GaN HEMTs technology
T
he pairing of GaN and AlGaN creates HEMTs that are renowned for high current densities, high
operating voltages and great performance over a wide frequency range. But this great set of attributes is of little practical benefit unless it can yield products with a guarantee of long-term reliability. Unfortunately, assessing whether this is the case in nitride HEMTs is far from straightforward, given the limited understanding of the degradation mechanisms of this device, plus the great deal of uncertainty relating to the handles that can slash testing times and are known as the accelerator factors.
The so-called three-temperature life test is the conventional approach for qualifying AlGaN/GaN HEMT reliability. A device population is stressed at three different (junction) temperatures, using the operating DC or RF bias conditions, and transistor failure is normally defined by the time it takes for the drain output current or power to fall by 10 or 15 percent.
It is possible to then extract the expected mean time to failure (MTTF) for a defined operating (junction) temperature, by first calculating the MTTF from the set of devices at each temperature. Such an approach often employs the Arrhenius law, which states that the device lifetime is inversely proportional to the exponential of its temperature.
Three-temperature life tests often yield lifetimes in excess of 100 years, indicating that AlGaN/GaN HEMTs have excellent reliability. But that is an incredibly optimistic view – it is certainly not the case that all the degradation mechanisms are strongly temperature-accelerated. The reality is that there are some failure mechanisms that are not brought to light with the three-temperature life test, and these could cause the device to fail far faster than the 100-year estimate.
Stressing the devices Recently, researchers from various institutions have shown that the increase in HEMT gate leakage current that results from high electric fields can rapidly reduce device lifetimes. Many researchers within the nitride community are blaming this premature ageing on defect generation below the gate edge located in the AlGaN barrier. This form of device ageing is normally studied by a step-stress test that involves ramping up the operating voltage. That’s because one of the most accepted degradation mechanisms is based on the inverse piezoelectric effect, which is electric field dependent. It is believed that defects form in the AlGaN layer when its elastic energy exceeds a critical value.
Figure 1: Device geometries and schematic structure of the Al0.3
Ga0.7
N/GaN epilayer grown on highly resistive 4-inch silicon (111)
substrate used in this study. The inset shows the TEM image of the embedded Ni/Au based T-gate
Figure 2: (a) A time-dependent breakdown experiment performed at VG
= -65 V and VD = VS
the device bias at VG (b) The tBD
= 0 V. After some time the gate leakage
current suddenly increases. The first current jump is indicated at time-to- breakdown (tBD
); the inset shows the emission microscopy image of = –65 V, VD
= VS stress. The dashed line is only a guide for the eye
One insight provided by step-stress experiments is the identification of the so-called critical voltage (VCRITICAL
).
Increase the bias beyond this value and the elastic energy in the AlGaN layer exceeds the critical value, leading to the formation of defects that cause a hike in gate leakage current. According to this theory, devices operating at voltages below |VCRITICAL degradation.
| should never show any June 2011
www.compoundsemiconductor.net 15 =0 V, before and after failure. relative to the reverse gate bias conditions used during the
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