Advanced Process and Device 3D TCAD Simulation of Split-Gate Trench UMOSFET

Lower conduction loss and fast switching characteristics for power devices are increasingly required in the more and more energy-conscious world. For the low to medium voltage ranges (12 V ~ 250 V), the split gate structures [1] have become prevalent in the power MOSFET technologies [2-4]. They allow to achieve the best trade-off between the breakdown voltage (BV) and specific on-state resistance (RSP) for the vertical discrete power MOSFETs. Most of these solutions are based on the RESURF (Reduced Surface Field) action of Split-Gate Resurf Stepped Oxide (SG-RSO) along the drift region.

The conventional Trench MOSFETs usually exhibit restively large switching losses due to a relatively high gate-to-drain capacitance (Cgd), also expressed by the Miller charge Qgd. In standard Trench MOSFETs the trench gates are isolated from the drain region only by the thin gate-oxide (at the bottom of the trenches). This combined with high trench density leads to high switching losses [1]. In RSO MOSFETs, the gate extension is shielded from the drain region by a thicker shield-oxide, which is significantly thicker than the gate-oxide [1-4]. Hence a reduction of this gate-to-drain capacitance Cgd in comparison to conventional Trench MOSFETs. This improvement is however partially cancelled out by the increase of the gate-to-drain overlap due to the gate extension in the drift region [1].

A split-gate (SG) version of the RSO device was proposed in [1], in which the bottom part of the gate, called Field Plate or Split Gate, is isolated from the gate so that the upper part next to the channel (the actual gate) and the lower part next to the drift region (the field plate) are connected independently, the field plate being usually connected to the source (grounded). This results in a drastic decrease in the capacitance between the gate and the drain (Cgd) while still maintaining the RESURF effect induced by the field plate. Such approach allows excellent switching performance even at high trench gate density and at high voltage applications [1-4].

In summary, Split-Gate RSO MOSFETs combine low channel resistance (due to a moderate gate density) and ultra-low drift region resistance (due to the RESURF effect) with a significantly reduced gate-to-drain capacitance, which improves switching performance.