conference report IEDM
resistance due to a new mechanism that is possibly related to hot carriers. The key message of their study is this: DC life tests underestimate RF reliability.
This important conclusion was drawn from a series of measurements on single-stage MMICs with 4 x 100 µm GaN HEMTs. Performance was evaluated at a current of 100 mA/mm, a source-drain voltage of 28 V, and a saturated power output (input power was 23 dB). The RF performance at this voltage is similar to that at 40 V, the designed operating condition for these MMICs.
Figure 1. MBE re-growth of heavily n-doped GaN contacts has helped to speed HRL’s HEMTs
Channel stoppers Modifications to transistor architectures were also behind the hike in blocking voltages of HEMTs produced by Panasonic’s Advanced Technology Research Laboratories. By introducing an array of ‘channel stoppers’ that terminate the leakage current at the interface with the silicon substrate, this team increased off-state breakdown voltage in HEMTs with GaN layers just 1.4 µm thick from 760 V to 1340 V. Turning to 1.9 µm-thick GaN bolstered the blocking voltage to 1900 V, and the team claims that additional thickening of GaN should increases this to 3 kV.
Simply increases the thickness of GaN layers in conventional devices – which could find application in power switching systems, such as inverters for industrial use and uninterruptible power supplies – has been widely touted as a route to increasing the HEMT’s blocking voltage. But in practice such efforts, which have the downside of increasing chipmaking costs, fail to deliver on this front. The reason for this was unclear until the recent efforts by this Japanese team. According to them, conventional devices suffer from a significant leakage current that stems from sheet electrons forming an inversion layer at the substrate-epilayer interface. The Kyoto team has confirmed the presence of an inversion layer by fabricating metal-insulator-semiconductor diodes and measured their capacitance-voltage characteristics.
To stem the flow of leakage current, Panasonic’s engineers insert channel stoppers. These stoppers, which are formed by selective boron ion implantation at the periphery of the chip, widen the depletion layer in silicon at high positive surface bias. This, in turn, increases the overall blocking voltage thanks to the addition of the breakdown voltage of the depletion layer.
Unreliable reliability tests Meanwhile, researchers Jungwoo Joh and Jesús del Alamo from MIT have revealed that it can be inappropriate to determine a HEMT’s RF reliability from DC tests. That’s because the degradation mechanisms for DC and RF operation are significantly different.
“Obviously life tests under RF conditions close to field, but mildly accelerated, would best represent the reliability of these devices,” said Joh to Compound Semiconductor. He and del Alamo found that RF stress degrades a device far more severely than DC stress at the same voltage, and this degradation gets more severe with increasing power compression. In addition, the pair of MIT researchers discovered that RF stress induces an increase in source
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www.compoundsemiconductor.net January / February 2011
The first experiment began by DC stressing the device for 5 hours, using a drain-source voltage of 40 V and a quiescent current of 100 mA/mm. This led to little change in device characteristics, aside from a small increase in current collapse. An RF stress test followed, involving increases in input power from 20 dB to 26 dB. This led to major changes in MMIC performance: significant increases in current collapse and sheet resistance; a permanent degradation in the maximum drain current; and a substantial cut in the output power.
Joh and del Alamo then looked in turn at three operating conditions that could potentially cause enhanced degradation at high compression: the “on” regime, the high-voltage “off” regime; or the “high-power” regime. They ruled out the first two, and although they couldn’t directly test the third scenario – in this condition there is very high power dissipation, which leads to incredibly high channel temperatures that kill the device – pulsed conditions revealed a sharp increase in sheet resistance beyond 40 V, especially for a high stress current.
Fluorine: a fine dopant
Reliability assessment was also the central theme of a study led by Kevin Chen from Hong Kong University of Science and Technology. He and his colleagues studied the behavior of enhancement-mode AlGaN/GaN HEMTs fabricated by fluorine plasma ion implantation. This technique offers a low-cost approach to making this class of transistor and has the merit of self-alignment between implantation and gate metallization.
In GaN and related materials, fluorine ions exhibit a negative charge state. “When incorporated in the AlGaN barrier, these ions can deplete the two-dimensional electron gas channel, shifting the threshold voltage to positive values and converting the device from depletion- mode to enhancement-mode,” explains Chen. The enhancement-mode form of the device, which is also referred to as ‘normally off’, is more desirable for power switching applications - it allows a simpler gate drive; and if it fails, the system is left in a safe state.
Fluorine plasma ion implantation technology has been previously used in other semiconductor materials, such as silicon and GaAs, where it has compromised reliability. The concern has been that this technology would also impair GaN transistor reliability, although preliminary results indicate that this does not impact the electrical and thermal reliability of device made from this wide bandgap semiconductor.
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