CONFERENCE REPORT IEDM
enlarged gate swing. However, this more promising class of transistor tends to be plagued by a high density of traps at the interface between the dielectric and the III-N. The dynamic charging and discharging of these traps has been blamed for instability in the transistor’s threshold voltage. To address this issue, engineers from The Hong Kong University of Science and Technology have developed a deposition process that yields a high-quality interface. “We believe that we have gotten to the heart of the problem and developed an effective solution,” claims team-leader Kevin Chen.
At IEDM, this group unveiled its deposition process for forming a MIS structure, and also detailed a new approach to characterising the interface traps in MIS-HEMTs. The latter is a tricky task, due to the presence of two interfaces: that between the dielectric and III-N, and that between AlGaN and GaN.
Figure 1. GaN-based high-voltage switching devices developed at FBI. The normally on and normally off devices are fabricated using the same technology, with the exception of the gate module. The inset show details related to the p-type GaN technology, which renders the devices normally off
this approach, but the breakdown strength is just 40 or 50 V/µm, about a third of what is theoretically possible. Simply doping the buffer with carbon is also inappropriate, because although this boosts breakdown strength, it also increases dynamic RON
. To realise a high field strength and a low RON , the team from
FBH has drawn on both of these approaches. Its best test structure features a graded AlGaN barrier with an average aluminium concentration of 5 percent, which is positioned close to the channel, and a carbon buffer that is doped to 4 x 1018
cm-3
to large field changes during switching. With this carefully selected combination, HEMTs that switch at 500 V can exhibit a breakdown field strength of 80 V/µm and a produce a dynamic RON
that is just two-and-a-half times that of static RON .
Time dependency measurements reveal that immediately after switching from the off-state, the dynamic RON
of the HEMT
decreases rapidly, before flattening out and getting fairly close to its static value after 100 µs. Decline in dynamic RON
is not
exponential, but can be fitted by multiple exponential functions, suggesting that several different trap states are contributing to charging and discharging within the device.
Würfl and his co-workers are now trying to make further improvements to dynamic RON
. This effort forms part of a project
involving a large European company, which requires further gains to deliver a competitive product. “Otherwise the difference between [GaN HEMTs and] the very-good-performing silicon is getting smaller and smaller, and there would be a question whether someone would pay the higher price, even if it were for GaN-on-silicon,” says Würfl. The team also hopes to increase the breakdown strength of its devices. “I think we could go to 120 V per micrometre,” says Würfl, who believes that this is realistic while maintaining the value for dynamic RON
achieved in HEMTs with a breakdown of 80 V/µm.
Interface traps For high-voltage power switches, the MIS-HEMT is favoured over the more common Schottky-gate HEMT because it is capable of delivering a lower gate leakage current and an
38
www.compoundsemiconductor.net January / February 2014
Figure 2. Engineers at The Hong Kong University of Science and Technology have produced monolithic integrated circuits with E-mode and D-mode MIS-HEMTs
To form their monolithic circuit, the engineers used E-mode and D-mode HEMTs, rather than the more conventional Schottky- gate HEMTs (see Figure 2). According to Chen, two of the strengths of the E-mode MIS-HEMT over its conventional cousin
Chen’s team form a high-quality MIS structure by employing an in-situ, low-damage remote plasma treatment to remove the native oxide and add a nitridation inter-layer. After this, they deposit a gate dielectric. Components of the plasma, NH3
-Ar-N2 , perform different roles: NH3 oxide, while the N2 O3 under oxygen.
Interface traps in the MIS-HEMTs were uncovered by analysing the frequency- and temperature-dependent onset of the second slope in the current-voltage characteristics. Using frequencies varying from 400 Hz to 10 MHz and temperatures from ambient to 200 °C, engineers determined an interface trap density of 1012
– 1013 and inserted underneath regions that are not prone cm-2 eV-1 . Monolithic circuits
In a separate paper presented at IEDM, Chen and his co- workers claimed the first demonstration of a form of GaN-based high-voltage start-up circuit – one with low standby-power consumption and designed for off-line switch-mode power supplies. This mode of power supply is used in personal computers, battery charges, central power distribution systems, consumer electronics and LED lighting.
-Ar removes the native enables deposition of the nitridation inter-
layer. On top of this, engineers deposit, in-situ, a 25 nm-thick layer of Al2
, before the entire structure is annealed at 500 °C
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