MOSFET Performance Improvement: Decision Factors of RDS(ON)
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(1) The MOSFET device structure is selected according to the required voltage rating(VDSS). For example, many middle and high voltage MOSFETs (VDSS=250V or higher) have planar gate MOS(π-MOS) structure, and products with less than VDSS=200V have more trench gate MOS(U-MOS).
(2) The factor that determine the on-resistance RDS(ON) are shown in Fig.3-7 and Equation 3-(1). Depending on the structure and VDSS of MOSFET, the ratio of factors determining RDS(ON) will change. When the π-MOS structure is chosen, Rdrift becomes the dominant factor in VDSS=600V products, while the ratio of Rch+RJ-FET is high in VDSS=30V products. However by applying U-MOS structure, Rch will be very small.
RDS(ON)=Rsub+Rdrift+RJ-FET+Rch+RN+ --- Equ.3-(1)a
RDS(ON)= Rsub+Rdrift+Rch+RN+ --- Equ.3-(1)b
@ VDSS=600 V : Rdrift >> Rch > RJ-FET , RN+ , Rsub → RDS(ON) depends on Rdrift
@ VDSS= 30V : ●π-MOS --- Rch + RJ-FET > Rdrift >> RN+ , Rsub. ●U-MOS --- Rch≈ Rdrift >> Rsub, RN+
→ Dependence of RDS(ON) on Rch can be minimized by fine patterning of U-MOS structure.
Chapter III : Transistors
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