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(1) The MOSFET device structure is selected according to the required withstand voltage. The factors that determine the on-resistance RDS (ON) are as shown in Figure 3-7 and Equation 3- (1). Depending on the structure of the device, the ratio of factors determining on-resistance will change.
(2) For example, many middle- and high-voltage MOSFETs (250 V or higher) have planar MOS (π-MOS) structure, and products with less than 200 V have more trench MOS (U-MOS). Therefore, when the withstand voltage VDSS = 600 V, Rdrift becomes the dominant factor, and in the case of 30 V, the ratio of Rch is high.
RDS(ON）= Rsub + Rdrift + RJ-FET + Rch + RN+
RDS(ON）= Rsub + Rdrift + Rch + RN+ ・・・ Equation 3-(1)
In the case of VDSS=600 V, the order is Rdrift >> Rch > RJ-FET , RN+ , Rsub and RDS(ON) depends on Rdrift
In the case of VDSS=30V, the order is Rch >> Rdrift > RN+ , Rsub. Dependence of RDS(ON) on Rch can be minimized by fine patterning of trench MOS structure.