In a similar way to the JFET transistors, it can be used to identify, by graphical methods, the bias point of the transistors. In this region, the quadratic relationship between VGS and ID is shown in the left part of the picture. The NMOSFET transistor behaves as a voltage controlled current source VGS. Is a characteristic parameter of the MOS transistor, which depends on the k constant and the size of the transistor gate (width W and length L). The transistor behaves as a nonlinear resistive element, controlled by voltage. We can verify that VGS < VT and the current ID is zero. These regions of operation are briefly described below. The image shows the curves of electrical characteristics of an NMOS transistor with the different regions of operation. In the MOSFET transistors, there are defined the same regions of operation: cutoff, linear, saturation and breakdown. JFET and MOSFET transistors have a very different physical structure, but their analytical equations are very similar. Typical values for this voltage are between 0.5 and 3 volts. If VGS < VT, the drain-source current is zero. This is a characteristic feature of the transistor. ![]() The minimum voltage needed to create the inversion layer is called threshold voltage (VT). ![]() If a positive voltage is applied to the gate, negative charges are induced (inversion layer) on the substrate surface and they create a conduction path between the Drain and Source terminals. The Gate with W and L dimensions is separated from the substrate by a dielectric (SiO 2), creating a similar structure of the capacitor plates. Normally the Source and the substrate are connected together. The next image shows the N channel MOSFET transistor physical structure with its four terminals: Gate, Drain, Source and Substrate. MOSFET transistors (NMOS) physical structure
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