Finally, gate metal area is overlaid the oxide layer covering the entire channel. Similar to enhancement MOSFET, a thin layer of SiO 2 layer if formed allowing contacts with the source and the drain. An n-channel is diffused between the source and the drain. If consists of a lightly doped p-type substrate into which two highly doped n+ regions are diffused forming the source and the drain. For a p-channel MOSFET, typical value of V t is -4 volts and the corresponding value of drain supply voltage V DD is -12 volts.įigure 4 shows the basic structure of n-channel depletion MOSFET. Gate-source Threshold Voltage V GST or V T It is the value of V GS at which drain current |I D| reaches some prescribed small value typically chosen as 10 uA. Manufacturer’s data gives I D,ON and also the voltage V GS needed to get I D,ON. But as V GS is made more and more negative, the magnitude of drain current first increases slowly and then relatively fast with increases of |V GS| beyond a few volts.Ĭurrent I D,ON This is the maximum permitted drain current. Transfer characteristic of figure 3 shows that the saturation current is extremely small, of the order of a few nano-amperes for V GS = 0 or positive. Figure 2 shows that the drain characteristic of enhancement MOSFET are similar to those of junction FET. Figure 3 gives a transfer characteristic for V DS = -15 volts for the same device. Static Characteristic Curves | MOSFETįigure 2 gives the static drain current versus drain voltage characteristic of a typical p-channel enhancement MOSFET, using V GS as the parameter. Thus, increased negative voltage at the gate increases or enhances the drain current and this MOSFET is, therefore, called enhancement MOSFET. This in turn increases the conductivity of the region just below SiO 2 layer and increases the current from source to drain. These positive charges from minority carrier in the n-type substrate and result in an inversion layer in the region between source and drain.Īs the magnitude of the negative voltage at the gate is increased, the concentration of induced positive charges in the semiconductor increases. This electric field terminates on induced positive charges in the semiconductor material between source and drain at the sites shown in figure 1. This results in an electric field normal to the SiO 2 layer. Now let us ground the substrate and apply a negative voltage at the gate. The SiO 2 layer results in an extremely high input impedance of the MOSFET lying in the range 10 10 to 10 15 ohms. Hence, the device is called insulated gate FET. The gate Layer has been insulated from the rest of the device by means of SiO 2 layer. The metallic gate layer and the semiconductor channel from a parallel plate capacitor with SiO 2 as the dielectric in between. Junction Field Effect Transistor (JFET) | Working.FET Parameter | Small Signal Models for FET.You can checkout other article related to MOSFET This area is about 5% of that required for BJT. The chip area is extremely small, being only about 5 sq. Metal contacts are made to the source, drain and get regions as shown in figure 1. Subsequently aluminum layer is formed over the entire SiO 2 layer covering the entire channel from source to drain. Holes are cut into the SiO 2 layer and aluminum contacts are made to p+ source and drain regions. A thin insulating layer of SiO 2 of thickness 1000 to 2000 A is grown over the entire surface of the device. One region, say the left-hand region, acts as the source while the other region acts as the drain. It consists of a lightly doped n-type substrate into which all diffused two highly dipped p + region spaced 10 to 20um apart. Figure 1 gives the cross-sectional view of a p-channel enhancement MOSFET.
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