CONFERENCE REPORT IEDM
origin, however, has been something of a mystery – but it is starting to unravel, with a team from MIT and Texas Instruments claiming at IEDM that current collapse stems from high-field, tunnelling-induced electron trapping. These researchers studied this phenomenon in AlGaN/GaN MIS-HEMTs with a breakdown voltage greater than 600 V. Devices were fabricated from a 6-inch III-N-on-silicon epiwafer supplied by a commercial vendor.
Step-stress measurements were made on these transistors by increasing the drain-source voltage by 20 V every 10 s. Linear drain current, which is inversely proportional to RON
, degraded
Figure 3. The switch mode power supply produced by the team at The Hong Kong University of Science and Technology contains three GaN MIS-HEMTs
are: a larger positive threshold voltage that enables enhanced electromagnetic interference immunity; and a larger gate swing, which increases the compatibility with existing silicon-based gate driver ICs. The D-mode MIS-HEMT also has its merits, being more suitable than a conventional equivalent for the large negative threshold voltage – typically -5 V or more – needed in the start-up circuit.
The epitaxial structure used for making the 600 V MIS-HEMTs for the IC was deposited on silicon and contained a 21 nm-thick GaN/Al0.25
Ga0.75 N/AlN barrier layer and a 3.8 µm-
thick GaN buffer. Device fabrication involved electron-beam evaporation of Ti/Al/Ni/Au source and drain contacts, deposition of an AlN/SiN passivation layer and planar isolation of active regions by fluorine ion implantation. After opening the gate window, ions were applied to the gate region of E-mode devices. This was followed by the NH3
-Ar-N2 plasma treatment
process previously decsribed, deposition of a 17 nm-thick film as the gate insulator, and the addition of a Ni/Au gate electrode.
Chen says that one of the highlights of the team’s device technology is its passivation process, which leads to effective suppression of current collapse and a very small dynamic on- resistance. E-mode MIS-HEMTs have a high threshold voltage of 3.6 V and a large gate swing of 14 V, thanks to integration of fluorine implantation, surface nitridation and the gate dielectric processes. Supplied with an input of 200 V, the power supply circuit (see Figure 3) has a start-up current of 1.07 mA, a start- up time of 65 ms (with a 10 µF output capacitor), and a stand-by power that is calculated to be 2.1 mW. “The performance is comparable to a silicon-based circuit in terms of the start-up current and standby power consumption,” says Chen.
According to him, the ultimate goal for his team is to build a circuit with GaN for a highly efficient, compact, off-line switch- mode power supply. “The hysteresis comparator in the start- up circuit can be achieved using GaN E/D-mode HEMTs or MIS-HEMTs, which has not been done in this work yet.” If the circuit is to demonstrate commercial viability, it must also pass reliability tests involving operation under extreme conditions for lengthy periods.
Zener trapping
The weakness referred to as current collapse, the temporary increase in a GaN transistor’s RON
after high-voltage off-state biasing, is known to result from excessive trapping. Its precise Devices made by FBI on SiC substrates January / February 2014
www.compoundsemiconductor.net 39
A study of the dynamics of trapping followed, involving plotting of trapping time as a function of the reciprocal of the peak electric field, which occurs inside the AlGaN barrier and under the edge of the third field plate. The shape of this graph strongly suggests that the origin of current collapse is a valence-band- to-trap tunnelling process, which the team refer to as Zener trapping. Substitutional carbon on nitrogen sites is suggested to be a primary culprit. Team-member Donghyun Jin says that if carbon is to blame, it should be minimised in the GaN channel and AlGaN barrier to reduce Zener tunnelling. “However, its concentration in the buffer should be retained high enough to achieve high resistivity and a high level of breakdown voltage.”
So, a delicate balancing act is recommended, just as it is to realise the combination of a high breakdown voltage and low on- resistance in conventional HEMTs. Optimising these trade-offs will take time, but progress may well be reported at papers given at the next IEDM, which takes place in December in San Francisco.
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by about 10 percent as the voltage approached 200 V, before abruptly dropping to 10 percent of its initial value. Further degradation occurred as the voltage was cranked higher, with the linear drain current becoming negligible and RON
increasing
by around 10 orders of magnitude. However, this damage is fully recoverable – initial device characteristics recovered with strong UV illumination or moderate thermal treatment, such as heating for 3 hours at 100 °C.
To increase the operating voltage of GaN MIS-HEMTs, many developers have turned to multiple gates that may also prevent current collapse – although their effectiveness to address the later issue is unclear. To try and resolve this, the US team investigated the influence of device geometry. They found that the lengths of the three field plates used in the transistor do not impact trapping characteristics.
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