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    The working principle of N-channel enhancement mode MOS tube

    The working principle of N-channel enhancement mode MOS tube


            (1) The control effect of vGS on iD and channel


            ① The case of vGS=0


           There are two back-to-back PN junctions between the drain d and the source s of the enhanced MOS tube. When the gate-source voltage vGS=0, even if the drain-source voltage vDS is added, and regardless of the polarity of vDS, there is always a PN junction in the reverse bias state, and there is no conductive channel between the drain and the source. So the drain current iD≈0 at this time.


            ② When vGS>0


            If vGS>0, an electric field is generated in the SiO2 insulating layer between the gate and the substrate. The direction of the electric field is perpendicular to the electric field directed from the gate to the substrate on the semiconductor surface. This electric field can repel holes and attract electrons.


    Repelling holes: the holes in the P-type substrate near the gate are repelled, leaving immobile acceptor ions (negative ions) to form a depletion layer. Attracting electrons: The electrons (minority carriers) in the P-type substrate are attracted to the surface of the substrate.


            (2) The formation of conductive channel:


            When the value of vGS is small and the ability to attract electrons is not strong, there is still no conductive channel between the drain and the source, as shown in Figure 1(b). When vGS increases, the electrons attracted to the surface layer of the P substrate increase. When vGS reaches a certain value, these electrons form an N-type thin layer on the surface of the P substrate near the gate, which is connected to two N+ regions Through, an N-type conductive channel is formed between the drain and the source. Its conductivity type is opposite to that of the P substrate, so it is also called the inversion layer. The larger the vGS, the stronger the electric field acting on the semiconductor surface, which attracts P The more electrons there are on the surface of the substrate, the thicker the conductive channel and the lower the channel resistance.


        The gate-source voltage at the beginning of the formation of the channel is called the turn-on voltage and is represented by VT.




            The N-channel MOS tube discussed above cannot form a conductive channel when vGS<VT, and the tube is in an off state. Only when vGS≥VT, the channel is formed. This kind of MOS tube that can form a conductive channel when vGS≥VT is called an enhanced MOS tube. After the channel is formed, a forward voltage vDS is added between the drain and the source to generate a drain current.


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