How does the MOS field effect transistor work?

Last Update Time: 2018-12-14 11:19:54

MOS field-effect transistor is metal oxide semiconductor field effect transistor. It is a field effect device which forms channel conduction by using semiconductor surface layer.


(1) The depletion type MOS tube is not pure because the structure of the silicon dioxide film under the gate is not pure. It often has some impurities or defects with positive charge, and the positive charge itself will produce the fixed electric field, which affects the silicon surface under the oxide film. If the positive charge in the oxide film is much more, or the doping depth of the P type silicon is very low, even if there is no positive voltage on the gate, the positive brief in the oxide film can also attract enough electrons and produce a reverse layer on the surface of the silicon wafer, so that there is a conducting channel between the source and the drain. This tube is called a depleted MOS tube.

The relationship between the leakage current Io and the gate source power Vgs is usually works with the negative voltage, and the conduction performance of the channel is gradually weakened after the negative voltage is added, and the electrical conductivity will disappear after the negative voltage is large to a certain extent.


(2) The enhanced MOS tube N channel MOS tube is based on P silicon as the bottom of the village and diffuses two N+ regions on the surface, respectively, as the source and drain. There are thin silicon dioxide films on the silicon surface between the two diffusers, and the metal electrodes on them are grid. In this structure, the N source diffusion zone and the leaky diffusion zone are separated from the P zone, so it seems to be two "back-to-back" connected diodes, and if the voltage is added between the source and the drain, there will be no current flow (only a very small PN junction reverse current), which is in the absence of a voltage added to the gate. When the gate is loaded with a positive voltage (to the source), an electric field is generated under the grid to attract electrons from the P silicon to the surface, so that a thin layer of electrons is formed on the surface of the silicon under the gate, that is, a conductive reverse layer is formed on the surface of the P type silicon. This reverse layer becomes a conducting channel. When the positive gate voltage increases, the electrons that are absorbed into the trans layer increase, the resistance of the conducting channel decreases and the current passing through the channel increases; on the contrary, the current decreases.

 When VcsV+, the number of Vcs increases with the increase of Vcs. Vr is called the valve threshold voltage of the tube. When the pipe is at Vcs=0, the source and leakage does not conduct electricity. Only when the Vcs is larger than the positive open voltage V7, the reverse layer is formed under the gate to have the conductivity, called an enhanced MOS tube.