Breaking down resistance

14 March 2013

A method to reduce specific on-resistance in AlGaN/GaN HEMTs has been developed in Korea. Requiring no special growth techniques the devices produced also have excellent breakdown voltage, on/off current ratio and figure-of-merit.

Lows and highs

AlGaN/GaN high electron mobility transistors (HEMTs) are of particular interest for high-power applications like power conversion systems. This is due both to the high critical electric field of GaN and high mobility, high-density channel between AlGaN and GaN formed by piezoelectric polarisation. This channel layer induces a low on-resistance and high switching speed and gives power conversion systems using such HEMTs a higher-conversion efficiency than similar systems using silicon-based devices.

GaN based HEMTs are also cheaper to fabricate than other wide bandgap based devices like SiC MOSFETs. GaN can be grown on low-cost Si substrate and the equipment required is compatible with silicon devices. It can also allow chip size reduction because of its low on-resistance.

Specific on-resistance is key in deciding the maximum current, size and cost of devices. However, there is often a trade-off between specific on-resistance and breakdown voltage, which is important for stability and blocking capability in high voltage devices, to allow high-power conversion without device failure.

In addition, with devices using other materials, for example Si DMOS, specific on-resistance can be controlled by using wafer thinning techniques and doping in the buffer layer. However, controlling on-resistance in the case of AlGaN/GaN HEMTs is not as easy due to difficulties in doping and the lateral structure of HEMTs.

Isolated reduction

In their current Letter, the team from Seoul National University and Korea Electronics Technology Institute, Seongnam, present a method for reducing specific on-resistance in AlGaN/GaN HEMTs that is both simple and doesn’t compromise the breakdown voltage. They have achieved specific on-resistance of 2.28 mΩcm2 in devices with 1410 V breakdown voltage and on/off current ratio and figure-of-merit values of 4.97 x 1010 and 872 MW/cm2 respectively.

They have accomplished this through simple structural changes to the HEMT design. By extending the TaN gate structure of a conventional AlGaN/GaN MOS-HEMT they remove the gate-source space. Gate-source space increases the series resistance between source and drain so its removal reduces the specific on-resistance. By doing this in such a way that the gate-drain distance is maintained, the breakdown voltage, which is dependent on this distance, isn’t compromised and the use of a HfO2 gate insulator gives a high breakdown value.

This structure also reduces drain-source distance and the effective gate length for the photo lithography equipment used because part of the gate region is overlapped with the HfO2 gate insulator on the source. None of the changes require any new fabrication processes compared to a conventional AlGaN/GaN HEMT.

Team member Ogyun Seok explained that the selection of the gate insulator was a key factor in the success of their design: “The stable blocking characteristic of the gate insulator at on and off states is most important in these devices. Thick gate insulators cause negative shift of threshold voltage so that higher gate-source voltage is required to turn off the devices. We successfully fabricated the AlGaN/GaN MOS-HEMTs with the extended TaN gate by applying a high-quality HfO2 gate insulator via RF-sputtering.”

Application methods

The team expect that this extended gate structure will be of great use in large AlGaN/GaN MOS-HEMTs for high-current capability, where employing this structure will allow the maximum drain current to be increased for a given device size. This kind of structure will also allow simplification of the layout of multi-finger AlGaN/GaN MOS-HEMTs by using source/gate-insulator/gate stacks. The structure is also relatively insensitive to misalignment compared to conventional MOS-HEMTs.

However, there are still possible problems to investigate, such as whether high gate-source voltage will cause a tunnelling gate-source leakage current through a source/gate-insulator/gate stack. The switching speed may also be decreased by increased gate-source capacitance. The team are now looking at these issues to try to guarantee stability of such structures and hope the extended gate structure can be used commercially in the near future.

Seok explained “We plan to perform recess etching underneath the gate to shift threshold voltage to a positive direction for stable operation with the extended gate. In the proposed device in this Letter, we cannot apply highly negative gate-source voltage because of dielectric breakdown. The recessed gate structure may induce more stable operation by shifting the operation point to the positive direction.”

The wider goals that the team are working toward are a fully-sputtering, gold-free process. In this work they have used an RF-sputtered gate and gate insulator, and they tell us that they plan to report a gold-free process in the near future.

Further reading

This article is based on the Letter: High-breakdown voltage and low on-resistance AlGaN/GaN on Si MOS-HEMTs employing an extended TaN gate on HfO2 gate insulator (new window).

A PDF version (new window) of this feature article is also available.

Journal content

Cover of Electronics Letters, Volume 49, Issue 25

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