research review
Sapphire cuts current leakage in high-speed HEMTs
A 2 nm-thick layer of sapphire can drastically reduce the leakage current in high-speed, highly scaled HEMTs
RESEARCHERS from HRL Laboratories in Malibu, CA, have fabricated 40 nm gate- length GaN HEMTs that combine operating speeds of hundreds of gigahertz with incredibly low gate leakage currents.
It is well known that slashing the size of the gate length holds the key to producing GaN transistors operating at really high speeds, which could be used for applications including mm-wave imaging. However, reductions in gate size often come at the expense of a hike in leakage current. To combat higher gate leakage, engineers can insert a dielectric gate material, often SiN, into the device. But the dielectric constant for SiN is only 6-8 – not high enough for deeply scaled HEMTs with a gate length of 40 nm. At these dimensions, the West-coast team has shown that a far better approach is to use a 2 nm-thick layer of sapphire, which has a dielectric constant of 8-10.
These engineers have fabricated transistors containing this layer on 3-inch SiC substrates, and compared their performance to conventional HEMTs (see figure for both device designs). MBE was employed to deposit the device layers, with atomic layer deposition used to add the Al2
film. These HEMTs included AlGaN buffers that provide a back barrier for electrons and deliver two benefits – enhanced carrier confinement and a reduction in short-channel effects.
O3
The novel HEMT had a forward-bias gate current that was more than two orders of magnitude smaller than the conventional equivalent. Although the addition of this sapphire layer reduced the performance of the HEMT in some areas, impact was minimal: Intrinsic transconductance fell from 622 mS/mm to 552 mS/mm; peak drain currents at a gate-source voltage of 2V decreased from 0.9 A/mm to 0.86 A/mm; and values for the cut-off frequency and maximum oscillation frequency reduced from 138 GHz and 286 GHz, respectively, to 134 GHz and 261 GHz, respectively.
Based on the results, the researchers claim that ultra-thin Al2
O3 gate dielectrics
are a promising addition to highly scaled GaN HEMTs.
A. Corrion et al. accepted for publication in IEEE Electron Dev. Lett.
46
www.compoundsemiconductor.net August / September 2011
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