Power Management 101: Power MOSFETs

May 15, 2009 11:43 AM

What are Power MOSFETs ?

Power MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors) are three-terminal silicon devices that function by applying a signal to the gate that controls current conduction between source and drain. Their current conduction capabilities are up to several tens of amperes, with breakdown voltage ratings (BVDSS) of 10V to 1000V.

What type of power MOSFET is used in integrated circuits?

MOSFETs used in integrated circuits are lateral devices with gate, source and drain all on the top of the device, with current flow taking place in a path parallel to the surface. The Vertical Double diffused MOSFET (VDMOS) uses the device substrate as the drain terminal. MOSFETs used in integrated circuits exhibit a higher on-resistance than those of discrete MOSFETs.

What package styles are used for power MOSFETs?

MOSFETs are available in Small Outline IC (SOIC) packages for applications where space is at a premium. Larger through-hole TO-220, TO-247 and the surface mountable D2PAK or SMD-220 are also available. Newer package styles include chip scale devices and also the DirectFET™ and PolarPak™ packages.

What fabrication processes are used for power MOSFETs ?

The fabrication processes used to manufacture power MOSFETs are the same as those used in today's VLSI circuits, although the device geometry, voltage and current levels are significantly different. Discrete monolithic MOSFETs have tens or hundreds of thousands of individual cells paralleled together in order to reduce their on-resistance.

Is there an SiC power MOSFET?

Cree is the first to come up with a viable MOSFET. The ability to make these parts rests on the gate structure, which requires a physics and chemistry solution. The company still has some "tweaking" to do with the process, but they appear to be well ahead of the other companies that have ventured into this technology.

The commercial production of 1200 V SiC  power MOSFETs is now feasible because of recent advances in substrate quality, improvements in epitaxy, optimized device design, advances made in increasing channel mobility with nitridation annealing, and optimization of device fabrication processes. SiC is a better power semiconductor than silicon (Si) because SiC has a much higher electric field breakdown capability (almost 10x), higher thermal conductivity, and higher temperature operation capability (wide electronic band gap).

SiC excels over Si as a semiconductor material in 600V and higher rated breakdown voltage devices. SiC Schottky diodes at 600V and 1200V ratings are commercially available today and are already accepted as the best solution for efficiency improvement in boost converter topologies as well as in solar inverters by substituting them for the previously-used Si PiN free-wheeling diodes that have significant switching losses

The SiC MOSFET being discussed here is a 1200V, 20A device from Cree that has a 100mW RDS(on) at a +15V gate-source voltage. Besides the inherent reduction in on-resistance, SiC also offers a substantially reduced on-resistance variation over operating temperature. From 25°C to 150°C, SiC variations are in the range of 20% versus 200% to 300% for Si. The SiC MOSFET die is capable of operation at junction temperatures greater than 200°C but for this particular example it is limited by its TO-247 plastic package to 150°C.

How does a power MOSFET turn on?

The gate turns the MOSFET on when its gate-to-source voltage is above a specific threshold. Typical gate thresholds range from 1 to 4 V. When a positive bias greater than the gate-to-source threshold voltage (VGS(th) ) is applied to the gate, a current flows between source and drain. For gate voltages less than VGS(th) the device remains in the off-state.

Acceptable Use Policy blog comments powered by Disqus