Operating principle of a MOSFET
Operating principle of a MOSFET:
- It would appear that there is no path for any current to flow between the source and the drain terminals since at least one of the p n junctions (source – body and body-Drain) will be reverse biased for either polarity of the applied voltage between the source and the drain.
- There is no possibility of current injection from the gate terminal either since the gate oxide is a very good insulator. However, application of a positive voltage at the gate terminal with respect to the source will convert the silicon surface beneath the gate oxide into an n type layer or “channel”, thus connecting the Source to the Drain as explained next.
- The gate region of a MOSFET which is composed of the gate metallization, the gate (silicon) oxide layer and the p-body silicon forms a high quality capacitor. When a small voltage is application to this capacitor structure with gate terminal positive with respect to the source (note that body and source are shorted) a depletion region forms at the interface between the SiO2 and the silicon.
- The positive charge induced on the gate metallization repels the majority hole carriers from the interface region between the gate oxide and the p type body. This exposes the negatively charged acceptors and a depletion region is created.
- Further increase in VGS causes the depletion layer to grow in thickness. At the same time the electric field at the oxide-silicon interface gets larger and begins to attract free electrons as shown in Fig 6.4 (b). The immediate source of electron is electron-hole generation by thermal ionization. The holes are repelled into the semiconductor bulk ahead of the depletion region. The extra holes are neutralized by electrons from the source.
- As VGS increases further the density of free electrons at the interface becomes equal to the free hole density in the bulk of the body region beyond the depletion layer. The layer of free electrons at the interface is called the inversion layer.
- The inversion layer has all the properties of an n type semiconductor and is a conductive path or “channel” between the drain and the source which permits flow of current between the drain and the source. Since current conduction in this device takes place through an n- type “channel” created by the electric field due to gate source voltage it is called “Enhancement type n-channel MOSFET”.
- The value of VGS at which the inversion layer is considered to have formed is called the “Gate – Source threshold voltage VGS (th)”. As VGS is increased beyond VGS(th) the inversion layer gets somewhat thicker and more conductive, since the density of free electrons increases further with increase in VGS. The inversion layer screens the depletion layer adjacent to it from increasing VGS. The depletion layer thickness now remains constant.