Mesh overlay technology boosts MOSFET performance

SGS-THOMSON Microelectronics is now delivering the first MOSFETs,made with Mesh Overlay technology, a revolutionary new power MOSFET technology that yields higher packing densities, higher reliability, dramatic reductions in gate charge and capacitance and the industry's lowest RDS(on)/Area figure. The STW15NB50 and STW20NB50 are high voltage devices with 500V breakdown voltages and maximum current ratings of 15A and 20A, respectively, and also feature an extremely high dV/dt capability of 4V/ns, making them ideal for use in power supplies, motor drives, lamp ballasts and similar applications.

Said to be the most importance advance in MOSFET technology for twenty years, SGS-THOMSON's mesh overlay approach replaces the traditional cellular layout by a new approach in which a mesh of P-type material is diffused into the surface of an N-type substrate, effectively creating an array of thin, rectangular N-type islands in a sea of P-type material. The N-type strips are typically 100 microns long but only 10 microns wide. This 10:1 aspect ratio means that mask alignment is now ten times less critical along the Y axis than along the X axis, a major manufacturing benefit compared to cellular layouts. The effect on die size is also dramatic: with conventional cellular layouts, there is little scope for reducing the die area required to achieve a given combination of on-resistance and breakdown voltage and the cumulative reduction that has been achieved since the introduction of 500V MOSFETs is 20%. The mesh scheme allows another 20% reduction to be obtained immediately.

The mesh geometry also brings other important benefits: gate charge is halved as a result of the reduced gate/drain overlap, leading to faster switching speeds, and potential gradients at cell edges are reduced. This allows higher doping levels to be safely used, leading to significant improvements in resistivity for a given breakdown voltage.

The new devices have a typical switching rise time of only 14ns, compared to 20ns for the STW15NA50 made with SGS-THOMSON's ultrafast UHD cellular technology and 25ns for an industry standard IRFP450. Fall time for the new STW15NB50 is just 30ns, against 80ns for the industry standard part.

The STW15NB50 and STW20NB50 offer typical RDS(on) values of 0.33 and 0.22 Ohms, respectively, coupled with exceptional and dV/dt capabilities. Maximum avalanche current (IAR) is 14.6A for the STW15NB50 and 20A for the STW20NB50, at 25oC and is also specified at 100oC (9.2A and 12.7A, respectively). The superior dV/dt ruggedness ensures safe operation of the internal diode in bridge topologies.

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