What is a MOS tube?
MOS transistor (Metal Oxide Semiconductor Field Effect Transistor, MOSFET) is a semiconductor three-terminal device that uses electric field effect to control its current. Many characteristics and application directions are similar to triodes. This device is not only small in size, light in weight, low in power consumption, and long in life, but also has the advantages of high input impedance, low noise, good thermal stability, and strong radiation resistance. It is widely used, especially in large-scale integrated circuits. middle.
MOS tube structure and classification
According to the different conductive channels, MOS tubes can be divided into two categories: N-channel and P-channel, and each category is divided into two types: enhancement mode and depletion mode. Therefore, MOS tubes can be divided into four major categories: N-channel consumption type, N-channel enhancement type, P-channel consumption type, and P-channel enhancement type. Now take N-channel devices as an example to introduce the working principle of MOS tubes.
As shown in the figure above, the structural diagram of the N-channel enhancement MOS tube. It uses a low-doped P-type silicon material as a substrate, creates two highly-doped N-type regions on it, and leads out two electrodes respectively as the source electrode s and the drain electrode d, covering the surface of the P-type substrate A very thin oxide film (silicon dioxide) insulating layer and an electrode lead as the gate g. The gate g of this field effect transistor is insulated from the P-type semiconductor substrate, drain d, and source s, so it is also called an insulated gate field effect transistor.
Working principle of MOS tube
The basic working principle of the MOS tube is to use the gate-source voltage to control the drain current, but there is no original conductive channel between the drain and the source, so it needs to be established first during operation.
1. Create a conductive channel
As shown in the figure, when a positive gate-source voltage VGS>0 is applied, an upper positive and lower negative electric field appears on the oxide layer below the gate. This electric field will attract the free electrons in the P region and cause them to move in the oxide layer. Accumulation below will also repel the holes in the P area, causing them to leave the area. The larger the VGS, the greater the electric field intensity, and the more obvious this effect is. When VGS reaches VT, the concentration of free electrons accumulated in this region is large enough to form a new N-type region, like a bridge connecting the drain and source. This area is called the N-type conductive channel, or N channel for short, and Vt is called the turn-on voltage. VGS>VT is a necessary condition for establishing the conductive channel.
2. Establish drain current
After the channel is established, if there is a certain driving voltage VDS between the drains. When the drain voltage VDS appears, the drain potential is higher than the source, so VGS>VGD, so the electric field distribution on the oxide layer is uneven. The intensity is strong near the source and weak near the drain, and the corresponding conductive trench The channel also changes accordingly: it is wider near the source and narrower near the drain.
Therefore, the drain current Id of the MOS tube is mainly affected by the voltage VGS and VDS. The former affects Id by controlling the conductive channel, and the latter directly affects Id as a driver. But it needs to be emphasized again that if the conductive channel is not established, there will be only VDS and the drain current will not appear.
Main characteristics of MOS tube
1. High input impedance: There is an insulating layer between the gate electrode of the MOS tube and the source and drain regions, and there is only a weak gate current, so the input impedance of the MOS tube is very high, close to infinity.
2. Low output impedance: Since the MOS tube is a voltage-controlled device, its source-to-drain current can change with changes in the input voltage, so its output impedance is very small.
3. Constant current performance: When the MOS tube works in the saturation zone, even if the source-drain voltage changes, its current is almost unchanged, so the MOS tube has good constant current performance.
Common parameters of MOS tubes
Vgs: The maximum drive voltage of the gate and source, a limit parameter of the MOS tube, indicating the maximum drive voltage that the MOS tube can withstand. Once the driving voltage exceeds this limit, it will cause permanent damage to the gate oxide layer of the MOS tube.
VDS: Drain-source voltage, which represents the maximum voltage that the MOS tube can withstand between its drain and source. This parameter is related to the junction temperature. Generally, the higher the junction temperature, the largest VDS value.
RDS(on): Drain-source on-resistance, which is the equivalent resistance between drain and source when the MOS tube is fully turned on. This parameter is related to junction temperature and driving voltage Vgs. Within a certain range, the higher the junction temperature, the greater Rds; the higher the driving voltage, the smaller Rds.
The impedance of the MOS tube when it is in the conductive state. The greater the on-resistance, the greater the loss in the on state. Therefore, the conduction resistance of the MOS tube should be reduced as much as possible.
Power consumption during conduction:
Qg: Gate charge, which is the charging charge required to increase the gate voltage from 0V to the termination voltage under the action of the drive signal. That is, the charge that the drive circuit needs to provide when the MOS tube changes from the off state to the fully conductive state. It is a main parameter used to evaluate the driving capability of the MOS tube's drive circuit.
Id: drain current, which means that when the case temperature is at a certain value, if the MOS tube operating current is the above-mentioned maximum drain current, the junction temperature will reach the maximum value. This parameter is also related to device packaging and ambient temperature.
Eoss: Output capacitor energy, indicating the amount of energy stored in the MOS tube by the output capacitor Coss.
According to the working principle and characteristics of the MOS tube, it is not difficult to find that its characteristics are very similar to those of the triode, and both can be used as amplifier devices, such as forming an inverse amplifier, a voltage follower, a current follower, etc. The amplifier circuit composed of the two devices has its own characteristics. There are advantages. The MOS tube amplifier circuit has high input impedance and low noise, and the triode amplifier circuit has strong amplification capability. In practical applications, the two are often used in combination.
Switching characteristics of MOS tube
1. P-channel MOS tube switch circuit
The characteristics of PMOS, Vgs is less than a certain value will be turned on, suitable for the situation when the source is connected to VCC (high-side drive). It should be noted that Vgs refers to the voltage of the gate G and the source S, that is, the gate is turned on when it is lower than a certain voltage of the power supply, not the voltage relative to the ground. However, because the PMOS conduction internal resistance is relatively large, it is only suitable for low power situations. High power still uses N-channel MOS transistors.
2. N-channel MOS tube switch circuit
The characteristics of NMOS, when Vgs is greater than a certain value, it will be turned on. It is suitable for the situation when the source is grounded (low-side drive). As long as the gate voltage is greater than the Vgs given in the parameter manual, the drain D is connected to the power supply. , the source S is grounded. It should be noted that Vgs refers to the voltage difference between the gate G and the source S, so when the NMOS is used as a high-side driver, when the drain D and the source S are turned on, the potential of the drain D and the source S are equal, then The gate G must be higher than the voltage of the source S and the drain D, so that the drain D and the source S can continue to conduct.
MOS tube switch circuit example 1 (MOS tube is used to control the load)
Conduction condition: Vgs>Vth, the function of R1 and R2 is to create a Vgs voltage between G and S, and there is no need to care about the voltage relationship between G and D (as long as the breakdown voltage is not reached). In addition, the S pole does not necessarily need to be grounded, as long as the potential difference between Vg and Vs is greater than Vth, the MOS tube can still function as a switch.
Things to note:
1. The maximum driving peak current of the IO port, the driving capability of the IO port of different chips is different.
2. Understand the parasitic capacitance of the MOS tube. If the parasitic capacitance is large, the energy required for conduction is greater. If the peak value of the output current of the IO port is small, the conduction of the tube will be slower.
MOS tube switch circuit example 2 (MOS tube control power output)
Here we use NPN and NMOS transistors for switch design as shown in Figure 4. When Q2 inputs a low level, the triode Q2 does not conduct, the Vgs of the MOS transistor Q1=0, and the MOS transistor Q1 does not conduct. When the input level of Q2 is high, the triode is turned on, the Vgs and Vth of the MOS transistor Q1 are turned on, and the MOS transistor Q1 is turned on. Due to the conduction of the MOS tube
Matters needing attention:
1. Pay attention to the direction of the diode between the MOS tubes D and S. When it is not conducting, the direction of the diode should be opposite to the direction of the power output.
2. Since there is internal resistance when the MOS tube is turned on, the output voltage of the MOS tube is lower than the actual input voltage.
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