The difference between triode and MOSFET tube

The structure of a triode, also known as a semiconductor triode or bipolar transistor, is mainly composed of three regions: the emitter region, the base region, and the collector region. Its working principle is based on current control, that is, changes in the base current can control the size of the current between the collector and the emitter. This makes the transistor widely used in weak signal amplification and as a non-contact switch.

NPN type

The triode in the picture above is an NPN type, consisting of two PN junctions. When the B pole voltage is 0.7V higher than the E pole voltage, the lower PN junction is turned on, and the emitter will continuously send electrons to it. base area. When the transistor is in the amplified state, that is, when the collector voltage is relatively high, a large number of electrons are sent directly to the collector. The current flowing to the collector and the current flowing to the base will maintain a proportional relationship, β, which is the amplification factor we often mention.

what are the problems with this triode?

For example, under saturation, even under deep saturation, we say that Uce can reach 0.3V, but this voltage is still relatively high. Small power and small current do not matter. If the current is relatively large, such as 10A current, then the transistor will generate 3W. of losses. Therefore, transistors are not suitable for high-power scenarios. How to solve this problem?

MOSFET type

The MOSFET in the picture above is very similar to an NPN transistor. It is similar to creating an N region on the base of the original NPN transistor. Then this will connect the D to S poles because they are all N areas. If a channel is formed in this way, it can also be considered that the internal resistance from D to S becomes very, very small. Once the internal resistance is reduced, U=I*R, R is very small. Even if the current I is large, the Uds voltage drop will be very small. It will not be very large, so the loss of the MOS tube P=U*I will not be very large. This internal resistance is the conduction resistance of the MOS tube, generally expressed as Rdson, which is the resistance between DS during the turn-on time Ton. Rdson determines the power of the entire MOS.

This N area growing out of the P area is called a channel and connects the two N areas D and S. So how did this channel come about?

We have to look at the structure of the MOS tube.

N-type MOS tube

The picture above is the structural diagram of an N-type MOS tube. It uses a low-doped P-type silicon wafer as the base, which is the substrate.

Then it makes two highly doped N regions on it, namely D, and The S area is covered with a layer of SiO2 insulating layer.

Finally, a layer of metal aluminum is added, which leads to four polarities, D, S, G, and B, which are often called the drain D and the source S.

The gate G and the B electrode are derived from the P-type substrate and are directly short-circuited with the S electrode, which makes the S electrode and the P region equal potential.

Let’s continue to look at it. There is an insulating layer between the gate G and the substrate P. The aluminum of the gate and the P-type substrate are like two plates, so does this look like a capacitor?

So how does this MOS tube work?

Similar to a triode, when we add a forward bias voltage UGS between the G and S poles, the capacitor-like place just mentioned will start to charge, that is, electrons will continue to concentrate near the insulating layer. The more electrons gather, and the concentration is even the same as that of the N region, the more an N-type region will be established under the insulating layer, which is what we call the channel. This channel is N-type, and from D to S is N-type, so DS will be connected, so that current can flow through the channel.

In terms of driving mode, the triode is a current driving device, and its base current determines the current between the collector and emitter. MOSFET is a voltage-driven device, and its gate voltage determines the conductivity of the channel. This difference in driving methods also leads to differences in practical applications. For example, in switching applications that control larger currents, because the equivalent resistance of the MOSFET is very small when it is turned on and the voltage drop it produces is also very small, it is more suitable for this application.

In general, there are differences between transistors and MOSFETs in terms of structure, working principle, driving method, and application scenarios. The choice of which device to use depends primarily on the specific design needs and application environment.

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