A Novel Approach to Improve Heat Dissipation of AlGaN/GaN High Electron Mobility Transistors with a Backside Cu Via
Authors: Hwang Ya-Hsi , Kang Tsung-Sheng , Ren Fan , Pearton Stephen J. 
Published: February 2020

AlGaN/GaN HEMTs have gained more attention recently because of the superior mobility (~1400 cm2/V-s) and high breakdown voltage. However, there still are issues such as self- heating effect, current collapse. HEMTs are grown on sapphire, SiC or Si substrate. Of all the substrates, sapphire is the most common one. However, it is not compatible with the current Si technology. To grow HEMTs on Si, transition layer, grading AlGaN layer, was necessary to overcome the lattice mismatch between Si and GaN. This grading layer provided an additional resistive layer to HEMTs, making the self-heating effect more severe. Several studies have been done to understand the thermal behavior of HEMTs. However, none of them provide a method to increase the thermal dissipation. In this study, we propose a new structure by adding a backside Cu via under active area and demonstrate the thermal dissipation capability by simulation.
The structure used in the model consisted of a 25 nm AlGaN barrier layer, a 0.8 µm GaN buffer layer, a 1.4 µm AlGaN transition layer and a 28 nm AlN nucleation layer on 100 µm Si substrate. The LDS was 4 µm. The LG and WG were 0.45 and 100 µm, respectively. For reference, a square through wafer via of 50 µm ×50 µm was placed under the source pad. For our new approach, another rectangular through Si via of 6 µm× 110 µm was opened beneath the active area. Both via holes were filled with copper.
Figure 1 showed the temperature distribution along the source and drain direction. For the reference, the maximum junction temperature (Tmax) was 146oC, while 120oC for the new structure. The reduction of Tmax was attributed by the reduction of the thermal resistance, which is a combination effect of nucleation layer removal and filled Cu via.
Figure 2 showed vertical temperature distributions in the region directly under the gate for the reference and the new structure. In this figure one can find the temperature drop in each layer. The dimensions of the top AlGaN layer and 2DEG are too thin and the temperature changes across these regions are too small to be observed. For the reference, the temperature drops across the GaN layer, AlGaN layer and AlN layer were 9, 64 and 6°C, respectively. Although the defective AlN layer was only 28 nm, there is an obvious 6°C drop due to its high thermal resistivity. In the case where the defective AlN layer under the active layer was removed and the via was filled up with copper, not only was the Tmax much lower than the reference, but also the temperature at the bottom of the transition layer was 10°C lower than the reference. This indicates the effectiveness of heat removal from the copper filled via.
Figure 3 showed the relationship between Tmax with power consumption. As shown in Figure 3, Tmax is directly proportional to power consumption. For the reference, the Tmax increased rate was around 27°C/W while 22°C/W for the new structure. The effect of the copper thickness inside the via on the Tmax was also investigated. From Figure 3, the temperature increase rate was 24°C/W and 23 °C/W of 2 and 1 µm filled Cu via device respectively.
The effect of the dimensions of the via on the Tmax were also studied. The results so far were based on the via covering the regions of 4 µm between source and drain contacts as well as 1 µm of the transfer length on each side of the Ohmic metal. The majority of the heat generation of the HEMT is in these two regions. Thus the Tmax increased dramatically when the opening of the via is symmetrically decreased around the gate, as shown in the region I in Figure 4. Region II showed the impact of expanding the width of the via under the source Ohmic metal contact on the Tmax. The Tmax continuously decreased as the width of the via increases due to larger areas of thermal resistive layer being removed and replaced with a less resistive copper layer. The Tmax can be further reduced by extending the via to the drain contact, as shown in region III.
In summary, a new approach of implementing a via under the active area of HEMT is proposed to reduce the maximal junction temperature. The via provided a way to remove the highly defective layer and replace it with metal. Because of the reduced thermal resistance, the maximum junction temperature can be significantly decreased from 146 ºC to 120ºC.

Figure 1