Important characteristics of MOS transistor compared to transistor
1) The source S, gate G, and drain D of a field-effect transistor correspond to the emitter e, base b, and collector c of the transistor, respectively, and their functions are similar.
2) Field effect transistor is a voltage controlled current device, controlled by VGS for ID, while ordinary transistor is a current controlled current device, controlled by IB for IC. The amplification factor of MOS pipeline is (transconductance gm), which is the amount of ampere change in drain current that can be caused when the gate voltage changes by one volt. A transistor is a transistor with a current amplification factor (beta β) that changes the collector current by one milliampere when the base current changes.
3) The gate and other electrodes of the field-effect transistor are insulated and do not generate current; The base current IB determines the collector current IC during the operation of the transistor. Therefore, the input resistance of field-effect transistors is much higher than that of transistors.
4) Field effect transistors only involve the majority of charge carriers in conduction; There are two types of charge carriers involved in conduction in a transistor: majority charge carriers and minority charge carriers. Due to the significant influence of temperature, radiation, and other factors on the concentration of minority charge carriers, field-effect transistors have better temperature stability than transistors.
5) When the source and drain of a field-effect transistor are not connected to the substrate, they can be used interchangeably with little change in characteristics. However, when the collector and emitter of a transistor are used interchangeably, their characteristics differ greatly, and the b value will decrease significantly.
6) The noise coefficient of field-effect transistors is very small, so field-effect transistors should be used in the input stage of low-noise amplifier circuits and circuits that require high signal-to-noise ratios.
7) Both field-effect transistors and ordinary crystal transistors can be used to form various amplifier circuits and switch circuits. However, field-effect transistors have simple manufacturing processes and excellent characteristics that ordinary crystal transistors cannot match. They are gradually replacing ordinary crystal transistors in various circuits and applications. Currently, field-effect transistors are widely used in large-scale and ultra large scale integrated circuits.
In switch mode power supply circuits, the advantages of high-power MOS transistors and high-power transistor transistors over MOS transistors
1) High input impedance and low driving power - due to the silicon dioxide (SiO2) insulation layer between the gate and source, the DC resistance between the gate and source is basically the SiO2 insulation resistance, usually around 100M Ω, while the AC input impedance is basically the capacitive impedance 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 with voltage, resulting in extremely low driving power (high sensitivity). A typical transistor must have a base voltage Vb and generate a base current Ib to drive the generation of collector current. The driving of a transistor requires power (Vb × Ib).
2) Fast switching speed - The switching speed of MOSFET is closely related to the capacitive characteristics of the input. Due to the presence of the capacitive characteristics of the input, the switching speed slows down. However, when used as a switch, it can reduce the internal resistance of the driving circuit and accelerate the switching speed (the input is driven by a "charging circuit" described later, which accelerates the charging and discharging time of the capacitive). MOSFETs rely solely on multi carrier conduction and do not have a few carrier storage effect, resulting in a very rapid turn off process. The switching time is between 10-100ns, and the operating frequency can reach over 100kHz. Ordinary transistor transistors always have hysteresis due to the storage effect of minority carriers, which affects the improvement of switching speed (currently, switching power supplies using MOS transistors can easily achieve a working frequency of 100K/S to 150K/S, which is unimaginable for ordinary high-power transistor transistors).
3) No secondary breakdown - due to the phenomenon that ordinary power crystal transistors have a positive temperature current characteristic where the collector current increases as the temperature rises, and the increase in collector current leads to further temperature rise, which further leads to a vicious cycle of increasing collector current. The withstand voltage VCEO of the transistor gradually decreases with the increase of tube temperature, which leads to the breakdown of the transistor as the tube temperature continues to rise and the withstand voltage continues to decrease. This is a destructive thermoelectric breakdown phenomenon that causes a 95% damage rate to the switching power supply tube and row output tube of the television, also known as the secondary breakdown phenomenon. MOS transistors have temperature current characteristics opposite to ordinary transistor, that is, when the transistor temperature (or ambient temperature) rises, the channel current IDS actually decreases. For example; A MOS FET switch with IDS=10A, when the VGS control voltage remains constant, IDS=3A at 250C temperature. When the chip temperature rises to 1000C, IDS decreases to 2A. This negative temperature current characteristic, which causes a decrease in channel current IDS due to temperature rise, prevents a vicious cycle and thermal breakdown. That is to say, there is no secondary breakdown phenomenon in MOS transistors. It can be seen that using MOS transistors as switching transistors significantly reduces the damage rate of switching transistors. In the past two years, the use of MOS transistors instead of ordinary crystal transistors in TV switching power supplies has greatly reduced the damage rate of switching transistors, which is also an excellent proof.
4) After the MOS transistor is turned on, its conduction characteristics are purely resistive - ordinary crystal transistors have almost straight through saturation conduction, with an extremely low voltage drop called saturation voltage drop. Since there is a voltage drop, it is; After saturation conduction, a regular transistor is equivalent to a very small resistance, but this equivalent resistance is a nonlinear resistance (the voltage on the resistance and the current flowing through it cannot conform to Ohm's law). As a switching transistor, a MOS transistor also has a very small resistance after saturation conduction, but this resistance is equivalent to a linear resistor. The resistance value of the resistor is related to the voltage drop at both ends and the current flowing through it, which conforms to Ohm's law. If the current is large, the voltage drop will be large, and if the current is small, the voltage drop will be small. After conduction, since it is equivalent to a linear element, the linear element can be applied in parallel. When these two resistors are connected in parallel, there is an automatic current balancing effect. Therefore, MOS transistors are used in a When the power of a single tube is insufficient, multiple tubes can be used in parallel, And there is no need to add additional balancing measures (nonlinear devices cannot be directly applied in parallel).
Compared with ordinary transistor, MOS transistor has the above four advantages, which are sufficient to completely replace ordinary transistor in switch operation. At present, the MOS pipeline VDS can achieve 1000V and can only be used as a switch tube for switching power supplies. With the continuous improvement of manufacturing processes and the continuous improvement of VDS, it is also possible to replace the row output tube of cathode ray tube televisions in the near future.
