MOS tube (1)
I.What is MOS tube?
The full name of MOS transistor is metal oxide semiconductor field effect transistor or metal insulator semiconductor field effect transistor, which is called metal oxide semiconductor in English. It belongs to the insulating gate type of field effect transistor. Therefore, MOS transistor is sometimes called insulating gate field effect transistor.
II.the structure of MOS transistor.
MOS transistor has two electrodes, drain D and source S. both the n-type and p-type in Figure 1 and Figure 2 are p-type silicon substrates with low doping concentration. Two N + / P + regions with high doping concentration are fabricated by semiconductor lithography and diffusion process, and drain D and source s are extracted by aluminum. Then a thin silicon dioxide (Si02) insulating layer is covered on the surface of N / P semiconductor between drain and source, and an aluminum electrode is installed on the insulating layer as gate G. This constitutes an N / P channel (NPN type) enhanced MOS transistor.
III.Characteristics of MOS transistor.
MOS transistor has the advantages of 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.
IV. voltage polarity and symbol rules of MOS transistor:
Figure 1 is the symbol of n-channel MOS transistor. In the figure, D is the drain, s is the source, G is the gate, and the arrow in the middle represents the substrate. If the arrow in represents the n-channel MOS transistor, the arrow out represents the p-channel MOS transistor.
In the actual production process of MOS transistor, the substrate is connected with the source before leaving the factory, so in the rules of symbols, the arrow representing the substrate must also be connected with the source to distinguish the drain and the source. Figure 3 is the symbol of p-channel MOS.
The polarity of the applied voltage of MOS transistor is the same as that of our common crystal triode. The n-channel is similar to NPN crystal triode. Drain D is connected to the positive pole, source s is connected to the negative pole, and the conduction channel is established when grid G is positive. The n-channel MOS transistor starts to work, as shown in Figure 2. When the drain D is connected to the negative pole, the source s is connected to the positive pole, and the grid G is under negative voltage, the conductive channel is established, and the p-channel MOS tube starts to work, as shown in Figure 4.
V. the working principle of MOS transistor.
It can be seen from Figure 1 that there are two back-to-back PN junctions between drain D and source s of the enhanced MOS transistor.
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 and source (no current flows), so the drain current id = 0.
If a positive voltage is applied between the gate and the source, as shown in Figure 2, i.e. VGS > 0, an electric field is generated in the SiO2 insulating layer between the gate and the silicon substrate, which points to the p-type silicon substrate. Because the oxide layer is insulated, the voltage applied to the gate, VGS, cannot form a current, and a capacitor is formed on both sides of the oxide layer. VGS is equivalent to charging the capacitor and forming an electric field. Field, with the gradual rise of VGS, attracted by the positive voltage of grid, a large number of electrons are gathered on the other side of this capacitor and a n-type conductive channel is formed from drain to source. When VGS is greater than the opening voltage Vt of the tube (generally about 2V), the n-channel tube begins to conduct and form the drain current ID. we call the gate source voltage when the channel begins to form the opening voltage. It is generally expressed in vt. The size of the control gate voltage VGS changes the strength of the electric field, which can control the size of the drain current ID. this is also an important feature of the electric field used to control the current of MOS transistor, so it is also called field effect transistor.
VI. advantages of MOS:
1. The source s, gate g and drain D of the FET correspond to the emitter e, base B and collector C of the triode respectively. Their functions are similar. Figure 1 shows the pins of n-channel MOS transistor and NPN type crystal triode, and Figure 2 shows the pins of p-channel MOS transistor and PNP type crystal triode.
2. FET is a voltage controlled current device controlled by VGS, ID is controlled by VGS, common crystal triode is a current controlled current device controlled by IB. The amplification factor of MOS pipes is how many amperes can the drain current change when the gate voltage changes by one volt. The crystal triode is the current amplification coefficient (beta) how much the collector current changes when the base current changes one milliampere.
3. The gate and other electrodes of the FET are insulated and do not produce current, while the base current IB determines the collector current IC when the triode is working. Therefore, the input resistance of FET is much higher than that of triode.
4. Most of the carriers in the FET participate in the conduction; most of the carriers and a few of the carriers in the triode participate in the conduction. Because the concentration of a few carriers is greatly affected by temperature, radiation and other factors, the temperature stability of the FET is better than that of the triode.
5. When the source of FET is not connected with the substrate, the source and drain can be used interchangeably, and the characteristics change little. When the collector and emitter of triode are used interchangeably, the characteristics are very different, and the b value will be much reduced.
6. The noise coefficient of the FET is very small, so the FET should be selected in the input stage of the low noise amplifier circuit and the circuit requiring high signal-to-noise ratio.
7. Field effect transistor and common crystal triode can form all kinds of amplifying circuits and switching circuits, but the manufacturing process of field effect transistor is simple, and it has outstanding characteristics that common crystal triode can't match. It is gradually replacing common crystal triode in all kinds of circuits and applications. At present, field effect transistor has been widely used in large-scale and ultra large scale integrated circuits.
8. High input impedance and small driving power: as the silicon dioxide (SiO2) insulation layer is between gate sources, the DC resistance between gate sources is basically SiO2 insulation resistance, generally up to about 100m Ω, and the AC input impedance is basically the capacitive reactance of input capacitance. Because of the high input impedance, there will be no voltage drop to the excitation signal, so the driving power is very small (high sensitivity). In order to drive the collector current, the base voltage VB and the base current IB are needed in general transistor. The driving of the transistor needs power (VB × IB).
9. Fast switching speed: The switching speed of MOSFET is closely related to the capacitive characteristics of input. Because of the existence of the capacitive characteristics of input, the switching speed becomes slower, but when used as a switch, the internal resistance of driving circuit can be reduced and the switching speed can be accelerated (the input is driven by the "perfusion circuit" described later, which speeds up the charging and discharging time of capacitance). MOSFET only relies on multi carrier conduction, and there is no storage effect of few carriers, so the switching process is very fast. The switching time is between 10-100ns, and the operating frequency can reach above 100kHz. Due to the storage effect of a few carriers, the general crystal triode always has hysteresis phenomenon, which affects the improvement of switching speed (at present, the operating frequency of the switching power supply with MOS transistor can be easily achieved 100k / S ~ 150k / s, which is unimaginable for ordinary high-power transistor).
10. No secondary breakdown: the common power transistor has the phenomenon that when the temperature rises, the collector current will rise (positive temperature current characteristic), and the rise of collector current will lead to further rise of temperature, further rise of temperature, and further rise of collector current. The Vceo of the transistor decreases with the increase of the tube temperature, which leads to the breakdown of the transistor. This is a kind of ring breaking thermal electric breakdown phenomenon, also known as the secondary breakdown phenomenon, which leads to 95% of the damage rate of the switching power supply tube and the line output tube of the television. MOS transistor has the opposite temperature current characteristic of common crystal triode, that is, when the tube temperature (or ambient temperature) increases, the channel current IDS decreases. For example, for a MOS FET with IDS = 10a, when the VGS control voltage is constant, IDS = 3A at 250C, when the chip temperature rises to 1000C, IDS drops to 2a, which is a negative temperature current characteristic that the channel current IDS drops due to the temperature rise, so that it will not generate malignant cycle and thermal breakdown. That is to say, there is no secondary breakdown phenomenon in MOS transistor. It can be seen that the damage rate of the switch transistor is greatly reduced when MOS transistor is used as the switch transistor. In recent two years, the damage rate of the switch transistor is greatly reduced when MOS transistor is used to replace the common transistor in TV switching power supply.
11. After the MOS transistor is turned on, its conduction characteristic is pure resistance: the common crystal triode is almost through in saturation conduction, with a very low voltage drop, which is called saturation voltage drop. Since there is a voltage drop, that is to say, the common crystal triode is equivalent to a resistance with a very small resistance value after saturation conduction, but the equivalent resistance is a non-linear resistance. The voltage and the current flowing can't conform to Ohm's law), and the MOS transistor is used as a switch. There is also a resistance with a very small resistance value after saturated conduction, but this resistance is equivalent to a linear resistance. The resistance value of the resistance and the voltage drop at both ends and the current flowing conform to Ohm's law. If the current has a large voltage drop, the current has a small voltage drop. After conduction, since the equivalent is A linear element can be used in parallel. When two resistors are connected in parallel, there will be an automatic current balance function. Therefore, MOS tube can be used in parallel with multiple tubes when the power of one tube is not enough, and there is no need to add additional balance measures (non linear element can not be directly used in parallel).
Compared with the common crystal triode, MOS transistor has the above 11 advantages, which is enough to replace the common crystal triode completely in the switch operation state. At present, the technology of MOS pipeline VDS can achieve 1000V, which can only be used as the switch tube of switching power supply. With the continuous progress of manufacturing technology and the continuous improvement of VDS, it can also be realized in the near future to replace the line output tube of CRT TV.
For more details on MOS tube, please look forward to the next MOS tube encyclopedia knowledge: detailed introduction to MOS tube II
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