Improving The Performance Of Power Transistors
PHOTO: Vertical fin power FETs on a 2-inch GaN-on-GaN wafer. Researchers at IEDM offers insights into how to make GaN transistors switch faster, produce higher blocking voltages, exhibit a very low on-resistance and completely supress dynamic on-resistance by Richard Stevenson
There is still much work to do to improve the performance of the GaN power transistor. While its commercialisation is netting sales, far greater revenues could follow if this device switches more efficiently, handles higher voltages and currents, and offers greater stability.
Options for succeeding on all these fronts were outlined at the recent International Electron Devices Meeting (IEDM), held in San Francisco from 2-6 December 2017. At that gathering researchers detailed: improvements in the switching performance of transistors that feature a gate insulator over the junction; the tremendous capability of vertical power transistors with fins and trenches; and the elimination of dynamic on-resistance through proton irradiation.
A device architecture for providing superior switching was presented by Satoshi Nakazawa from Panasonic Corporation. Working with colleagues from Osaka University and Hokkaido University, he and his co-workers developed a GaN transistor with an AlON gate insulator that combines fast switching with a low leakage current.
"The proposed device exhibits a two-to-four times faster switching speed compared to a typical silicon super-junction-MOS with the same drain current rating," says Nakazawa. "This suggests a reduction in switching loss to half or less in high-frequency switching operations."
Figure 1. A team from Panasonic, Osaka University and Hokkaido University have developed a GaN transistor with an AlON gate insulator. This device combines fast switching with low leakage.
What's more, improvements wrought by the addition of the AlON gate should be applicable to all forms of GaN transistor, argues Nakazawa. "Those with a vertical architecture should expect similar results "“ such as stable gate characteristics with low gate leakages and a large gate voltage swing "“ ensuring gate-driving compatibility with conventional silicon power devices."
The team from Japan had to modify the standard HFET design to realise high-current, high-voltage switching. Their approach, introducing an insulator over the gate, has often been discussed as a way to to reduce leakage current and enable normally-off operation. However, success with this approach has not been reported prior to the work of Nakazawa and co-workers "“ instability of gate characteristics has probably hampered previous efforts.
Fabrication of the team's device (see Figure 1) began with growth, by MOCVD, of GaN and AlGaN layers on a silicon substrate (see Figure 2 for an overview of the fabrication process). Subsequent etching created a recessed gate structure, introduced to trim series resistance. After this, a thin AlGaN layer was added by MOCVD "“ this is highly beneficial, removing damage on the grooved structure caused by dry etching.
The next step involved adding AlON by atomic layer deposition, a growth process that combines great uniformity with an absence of processing damage. Annealing followed, to remove dangling bonds of gallium and/or aluminium at the AlGaN surface and ultimately ensure a positive shift of the threshold voltage. To realise a breakdown voltage in excess of 600 V, transistors were formed with a 2 Î¼m gate length and a 10 Î¼m gate-to-drain spacing.
Figure 2. The fabrication steps employed for the fabrication of a GaN transistor with an AlON gate insulator. This transistor is being pioneered by a team from Panasonic, Osaka University and Hokkaido University
Measurements of transfer characteristics reveal that the hysteresis of the team's annealed device is far less than that for a transistor with a gate made from Al2O3, a more conventional gate dielectric. The curves for transfer characteristics for the device with an AlON insulator do not change for gate voltages up to 10 V, leading Nakazawa and co-workers to claim that a high gate voltage can be applied to realise high-speed, on-state switching.
Further evidence for the benefits of AlON over Al2O3come from capacitance-voltage measurements. They show that dispersion is smaller for the novel oxide, and thus suggest a superior interface. Further encouraging signs come from values for interface trap densities, extracted from the capacitance-voltage measurements, that show that imperfections are lower for AlON, especially in the mid-gap.
Devices with a chip size of 2.3 mm by 2.3 mm are capable of a maximum drain current of 20 A, a breakdown of 730 V, and turn-on and turn-off transitions at 78 V/ns and 169 V/ns, respectively. "We are planning to demonstrate some practical applications," says Nakazawa. This will include converters formed with the team's devices.