Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET)

Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is a widely used in analog circuit and digital circuit of field effect transistor ( field-effect transistor ). MOSFET in accordance with the" channel" of different polarity, can be divided into n-type and p-type MOSFET, are often called NMOSFET and PMOSFET, including NMOS FET, other abbreviation is PMOS FET, nMOSFET, pMOSFET etc..

The working principle of MOSFET

To make the enhanced N channel MOSFET, in G, S and positive voltage VGS and in D, S is added between the positive voltage VDS, it has a positive current ID. Change the VGS voltage controlled current ID.

If you don't pick up VGS (VGS = 0 ), in D and S between the pole and a positive voltage is VDS, the drain D and between the substrate and the PN junction in reverse, therefore the leakage source cannot be conductive. If the grid G and a source S and a voltage VGS. This can be the gate and the substrate as the two pole plates of the capacitor, and the oxide insulating layer as the capacitor dielectric. When coupled with VGS, the insulating layer and the gate interface induction positive charge, and the insulating layer and the P type substrate interface induces a negative charge ( Figure 3). The induction of negative charge and the P type substrate of majority carriers ( holes) of opposite polarity, so called" inversion", the inversion layer is likely to drain and source of two N area are connected together to form a conductive channel. When VGS when the voltage is too low, the induction of negative charge less, it will be the P type substrate of the holes in the neutralization, so in this case, the source of leakage between still no current ID. When VGS increases to a certain value, the induction of negative charge to two separate N communication form N channel, the threshold voltage is known as open voltage ( or threshold voltage, voltage threshold ), denoted by VT ( general provisions in the ID = 10uA VGS as VT ). When VGS increases, increased negative charges, the conductive channel is enlarged, resistance is reduced, ID also increases, and showed a good linear relationship, as shown in figure 4. This curve is called the conversion characteristics. Therefore, in a certain range can think, change the VGS to control the source of leakage resistance between, to control the ID effect.

Due to this structure, when VGS = 0, ID = 0, called the MOSFET for enhanced. Another class of MOSFET, when VGS = 0 also has a certain ID ( called IDSS), the MOSFET called depletion type. Its structure as shown in Figure 5, the transfer characteristics such as shown in figure 6. VP pinch-off voltage ( ID = 0 ).

The depletion and enhanced the main difference is in the manufacture of SiO2 insulation layer has a number of positive ions, so that the P type substrate interface on induction of more negative charge, i.e. in two type N area in the middle of P type silicon to form a N type silicon layer to form a conductive channel, so in VGS = 0, VDS effect also has some ID ( IDSS ); when the VGS voltage ( can be positive or negative voltages ), sensing changes in negative charge amount, thereby changing the size of the ID. VP ID = 0 - VGS, called pinch off voltage.