4H-Silicon Carbide for High Temperature Power MOSFETs

THE RAPID DEVELOPMENT OF THE TECHNOLOGY FOR PRODUCING HIGH QUALITY SINGLE CRYSTAL SiC WAFERS AND THIN FILMS PRESENTS THE OPPORTUNITY TO FABRICATE SOLID-STATE DEVICES WITH POWER-TEMPERATURE CAPABILITY FAR GREATER THAN DEVICES CURRENTLY AVAILABLE. THIS CAPABILITY IS IDEALLY SUITED TO THE APPLICATION OF POWER CONDITIONING ON THE ALL-ELECTRIC AIRPLANE, TURBINE ENGINE ACTUATORS, AND SPACE-BASED POWER SYSTEMS. THESE APPLICATIONS REQUIRE SWITCHES AND AMPLIFIERS CAPABLE OF LARGE CURRENTS WITH RELATIVELY LOW VOLTAGE DROPS. WHILE CONVENTIONAL SILICON POWER DEVICES ARE ALREADY BEING USED NEAR THEIR LIMITS OF OPERATING TEMPERATURE AND POWER, THE POTENTIAL OF SiC IS JUST BEGINNING TO BE DEMONSTRATED. VERTICAL POWER MOSFET STRUCTURES FABRICATED IN 6H-SiC WITH 50 V CAPABILITY HAVE ALREADY SHOWN CURRENT AND POWER DENSITIES AS HIGH AS 191 A/CM2 AND 5.4 KW/CM2. THE SPECIFIC ON-RESISTANCE (R Ds(on)) OF THESE DEVICES WERE AS LOW AS 37.5 M -CM2. THESE DEVICES SHOWED GOOD CHARACTERISTICS AT TEMPERATURES UP TO 300 0C. WHILE THESE ARE PROMISING RESULTS, IT IS PREDICTED THAT MUCH BETTER RESULTS COULD BE ACHIEVED IF THESE DEVICES WERE FABRICATED IN 4H-SiC. THE 4H POLYTYPE OF SiC HAS BEEN SHOWN TO HAVE AN ELECTRON MOBILITY ALMOST TWICE THAT OF 6H-SiC, WHICH WOULD GREATLY REDUCE THE R ds(on) OF A SiC POWER DEVICE. ADDITIONALLY, THE WIDER BANDGAP (3.26 eV) OF 4H WILL RESULT IN AN EVEN HIGHER BREAKDOWN FIELD THAN 6H-SiC, WHICH IN TURN ALLOWS ANOTHER REDUCTION IN R ds(on). THEREFORE, IT IS PROPOSED THAT THE USE OF 4H-SiC BE EVALUATED FOR A HIGH TEMPERATURE POWER MOSFET. THIS EFFORT WILL INCLUDE THE CHARACTERIZATION OF THE ELECTRON MOBILITY OF 4H-SiC, PARALLEL TO THE C-AXIS, USING HALL EFFECT MEASUREMENTS. PLANER MOSFETS WILL ALSO BE FABRICATED IN 4H-SiC IN ORDER TO DETERMINE THE CHANNEL MOBILITY OF THESE DEVICES. ADDITIONALLY, ONE BATCH OF VERTICAL POWER MOSFETS WILL BE FABRICATED TO PROVE THE VIABILITY OF THE STRUCTURE. BOTH THE PLANAR AND VERTICAL DEVICES WILL ALSO BE MEASURED AT HIGH TEMPERATURE.