Difference between triode and FET

The three legs of BJT are named as base (B), collector and emitter (C).

The three pins of field effect transistor (FET) are named as grid, source and drain. Field effect transistors (FETs) can be divided into junction field effect transistors (JFET) and insulated gate field effect transistors (MOS). MOSFET can be divided into N-channel depletion type and enhanced type; There are four types of P-groove depletion type and enhanced type.

The collector and source are grounded, and the base and grid are control electrodes.

In the past, I always forgot these poles, so I asked about the working principles of the next two tubes.

1. The triode is current driven and needs to consume base current. Therefore, the amplification coefficient of the triode is obtained through Ic/Ib, which means that the amplification function of the triode is realized through the current of the base electrode.

2. FET is voltage driven, the grid is not conductive, no current passes through, and no current is consumed. It enables the effect tube to gather electrons through voltage to form an electronic channel, and then the drain and source are connected. Therefore, FET realizes amplification function by conducting electron tunnel. The higher the grid voltage is, the greater the conducting current is, but at the same time, the grid does not consume current.

Through the above comparison, the advantages and disadvantages of the FET are summarized:

The advantage is that the current consumption is relatively small, and the conduction speed is fast (as long as the voltage is added, the conduction is faster than that of the triode through the formation of current conduction).

The disadvantage is easy to be broken down by static electricity.

Application scenario

FETs are generally expensive. This technology is used in memory. For example, EPROM and FLASH use FETs to save data.

Operating principle of junction field effect transistor (N-channel JFET):

The N-channel JFET can be regarded as a faucet with "artificial intelligence switch". There are three parts: water inlet, AI switch and water outlet, which can be regarded as the d pole, g pole and s pole of JFET respectively.

"Manual" reflects the "control" function of the switch, namely vGS. When JFET is working, a negative voltage (vGS<0) shall be added between the grid and the source to reverse the bias of the PN junction between the grid and the channel. The grid current iG ≈ 0, and the FET presents an input resistance of more than 107 Ω. A positive voltage (vDS>0) is applied between the drain and the source to make most carriers (electrons) in the N-channel move from the source to the drain under the effect of electric field, forming a current iD. The size of the iD is controlled by the "manual switch" vGS. When vGS increases from zero to negative, the depletion layer of the PN junction will widen and the conductive channel will narrow. The greater the absolute value of vGS is, the closer the manual switch is to closing, and the outflow of water (iD) must be smaller and smaller. When you turn off the switch to a certain range, the water will not flow.

"Intelligence" reflects the "influence" of the switch. When the pressure difference (vDS) at both ends of the faucet is greater, the manual switch will automatically "grow" intelligently. The greater the vDS value is, the faster the manual switch grows. The closer the water channel is to close, the smaller the outflow (iD) must be. When the manual switch grows to a certain extent, the water will not flow. Theoretically, with the gradual increase of vDS, on the one hand, the channel electric field strength increases, which is conducive to the increase of drain current iD; On the other hand, with vDS, a potential gradient along the channel is generated in the N-type semiconductor region consisting of the source through the channel to the drain. Since the potential of the N-channel increases gradually from the source end to the drain end, the potential difference between the drain and the channel is not equal at different positions from the source end to the drain end. The farther away from the source, the greater the potential difference, and the greater the reverse voltage of the PN junction at this point. The depletion layer also expands toward the center of the N-type semiconductor, making the conductive channel near the drain narrower than the source, and the conductive channel is wedge-shaped. Therefore, it is figuratively compared to that when the pressure difference (vDS) at both ends of the faucet is greater, the manual switch will automatically and intelligently "grow".

When the switch collides for the first time, it is in the pre pinch off state. After the pre pinch off, the id tends to be saturated.

When vGS>0, the PN junction will be in a positive bias, resulting in a large grid current, which destroys its control over drain current iD. That is, pull out the manual switch and add an inlet pipe at the switch, which will have no control over the faucet.

Operating principle of insulated gate field-effect transistor (N-channel enhanced MOSFET):

The N-channel MOSFET can be regarded as a faucet with an "artificial intelligence switch". The corresponding situation is the same as JFET. What is different from JFET is that the manual switch of MOSFET is turned off at the beginning, and water cannot flow out. When vGS>0 is added between gate sources and N-type induced channel (inversion layer) is generated, the manual switch is gradually turned on, and the water flow (iD) becomes larger and larger. The size of iD is controlled by the "manual switch" vGS. When vGS increases from zero to the positive direction, the gate and P-type silicon chip are equivalent to a flat capacitor with silicon dioxide as the dielectric. Under the action of positive gate source voltage, an electric field perpendicular to the semiconductor surface from the gate to the P-type substrate is generated in the medium, which repels holes and attracts electrons. The minority electrons in the P-type substrate are attracted to the substrate surface, These electrons form an N-type thin layer on the P-type silicon surface near the gate, that is, the N-type conductive channel between the conductive source and drain. The greater the gate source voltage vGS, the stronger the electric field on the semiconductor surface, the more electrons attracted to the P-type silicon surface, the thicker the induced channel, and the smaller the channel resistance. It is equivalent to that the closer the manual switch is turned on, the more water (iD) will flow out. When you turn the switch on to a certain extent, the water flow will reach the maximum. The "intelligence" of MOSFET is the same as that of JFET.

Operating principle of insulated gate field-effect transistor (N-channel depleted MOSFET):

Basically the same as N-channel JFET, except that when vGS>0, N-channel depletion MOSFET will not generate the positive current of the PN junction due to the existence of the insulation layer, but will induce more negative charges in the channel, making the control effect of the AI switch more obvious.