What are the differences between the triode and the MOS tube?

The bipolar tube is a current-controlled device (which controls a large collector current through a small base current), and the MOS transistor is a voltage-controlled device (the on-resistance between the source and the drain is controlled by the gate voltage). MOSFET (field effect transistor) under the conduction voltage drop, the on-resistance is small, the gate drive does not require current, the loss is small, the drive circuit is simple, the self-protection diode, the thermal resistance characteristics are good, suitable for high-power parallel, fault switch The speed is not high and it is expensive. The triode switch speed is high, the Ic of the large triode can be made very large, the disadvantage is large, the base drive current is large, and the drive is complicated. Generally speaking, in low-cost applications, triodes are considered for common applications, and MOS tubes are not considered. In fact, it is wrong to say that current control is slow and voltage control is fast. It is only necessary to understand the way in which bipolar and MOS transistors work. The triode is operated by the movement of carriers. Taking the npn tube emitter follower as an example, when the base is not applied with voltage, the pn junction composed of the base region and the emitter region is blocked by multiple ions (the base region is a hole). The emission region is an electron diffusion motion. At this pn junction, an electrostatic field (ie, a built-in electric field) directed from the emitter region to the base region is induced. When the base plus a positive voltage is directed to the base region, the emitter region is pointed to the emitter region. When the electric field generated by the applied voltage of the base is greater than the built-in electric field, the carrier (electron) in the base region may flow from the base region to the emitter region. The minimum value of this voltage is the forward conduction voltage of the pn junction (it is generally considered in engineering) 0.7v). However, at this time, there is a charge on both sides of each pn junction. At this time, if the collector-emitter is positively charged, the electrons in the emitter region move toward the base region under the action of the electric field (actually, the electrons move in the opposite direction. ), because the width of the base region is small, electrons easily cross the base region to reach the collector region, and recombine with the holes of the PN here (near the collector), in order to maintain the balance, under the action of the positive electric field, the collector region The electron accelerates the movement of the outer collector, while the cavity moves at the pn junction, a process similar to an avalanche process. The collector's electrons return to the emitter through the power supply, which is how the transistor works. When the triode is working, both pn junctions induce charge. When the switch is in the on state, the triode is saturated. If the triode is turned off, the charge induced by the pn junction should be restored to equilibrium. This process takes time. The MOS triode works differently and does not have this recovery time, so it can be used as a high speed switch. (1) The field effect transistor is a voltage control element, and the transistor is a current control element. In the case where only a small current is allowed from the signal source, the FET should be used; and when the signal voltage is low and the current is taken from the signal source, the transistor should be selected. (2) FETs use majority carriers to conduct electricity, so they are called unipolar devices, and transistors have both majority carriers and minority carriers. It is called a bipolar device. (3) The source and drain of some FETs can be used interchangeably. The gate voltage can also be positive or negative, and the flexibility is better than that of the transistor. (4) The FET can work under very low current and very low voltage conditions, and its manufacturing process can easily integrate many FETs on one silicon chip, so the FET is in large scale integrated circuit. It has been widely used. (5) Field effect transistors have the advantages of high input impedance and low noise, and are therefore widely used in various electronic devices. In particular, the field effect tube is used as the input stage of the entire electronic device, and the performance that is difficult to achieve by a general transistor can be obtained. (6) The FET is divided into two types: junction type and insulated gate type, and the control principle is the same. Other comparisons: 1. The triode is a bipolar tube, that is, the inside of the tube is operated by two carriers, a hole and a free electron. The FET is a unipolar tube, that is, the tube works with only holes, or only free electrons participate in conduction, only one type of carrier 2. The triode is a current control device that has an input current to have an output current. FETs are voltage-controlled devices that have an output current without input current. 3. The input impedance of the triode is small, and the input impedance of the FET is large. 4. Some FETs have source and drain interchangeable, and the collector and emitter of the transistor are not interchangeable. 5. The frequency characteristics of the FET are not as good as those of the triode. 6. The FET has a small noise figure and is suitable for the pre-stage of the low noise amplifier. 7. If you want the signal source current to be small, you should use the FET. Otherwise, it is more suitable to use the triode.