Fig 1-2: Powdec’s breakthrough HFET structure.

Gallium Nitride is a next generation semiconductor that enables power devices to have lower power losses and higher energy efficiency compared to present silicon devices. Together with Powdec’s previously announced GaN diodes, these GaN transistors will be core devices enabling an energy efficient, green future.

Up until now GaN HFETs (Heterojunction Field Effect Transistors) with high-voltage, high energy efficiency have been developed, however they suffer from a major problem of current collapse where current decreases and on-resistance increases during operation. To suppress this phenomenon, various techniques have been developed including metal field plates attached to the gate electrode to decrease the electric field and surface passivation to suppress gate leakage; however improvement in device performance is still not sufficient.

In Powdec’s breakthrough solution, instead of a conventional metal field plate, which has hit it’s limits, a thin film of p-type GaN (p-GaN) is used (Figure 1). Powdec succeeded in realizing a polarization effect in the top and bottom interfaces of the AlGaN (aluminium gallium nitride) layer where equal negative and positive charge is generated, creating a 2 dimensional hole gas (2DHG) and 2 dimensional electron gas (2DEG) at the interfaces (Figure 1). This polarization effect almost completely suppresses both the current collapse and current leakage at the gate of the HFET. This is a similar effect seen in silicon super junction devices where the entire length of the gate-drain channel is depleted (high resistance), and the electric field is distributed uniformly along the channel, enabling these GaN transistors to sustain high breakdown voltages. This accomplishment will be presented at the 23rd International Symposium on Power Semiconductor Devices and ICs (ISPSD) in May, 2011 at San Diego, USA.

Fig 2. Comparison of current collapse.

Powdec solved the difficult issue of realizing a high hole density in collaboration with University of Sheffield. This allowed a 2 dimensional hole gas of high hole density, 1.3 x 1013/cm2, to be achieved for the first time in the world.

These transistors with a gate-drain distance of 22mm, in the off-state, sustainable voltages of over 1,100 V are achieved, even without surface passivation. At 1,100 V, drain and gate leakage current is an extremely low 0.3mA/mm and on-resistance is at a world leading level of 6 mW.cm2. In a dramatic comparison to conventional HFETs, there is no current collapse at a stress voltage of 350 V (Fig 2).

Fig 2. Comparison of current collapse showing virtually zero current collapse for Powdec’s new HFET

Going forward, Powdec’s breakthrough HFET technology can be easily combined with ‘normally-off’ structures. Powdec plans to shift growth of the devices to large diameter silicon wafers, with aims to have these energy efficient, high voltage products shipping in volume in 2 to 3 years. To accelerate the market adoption of these innovative, low-power GaN devices, Powdec is actively expanding its partnerships worldwide.

Contact Details:
Email: info2@powdec.co.jp
Address: 1-23-15 Wakagi-cho, Oyama-shi,
Tochigi, 323-0028, JAPAN

About Powdec K.K.
Powdec is one of the industry leaders in the development and production of high quality Gallium Nitride (GaN) semiconductor wafers and devices. Powdec focuses on delivering next-generation semiconductor power devices to create an energy efficient, green future. Powdec is based in Oyama City, Japan (40 mins by train from Tokyo). For more information, visit www.powdec.co.jp/e/; or email us at info2@powdec.co.jp

About the University of Sheffield
The University of Sheffield, founded in 1905, is one of the UK’s leading universities with an outstanding record in both teaching and research. Professor E.M.Sankara Narayanan is a professor in the Electronic and Electrical Engineering Department and a world leading expert on high voltage power semiconductor devices and technologies including device modelling. Dr Akira Nakajima is a Newton International Fellow at the University of Sheffield and an expert in gallium nitride devices. For more information, please contact: s.madathil@sheffield.ac.uk.