Field Effect Transistor (FET) stands for Field Effect Transistor. A typical transistor is called a bipolar transistor due to the participation of two types of polar carriers, namely the majority carrier and the reverse polarity minority carrier, in conduction. FET, on the other hand, is only composed of the majority carrier, which is the opposite of bipolar transistors and is also known as a unipolar transistor.
1. MOS field-effect transistor
MOS field-effect transistors are divided into N-channel transistors and P-channel transistors. Usually, the substrate is connected to the source electrode S. As mentioned before, MOSFETs are divided into enhanced and depleted types based on their different conductive methods. The so-called enhanced type refers to: when VGS=0, the tube is in a cut-off state, and after adding the correct VGS, most carriers are attracted to the gate, thereby “enhancing” the carriers in that region and forming conductive channels. The depletion type refers to the formation of a channel when VGS=0, and when the correct VGS is added, it can allow most carriers to flow out of the channel, thus “depleting” the carriers and causing the tube to turn towards cutoff.
2. The working principle of field-effect transistors
Field effect transistors are mainly composed of three regions: gate, drain, and source. Under normal operating conditions, the gate voltage controls the current between the source and drain, thereby achieving control over the field-effect transistor.
When the gate voltage is below the threshold voltage, there is no conductive channel between the gate and the channel, and electrons cannot flow from the drain to the source. The field-effect transistor is in a cut-off state, acting as an insulator.
When the gate voltage is higher than the threshold voltage, a conductive channel is formed between the gate and the channel, and electrons can flow from the drain to the source along the channel. At this point, the carrier concentration near the source increases, forming a conductive region called an inversion layer (N-type or P-type).
As the source voltage further increases, the width of the inversion layer will increase, and the electron concentration in the channel will also increase. In this way, more electrons can flow from the drain to the source, forming conductive channels. When the source voltage reaches a certain value, the electron concentration in the channel is large enough to make the entire channel conductive.
When the gate voltage is 0V, due to the disappearance of the insulation layer between the gate and the channel, electrons in the channel can freely flow between the drain and source. This means that the field-effect transistor is in a conductive state.
Taking the N-channel as an example, it creates two high doping concentration source diffusion regions N+and drain diffusion regions N+on a P-type silicon substrate, and then leads out the source electrode S and drain electrode D respectively. The source electrode and substrate are connected internally, and both maintain an equal potential. The leading direction in the symbol is from the outside, indicating an N-shaped channel from the P-type material (substrate). When the positive pole of the power supply is missed and the source pole is connected to the negative pole of the power supply and VGS=0, the channel current (i.e. drain current) ID=0. As VGS gradually increases and is attracted by the positive gate voltage, a few negatively charged carriers are induced between the two diffusion regions, forming an N-type channel from the drain to the source. When VGS is greater than the opening voltage VTN of the transistor (usually about+2V), the N-channel transistor begins to conduct, forming a drain current ID.
3. The role of field-effect transistors
1. Field effect transistors can be applied for amplification. Due to the high input impedance of field-effect transistor amplifiers, the coupling capacitor can have a smaller capacity and does not require the use of electrolytic capacitors.
2. The high input impedance of field-effect transistors is very suitable for impedance transformation. Commonly used for impedance transformation in the input stage of multi-stage amplifiers.
3. Field effect transistors can be used as variable resistors.
4. Field effect transistors can be conveniently used as constant current sources.
5. Field effect transistors can be used as electronic switches.
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