Cree SiC power MOSFETs enable higher blocking voltages in boost, high-frequency DC/DC conversion circuits for reduced system size, weight and cost through increased efficiency. They operate at switching frequencies three times faster than silicon devices for fast switching action.
Guy Moxey, Senior Director of Power products at Wolfspeed, A Cree Company, discusses UHAST results for SiC MOSFETs.
Ultra-Low On-State Resistance
SiC mosfets offer lower resistance than silicon devices and faster switching speeds, which enable larger power densities and improved efficiency for high-voltage applications. Design engineers can use these compact power inverters to minimize system costs.
As voltage ratings increase, critical breakdown fields (Vbf) of devices tend to diminish and their on-state resistance (Rds(on)) begins to drop due to charge injection and accumulation in gate oxide during surge current stress. This phenomenon is called charge injection/accumulation in gate oxide.
Due to this avalanche stress, cracks form in the gate dielectric layer, allowing aluminium source metals to diffuse into these cracks and short out the device.
These cracks lead to the degradation of body diodes, leading to reduced Vth of SiC MOSFETs as junction temperatures rise, as well as depletion of their buried channel regions.
Cracks in SiC MOSFETs can be avoided through higher maximum gate drive voltage and reduced parasitic inductances, but their reliability in harsh environments requires advanced packaging and interconnect technologies to improve thermal conditions, lower parasitic parameters and eliminate failure modes that impact device performance – for instance long cables between device terminals generate parasitic inductances that cause transient overvoltages that damage devices; they may even cause overcurrent flow through body diodes which ultimately leads to their breakdown and failure modes affecting performance of device.
Ultra-Low Leakage Current
SiC’s wider bandgap allows it to operate at higher currents with thinner materials than other semiconductor devices, reducing on-state resistance significantly and making its switching performance superior. This combination makes SiC an excellent choice for power electronics applications requiring high current/voltage while being cost effective per amp.
One major reason is that P atoms added during post-oxidation have improved the interface between SiC and gate oxide, creating a barrier against Fowler-Nordheim tunnelling charges from SiC silicon conduction or valence bands into oxide channels, leading to significantly less gate leakage current compared to dry and NO-annealed SiC MOSFETs.
SiC MOSFETs also boast more resilient drain-source PN junctions compared to Si IGBTs when exposed to short circuit stress; indeed, one 1200 V/12.5 A planar gate MOSFET can withstand up to five times more avalanche energy compared to one 600V/16A Si IGBT while having approximately five times smaller die sizes.
At Cree, we understand that optimizing any electronic component means taking full advantage of its potential. That is why our SiC power devices come equipped with tools designed to assist customers in designing safe and reliable systems that use them.
Ultra-Low Switching Losses
As SiC MOSFET switching losses decrease, they will become viable replacements for silicon devices in applications where high blocking voltage is necessary, leading to reduced component count, smaller package size and reduced system cost. Furthermore, higher switching frequencies enable more efficient power conversion and enhanced machine control performance for applications such as DC-DC converters, PFC and boost architectures, industrial motor drives and on-board chargers for electric vehicles (EV).
However, when using high-speed SiC MOSFETs in a circuit topology it must be approached with extreme care. As fast-changing voltages and currents can increase parasitic inductances and capacitances in a loop which could result in overvoltage or overheating issues. SiC devices are much faster than their silicon counterparts with di / di rates increasing by an order of magnitude; meaning that they may generate up to 10x the peak voltage or current at their terminals.
To address these challenges, the design and operation of SiC MOSFET gate drivers must be optimized. This can be accomplished by altering gate resistance to balance voltage spikes against switching losses; using input supply inductors to suppress high-frequency transmission losses that would otherwise lead to overvoltage at device gates; as well as using TCAD Sentaurus’ comprehensive finite element simulation model for simulating complex witching processes for planar and asymmetric trench MOSFETs which provides physical insight into limitations regarding switching loss improvement strategies.
Ultra-Low Operating Temperature
CoolSiC MOSFETs differ from silicon MOSFETs in that their operating temperatures can be kept much lower to protect their gate oxide layer, making them suitable for system thermal resistance reduction, size reduction and streamlining power distribution networks.
SiC MOSFETs can also help improve system efficiency by minimizing switching losses in alternative energy inverter designs requiring DC-to-DC conversion stages – and in some instances even reduce total system costs by 50% when compared with an IGBT solution.
Silicon-carbide MOSFETs provide significant energy efficiency gains when applied to buck and boost converters used for applications like industrial motor drives, high power data center power architectures, and PFC circuits. Their lower power loss also reduces system heat by significantly decreasing system weight and cost.
Power electronics design engineers can take advantage of these benefits and the higher blocking voltage of a 1200V Z-FET SiC MOSFET to implement all-SiC versions of critical high-power switching circuits and systems with impressive energy efficiencies, power densities and system size reduction not possible with commercial silicon power devices of comparable ratings. However, wide bandgap semiconductor material presents its own unique issues including threshold voltage variations and bias temperature instability (BTI) that must be understood and assessed to optimize device performance in practical applications.
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