(Invited) Nanocrystalline Diamond for Near Junction Heat Spreading in GaN Power HEMTs

As a wide-bandgap semiconductor, gallium nitride (GaN) is an attractive material for next-generation power devices, but capabilities have been limited by self-heating effects. Attempts to mitigate thermal impairment have been limited due to the difficulties involved with placing high thermal conductivity materials close to heat sources in the device. Heat spreading schemes have involved growth of AlGaN/GaN on single crystal or CVD diamond, or capping of fully-processed HEMTs using nanocrystalline diamond (NCD). All approaches have suffered from reduced HEMT performance or limited substrate size.

     Recently, we have successfully demonstrated a “gate after diamond” approach, which enables scalable, large-area diamond integration and improves the thermal budget of the process by depositing NCD before the thermally sensitive Schottky metal gate, shown in the cross-section schematic and FIB image in Figure 1 [1,2]. The diamond-capped device appeared to demonstrate improved DC I-V characteristics in most key performance areas, notably improved on-resistance, saturation current, and transconductance, and reduced off-state current and gate leakage, as summarized in Table I. In addition, diamond-capped devices demonstrated improved forward blocking characteristics, manifested by the reduced off-state leakage current and a nearly 200V improvement in breakdown voltage in forward blocking mode. RF devices have produced comparable f_T, f_MAX, and output power relative to a reference device. Furthermore, channel temperature measurements by Raman thermography, shown in Figure 2, indicate a 20% reduction in channel temperature. Thermal mapping has also indicated a change in the temperature profile through the source-drain region by pulling the hot spot away from the gate edge and broadening the temperature distribution. These results produce a good agreement when compared to a finite-element model.

      A significant drawback to this approach is that the passivation layer, however thin, acts as a thermal insulator between the hottest part of the device and the heat spreading layer. This process has recently been improved to enable the deposition of diamond directly on the GaN surface, which produced an enhanced heat spreading effect as well as reduced current collapse. In this process, the most critical step is the reliable formation of the gate opening without damaging the now unprotected GaN surface in the diamond etch, which involves a high power plasma etch in O₂-based chemistry. To mitigate this effect and produce a reliable gate opening, we have developed a sacrificial gate after diamond process, where the gate “recess” is formed as a SiN_Xpillar before diamond growth, followed by either a realignment of the gate recess or selective diamond growth outside of the gate region. This approach is expected to improve the yield and scalability of the diamond directly on GaN process. This presentation will review progress in simulation, process integration, channel temperature mapping, and electrical characterization of diamond-capped devices.

1. M.J. Tadjer, T.J. Anderson, K.D. Hobart, T.I. Feygelson, J.D. Caldwell, C.R. Eddy, Jr. F.J. Kub, J.E. Butler, B.B. Pate, J. Melngailis. IEEE Electron Dev. Lett. 33, 23-25 (2012)

2. T.J. Anderson, A.D. Koehler, M.J. Tadjer, K.D. Hobart, T.I. Feygelson, J.K. Hite, B.B. Pate, C.R. Eddy, Jr, F.J. Kub. 2013 CS Mantech Technical Digest

Figure 1. Schematic and FIB cross-section of AlGaN/GaN HEMT with NCD heat spreading film

Table I. Table of DC parameters for reference and diamond-capped HEMTs

Figure 2. Raman thermography estimation of channel temperature with and without NCD heat spreading films