Thermal aspects of GaAs power FET attachment using isotropically conductive adhesive

GaAs power transistors are preferred to Si transistors for wireless communication due to their superior high frequency performance. However, the lower thermal conductivity of GaAs (44 W/mK compared to 150 W/mK of Si at 300 K) requires a careful thermal design. For optimizing radio frequency (RF) performance and device reliability GaAs power transistors need the development of advanced large signal models taking into account the dependencies of electrical characteristics from the local temperature distribution within the gate and the heterojunction structure. However, the temperature distribution within the chip (die) is significantly influenced by the packaging and die attach technique. In our case an isotropically conductive adhesive ICA with a silver content of more than 90 percent (cured) was used for mounting the chip. In this paper the influence of bonding failures and thermal qualities of the ICA bonding layer on the temperature distribution in the gate structure of a GaAs heterojunction power transistor is investigated. For this purpose a thermal simulation tool developed at our institute (TRESCOM II) is applied. For proving the validity of the simulation some results are compared with those established experimentally at distinct measuring points. We found that in spite of using an adhesive with a high thermal conductivity, its value has a significant impact on the peak temperatures but is not known well enough. In order to obtain a good agreement in this comparison and to increase the reliability of the simulated results, the thermal conductivity of the ICA was experimentally determined using a carbon dioxide laser. The experimental procedure and the results of this measurement are also described.