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Impact of residual carbon on electrical properties of gallium nitride transistor structure

In short: A very precise amount of carbon atoms should be incorporated into gallium nitride based high electron mobility transistors for optimal performance and reliability. We demonstrate that the carbon concentration effectively can be controlled by the temperature in the fabrication process, letting us reach excellent uniformity of the electrical properties with very high free electron density and high mobility. Our result is relevant for developing energy efficient high power and high frequency electronic devices.

Over the past decade, many research groups have demonstrated the excellent performance of gallium nitride based high electron mobility transistors (HEMTs) in high power and high-frequency electronic devices. However, the bottleneck towards commercialization is still the device reliability. One of the critical parameters that would influence the final device reliability is the electrical property of gallium nitride layer in the HEMT structure.

Two requirements for the gallium nitride layer should be fulfilled. Firstly, it has to be non-conductive to prevent unwanted current paths underneath the current channel and also to obtain high breakdown voltage with low leakage current. One effective way to achieve this is to introduce some impurities, like carbon, during growth of the layer. This process appears to be straightforward since carbon commonly exists in the materials due to the use of metal-organic gases in the growth process. Secondly, in order to prevent the current in the channel from being trapped, causing the failure of device characteristics, known as current collapse, the level of carbon in the vicinity of the current channel should be minimized. Therefore, the level of carbon in the gallium nitride layer needs to be carefully defined and optimized. 

On the other hand, since carbon incorporation inevitably takes place in the growth process, it is important to know how low the carbon concentration in the gallium nitride layer needs to be to sufficiently minimize current collapse in the HEMT device. However, this information is still lacking in the literature.

In this work, we demonstrate the development of high tunability of residual carbon incorporation in our chemical vapor deposition reactor. The carbon concentration can be effectively controlled over two orders of magnitude by the growth temperature of the gallium nitride layer (from 2×1018 cm-3 down to 1×1016 cm-3). Excellent uniformity of electrical properties in the HEMT structure with very high average carrier density (1.1×1013 cm-2) and high mobility (2035 cm2/Vs) over a three-inch diameter substrate of semi-insulating silicon carbide is realized with our temperature-tuned carbon incorporation scheme. Reduction of carbon concentration below the level of 5×1016 cm3 is shown sufficient to minimize current collapse and also to achieve excellent electrical properties of the HEMT structure.

(left) The carbon concentration in gallium nitride buffer layer as a function of its growth temperature. (right) 2DEG mobility and carrier density in the HEMT structures versus growth temperature.

For more details about this research, please see Applied Physics Letters 102, 193506 (2013)


Jr-Tai Chen, Urban Forsberg and Erik Janzén


Jr-Tai Chen, phd student
Phone: +46 (0)13 28 26 99
E-mail: jrche@ifm.liu.se


  • Swedish Foundation for Strategic Research (SSF).
  • European project of High Quality European GaN-Wafer on SiC Substrates for Space Applications (EuSiC)


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Last updated: 03/10/14