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Posted on 12 April 2020

Development of MOSFET Technologies

 

 

 

 

 

 

 

Power electronics largely use the vertical structure, where gate and source terminals are located on the chip surface and the drain terminal is on the underside of the chip. The load current is conducted vertically through the chip outside the channel. The VDMOSFET version ( Vertical Double Diffused MOSFET) introduced at the beginning of the eighties is still being used today and is being continuously improved on, e.g. reduction in cell dimensions. Depending on the application focus on "low" or "high" drain-source voltage, the key developments on the power MOSFET front have gone in two directions with significant structural differences:

Trench-Gate MOSFET

The example shown in Figure 1 illustrates the continuous development towards the Trench MOSFET, which was introduced around 1997.

Trench MOSFET development

Figure 1. a) conventional VDMOSFET; b) MOSFET with trench-gate (Trench MOSFET)

Similar to the development of the Trench IGBT, the insulated gate plates - and thus the channel area - are arranged vertically here; the distance to be covered by the electrons in the n-region is thus shorter. This enables a significant reduction in RDS(on) mainly in the lower voltage range compared to conventional structures.

Superjunction MOSFET

The compensation principle used in superjunction components was developed for MOSFETs with blocking voltages between 500 V and 1000 V.

Layout and functional principle of a superjunction MOSFET

Figure 2. Layout and functional principle of a superjunction MOSFET (CoolMOS)

With the aid of several epitaxy steps or lateral diffusion from trenches, highly doped conducting columns are injected into the low doped n- drift area. These columns are connected with the p-wells. Column doping is dimensioned such that the n-doping of the drift area is compensated for, resulting in a very low effective doping level.

In blocking state, the field is almost rectangular and can take up the maximum voltage level in relation to the thickness of the n- region. Drift area doping can only be increased to such an extent that still allows for it to be compensated by the same amount of doping in the p-column ("compensation principle"). This overrides the interdependency of blocking voltage and doping density.

As a result, the thickness of the n- drift area can be reduced substantially compared to conventional MOSFETs, and drift area conductivity can be increased by way of higher doping. This means that the on resistance RDS(on) will no longer increase to the power of 2.4...2.6, but almost linear to the breakdown voltage V(BR)DSS. The on-state losses of high-blocking power MOSFETs can be reduced significantly (to 1/3 - 1/5). Chip area, switching losses, and gate capacitance / gate charge drop accordingly for the same current capability.

Insulated field plates

In order to be able to transfer the superjunction principle to MOSFETs for low voltages, technologies had to be developed that were much simpler and cheaper than those dealing with high volt components. OptiMOS transistors by Infineon have insulated field plates rather than p-columns built up in epitaxy processes. These field plates are arranged in trenches which are etched into the n- drift area, insulated by a layer of silicon oxide and alternately connected to the source region and the polysilicon gate.

Principle layout of an OptiMOS

Figure 3. Principle layout of an OptiMOS

Figure 3 shows the compensation effect of the field plates and the field intensity characteristics in y-direction compared to a conventional type blocked pn-junction.The p-charge on the field plates compensates the doping of the n- region so that it can be increased as described above. In off-state, the triangular field shape of the simple structure becomes almost rectangular, permitting the reduction of the n- layer thickness. Higher doping and reduced n- drift area thickness brings about the advantages that superjunction MOSFET have over conventional structures.

Field distribution in a conventional pn-junction and a pn-junction with field plates

Figure 4. Field distribution in a conventional pn-junction and a pn-junction with field plates

 

For more information, please read:

Introduction to Metal Oxide Semiconductor Field Effect Transistors (MOSFETs)

Criteria for Successful Selection of IGBT and MOSFET Modules

 

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