POWER ELECTRONICS HANDBOOK
Why wide bandgap HEMPTs excel at efficient power conversion High electron mobility transistors reduce power supply size thanks to a special make-up that
ANDREA BRICCONI | WEI DENG INFINEON TECHNOLOGIES AG
eliminates sources of energy loss.
IT
is an exciting time for power supply designers. We are learning how devices based on wide bandgap (WBG) materials such as silicon carbide (SiC) and gallium nitride-on-silicon (GaN-on-Si) open the door to new capabilities, often through new topologies. Broadly speaking, the advantages of both SiC and GaN-on-Si stem from lower on-resistance, the ability to operate at higher switching frequencies, and improvements in such important figures of merit as heat tolerance. The resulting advances in efficiency and lowering of losses bring higher power densities and better reliability in end products. That said, silicon will continue to be a mainstream power technology, providing a good combination of power transmission and switching speeds. SiC is more suitable for higher voltages in the region of 600 V to 1.7 kV, at switching frequencies approaching 1 MHz. GaN-onSi is excels at switching voltages between 100 and 600 V at frequencies far in excess of 1 MHz. One major difference between these two WBG materials is that transistor structures (i.e., MOSFETs, diodes, etc.) based on SiC resemble those for silicon . With GaN-on-Si, we can fabricate high electron mobility transistors (HEMT) that bring new advantages for power electronics. HEMT structures are significantly different than traditional MOSFETs, and it is worthwhile to explore the basis for this new structure. A material’s bandgap is fundamental to its ability to carry a charge. The bandgap refers to the difference in energy between the valence band and conduction band (hence bandgap), between which electrons must pass
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during current flow. It is effectively the property that determines whether a material is a conductor, insulator, or semiconductor. Bandgap is measured in units of electron volts (eV), where a smaller number indicates a smaller gap and a better conductor. Semiconductor materials including silicon have a bandgap of between 1 and 4 eV, while a bandgap above 4 eV would typically indicate the material is an insulator. While silicon is a narrow bandgap material with an eV of just over 1, WBG materials such as SiC and GaN have an eV of between 2 and 4. This gives them advantages over silicon semiconductors that can be exploited for power conversion applications. The mobility and abundance of charge carriers also determines the semiconductor properties of a material. (As a quick review, charge carrier mobility is the speed at which the charge carriers move in the material in a given direction, in the presence of an applied electric field.) If there is a great abundance of charge carriers, the material will conduct well even if its charge carriers have a low mobility. Conversely, if a material’s charge carriers exhibit high mobility, it can be a good conductor even if there are relatively few charge carriers present. In this respect, the wide bandgap GaN matched with a narrow-bandgap silicon substrate is an ideal platform for transistors with a high level of electron mobility, hence the term HEMT. DELIVERING HEMTS One of the main structural differences between MOSFETs and HEMTs involves the direction of current flow. In a MOSFET, the charge carriers predominantly flow
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2/19/19 3:36 PM
