input stage of the low-noise amplifier circuit and the circuit requiring high signal-to-noise ratio.
7. Both FETs and ordinary transistors can form various amplifying circuits and switching circuits, but the manufacturing process of FETs is simple, and they have excellent characteristics that ordinary transistors can’t match. They are gradually being replaced in various circuits and applications. Ordinary transistors, field effect transistors have been widely used in current large-scale and very large-scale integrated circuits.
8. High input impedance and low driving power: Since the gate source is an insulating layer of silicon dioxide (SiO2), the DC resistance between the gate source is basically the insulation resistance of SiO2, generally about 100MΩ, and the AC input impedance is basically The capacitive reactance of the input capacitor. Due to the high input impedance, there will be no voltage drop on the excitation signal, and it can be driven when there is voltage, so the driving power is extremely small (high sensitivity). A general transistor must have a base voltage Vb and then generate a base current Ib to drive the generation of the collector current. The driving of the transistor requires power (Vb×Ib).
9. Fast switching speed: The switching speed of MOSFET has a great relationship with the capacitive characteristics of the input. Due to the capacitive characteristics of the input, the switching speed becomes slower, but when used as a switch, the internal resistance of the drive circuit can be reduced. , To speed up the switching speed (the input is driven by the "supply circuit" described later to speed up the capacitive charge and discharge time). MOSFETs only rely on multi-carriers to conduct electricity, and there is no minority carrier storage effect, so the turn-off process is very fast, the switching time is between 10-100ns, and the operating frequency can reach more than 100kHz. Ordinary transistors have the storage effect of minority carriers. Switching always has hysteresis, which affects the increase of switching speed (current switching power supplies using MOS tubes can easily achieve 100K/S～150K/S, which is unimaginable for ordinary high-power transistors) .
10. No secondary breakdown: Because ordinary power transistors have the phenomenon that when the temperature rises, the collector current will rise (positive temperature ~ current characteristics), and the rise of the collector current will cause the temperature to rise further, A further rise in temperature will further lead to a vicious circle of rise in collector current. The withstand voltage VCEO of the transistor will gradually decrease with the increase of tube temperature, which results in the continuous rise of tube temperature and the continuous decrease of withstand voltage, which will eventually lead to the breakdown of the transistor, which is a kind of switch power tube and line output of the TV. The destructive thermoelectric breakdown phenomenon, which accounts for 95% of the tube damage rate, is also called secondary breakdown. The MOS tube has the opposite temperature to current characteristics of ordinary transistors, that is, when the tube temperature (or ambient temperature) rises, the channel current IDS drops instead. For example; a MOS FET switch tube with IDS=10A, when the VGS control voltage is unchanged, IDS=3A at a temperature of 250C, when the chip temperature rises to 1000C, the IDS drops to 2A, which is caused by the rise in temperature The negative temperature current characteristics of the channel current IDS decrease, so that it will not produce a vicious cycle and thermal breakdown. That is to say, there is no secondary breakdown of MOS tube. It can be seen that using MOS tube as a switching tube has greatly reduced the damage rate of the switching tube. In the past two years, the switching power supply of TVs has replaced ordinary transistors with MOS tubes. The greatly reduced damage rate is also an excellent proof.
11. After the MOS tube is turned on, its conduction characteristics are purely resistive: ordinary transistors are almost straight-through when they are saturated and have a very low pressure drop, called saturation pressure drop. Since there is a pressure drop, then That is; the ordinary transistor is equivalent to a resistor with extremely small resistance after saturation and conduction, but this equivalent resistance is a non-linear resistance (the voltage on the resistor and the current flowing through it cannot comply with Ohm's law), The MOS tube is used as a switch tube, and there is also a resistor with a very small resistance after saturation and conduction, but this resistance is equivalent to a linear resistance, and the resistance of the resistance, the voltage drop at both ends and the current flowing through it conform to Ohm's law When the current is large, the voltage drop is large, and the current is small. The voltage drop is small. Since it is equivalent to a linear element after being turned on, the linear element can be used in parallel. When such two resistors are connected in parallel, there is an automatic current balance Therefore, when the power of one tube is not enough, the MOS tube can be used in parallel with multiple tubes, and there is no need to add additional balancing measures (non-linear devices cannot be directly applied in parallel).
Compared with ordinary transistors, MOS transistors have the above 11 advantages, which are enough to make MOS transistors completely replace ordinary transistors in the switching state. The current technology MOS pipeline VDS can achieve 1000V, which can only be used as a switching tube of a switching power supply. With the continuous improvement of manufacturing technology and the continuous improvement of VDS, it is also possible to replace the line output tube of CRT TV in the near future.
For more details about the MOS tube, please look forward to the next article on MOS tube encyclopedia: "more details on MOS tube 2"
"Detailed MOS Tube Two" -The third MOS tube problem classification: What is an enhanced MOS tube?
The enhanced type is turned on by "thickening" the thickness of the conductive channel. As can be seen from the above figure, the lower the gate voltage, the closer the positive ions of the p-type source and drain are to the middle, and the negative ions of the n substrate The farther away from the grid, when the grid voltage reaches a value, called a threshold or bump, the positive ions released from the p-type are connected together to form a channel, which is the effect of the illustration. Therefore, it is easy to understand that the gate voltage must be low to a certain level to be turned on. The lower the voltage, the thicker the channel and the smaller the on-resistance. Since the strength of the electric field is proportional to the square of the distance, the channel thickening caused by the voltage drop will not be obvious after the electric field is strong to a certain extent. This is also because it is more and more difficult for the n-type negative ions to "give back". The depletion type is to make a conductive layer in advance, and use the gate to thicken or thin to control the conduction of the source and drain. However, this kind of pipe is generally not produced and is basically not seen in the market. Therefore, when everyone talks about mos tube, it is enhanced by default.