Circuit models of a MOSFET cell
Circuit Models of a MOSFET Cell:
- Like any other power semiconductor device a MOSFET is used as a switch in all power electronic converters. As a switch a MOSFET operates either in the cut off mode (switch off) or in the ohmic mode (switch on).
- While making transition between these two states it traverses through the active region. Being a majority carrier device the switching process in a MOSFET does not involve any inherent delay due to redistribution of minority charge carriers.
- However, formation of the conducting channel in a MOSFET and its disappearance require charging and discharging of the gate-source capacitance which contributes to the switching times.
- There are several other capacitors in a MOSFET structure which are also involved in the switching process. Unlike bipolar devices, however, these switching times can be controlled completely by the gate drive circuit design.
- The most prominent capacitor in a MOSFET structure is formed by the gate oxide layer between the gate metallization and the n type source region. It has the largest value (a few nano farads) and remains more or less constant for all values of vGS and vDS. The next largest capacitor (a few hundred pico forwards) is formed by the drain – body depletion region directly below the gate metallization in the n- drain drift region.
- Being a depletion layer capacitance its value is a strong function of the drain source voltage vDS. For low values of vDS (vDS < (vGS – vGS (th))) the value of CGD (CGD2) is considerably higher than its value for large vDS as shown in Fig (b). Although variation of CGD between CGD1 and CGD2 is continuous a step change in the value of CGD at vDS = vGS – vGS(th) is assumed for simplicity.
- The lowest value capacitance is formed between the drain and the source terminals due to the drain – body depletion layer away from the gate metallization and below the source metallization.
- Although this capacitance is important for some design considerations (such as snubber design, zero voltage switching etc) it does not appreciably affect the “hard switching” performance of a MOSFET.