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Home > Technology List > What is the difference between the Triode and FET

What is the difference between the Triode and FET

Published time: 2018-07-27

In the process of circuit design, many people are very distressed that what is the difference between them,when the triode and the FET are used as switches,? How should we use them reasonably? What is the correct choice when we use the FET and the transistor ? Jotrin Electronics limited will introduce to you the details of this project that what is the differencebetween the triode and FET.

First of all, considering their nature of work: the source S, the gate G, and the drain D of the FET correspond to the emitter E, the base B, and the collector C of the Triode, their functions are similar. 

The triode is a current-controlled device. The IB (or IE) controls the IC. The FET belongs to the voltage control device. The VGS controls the Id. The amplification factor gm is generally small, so the amplification effect of the FET is poor.

Considering the cost that they spend: the price of the triode is relatively cheap, and the FET is more expensive than it, so if the ordinary circuit can use the triode as much as possible,The triode cannot achieve the expected function, we consider to use FET.

Secondly, considering the problem of power consumption: the triode loss is larger than the FET, and the FET gate hardly takes current (Ig»0); while the triode is working, the base electrode always absorbs a certain amount of current. Therefore, the input resistance of the FET is higher than the input resistance of the transistor.

Finally, considering the perspective of driving capability: FETs are commonly used for power switches and high-current switching circuits. Triodes are commonly used in digital circuit switching control.

Jotrin Electronics thinks about that triodes are cheaper,they are easier to use.The triodes are commonly used in digital circuit switch control. FETs are used in high frequency, high speed circuits, high current applications, and the place where the base or drain control current is sensitive. 

Generally speaking, When we pay more attention to cost issues, we should give priority to the use of triodes, otherwise, FETs are good choice. in some cases, current control is slow and voltage control is fast.

Jotrin electronics’s engineers don't think so. We can only understand the specific characteristics of both sides when we really understand how to work the FETs and MOS triodes.The triode is operated by the movement of carriers. Taking an example of the NPN tube emitter follower, when the base is not applied with voltage, the PN junction composed of the base region and the emitter region blocks multiple sub-domains (the base region is a hole). 

The emission region is an electron diffusion motion, where the PN junction induces an electrostatic field (ie, a built-in electric field) directed from the emitter region to the base region. When the base is applied with a positive voltage, the base region points 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 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 (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 opposite direction of the electron moves. ), 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 go back to the emitter through the power supply, which is how the transistor works.

When the triode is working, both PN junctions will induce a charge. When the switch is in the on state, the triode is saturated. If the triode is intercepted, the charge induced by the PN junction should return to equilibrium. This process takes time. The FET works differently, and there is no such recovery time. It can be seen that the switching speed of the FET is much larger than that of the triode, so it can be used as a high-speed switching transistor.

We can summarize the advantages of FET

(1) The FET is a voltage control element, and the triode is a current control element. In the case where only a small current is allowed from the signal source, the FET should be selected;  In the case where the signal voltage is low and the large current is taken from the signal source, the triode should be selected.
(2) FETs use majority carriers to conduct electricity, so they are called unipolar devices. The triodes have multiple carriers and few carriers to participate in conduction. They are called bipolar devices. The minority concentration is greatly affected by temperature, radiation and other factors, so the FET has better temperature stability and strong radiation resistance than the triode. Field effect transistors should be used when environmental conditions (temperature, etc.) vary widely.
(3) The source and drain of some FETs can be used interchangeably and the characteristics change little, and the gate voltage can also be positive or negative. When the collector and emitter of the triode are used interchangeably, their characteristics are very different, and the b value is much reduced. Therefore, the flexibility of the FET is much better than the triode.
(4) The FET can work under the conditions of low current and low voltage.Futhermore, its manufacturing process is simple and easy to operate, we can integrate many FETs on a single piece of silicon conveniently, so FET tubes are widely used in large-scale integrated circuits.
(5) FETs have the advantages of high input impedance and low noise, Therefore FETs are
widely used in various electronic devices. In particular, the use of FETs as the input stage of the entire electronic device ,it can achieve performance that the triode is difficult to achieve this.
(6) The FET is divided into two types: junction type and insulated gate type, and their control principle is the same.

(7) Both FET and triode can form various amplifying circuits and open circuits. Because FETs have many advantages: simple manufacturing process, low power consumption, good thermal stability, wide operating voltage range, therefore FETs are widely used in large scale integrated circuits.

 

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