Power transistors fabricated using isotopically purified silicon (28Si)

It is well known that isotopic purification of group IV elements can lead to substantial increases in thermal conductivity due to reduced scattering of the phonons. The magnitude of the increase in thermal conductivity depends on the level of isotopic purification, the chemical purity, as well as the test temperature. For isotopically pure silicon (²⁸Si) thermal conductivity improvements as high as sixfold at 20 K and 10%-60% at room temperature have been reported. Device heating during operation results in degradation of performance and reliability (electromigration, gate oxide wearout, thermal runaway). In this letter, we discuss the thermal performance of packaged RF LDMOS power transistors fabricated using ²⁸Si. A novel technique allows the cost effective deployment of this material in integrated circuit manufacturing. A clear reduction of about 5°C-7°C in transistor average temperature and a corresponding 5%-10% decrease in overall packaged device thermal resistance is consistently measured by infrared microscopy in devices fabricated using ²⁸Si over natural silicon.