The difference between triode and MOS tube in circuit design

What is the difference between triode and MOS tube when used as switches in circuit design? Working nature:

1. The transistor is controlled by current, while the MOS tube is controlled by voltage.

2. Cost issue: Transistors are cheap, while MOS tubes are expensive.

3. Power consumption problem: transistor loss is large.

4. Driving capability: MOS tubes are often used for power switches and high-current local switching circuits.

In fact, triodes are relatively cheap, easy to use, and are often used in digital circuit switch control.

MOS tubes are used in high-frequency and high-speed circuits, high-current applications, and places that are sensitive to base or drain control current.

Generally speaking, for low-cost applications, triodes should be considered first, and if that doesn't work, MOS tubes should be considered.

In fact, it is wrong to say that current control is slow and voltage control is fast. To truly understand it, you need to understand the working methods of bipolar transistors and MOS transistors. The triode works by the movement of carriers. Take the npn emitter follower as an example. When there is no voltage applied to the base, the pn junction composed of the base and emitter regions prevents the diffusion movement of the majority carriers (holes in the base region and electrons in the emitter region). At this pn junction, an electrostatic field (i.e., built-in electric field) pointing from the emitter region to the base region will be induced. When the positive voltage applied to the base points from the base region to the emitter region, and when the electric field generated by the applied voltage to the base is greater than the built-in electric field, the carriers (electrons) 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 (generally considered to be 0.7v in engineering). However, at this time, there will be charges on both sides of each pn junction. If a positive voltage is applied to the collector-emitter, under the action of the electric field, the electrons in the emitter region move to the base region (actually, the electrons move in the opposite direction). Since the width of the base region is very small, the electrons can easily cross the base region to reach the collector region and recombine with the PN holes here (close to the collector). To maintain balance, the electrons in the collector region accelerate the movement of the outer collector under the action of the positive electric field, while the holes move at the pn junction. This process is similar to an avalanche process. The electrons in the collector return to the emitter through the power supply. This is the working principle of the transistor. When the triode is working, both pn junctions will induce charges. When the switch tube is in the on state, the triode is in the saturation state. If the triode is turned off at this time, the charge induced by the pn junction needs to be restored to the equilibrium state, and this process takes time. However, the MOS triode works differently and does not have this recovery time, so it can be used as a high-speed switch tube.

(1) Field effect tubes are voltage-controlled components, while transistors are current-controlled components. When only a small amount of current is allowed to be drawn from the signal source, field effect tubes should be used; when the signal voltage is low and more current is allowed to be drawn from the signal source, transistors should be used.

(2) Field effect tubes use majority carriers to conduct electricity, so they are called unipolar devices, while transistors have both majority carriers and minority carriers to conduct electricity, so they are called bipolar devices.

(3) The source and drain of some field effect transistors can be used interchangeably, and the gate voltage can be positive or negative, which makes them more flexible than transistors.

(4) Field effect transistors can operate under very small current and very low voltage conditions, and their manufacturing process can easily integrate many field effect transistors on a silicon wafer. Therefore, field effect transistors have been widely used in large-scale integrated circuits.

(5) Field effect transistors have advantages such as high input impedance and low noise, and are therefore widely used in various electronic devices. In particular, using field effect transistors as the input stage of the entire electronic device can achieve performance that is difficult to achieve with ordinary transistors.

(6) Field effect transistors are divided into two categories: junction type and insulated gate type, and their control principles are the same.