Technology Integration of GaN-on-Si HEMTs for Power Electronics Applications

In this PhD thesis, GaN-on-Si HEMTs (high electron mobility transistors) have been studied. III-N materials (GaN, AlGaN, AlN) are very useful systems to be used at high voltage and under high ambient temperature owing to their excellent intrinsic properties, such as wide band gap and low intrinsic carrier concentration. The III-N hetero-structure (AlGaN/GaN) leads to the formation of a 2-DEG (two dimensional electron gas) with high carrier density, high mobility and high saturation velocity which can be utilized to realize an efficient compact power conversion system. GaN-on-Si HEMTs are cost effective solutions but in the scope of this PhD, it is sketched out that the parasitic conduction across the AlN/Si interface leads to device breakdown voltage saturation after a certain gate-drain distance. This saturation could be elevated by using thick buffers on Si; however, growing thick buffers are problematic due to the large lattice and thermal mismatch between III-N and Si. 

In this PhD work, novel Si substrate removal approaches to terminate the AlN/Si interface conduction to enhance the breakdown voltage of AlGaN/GaN/AlGaN-DHFETs has been studied extensively. Various technological approaches starting from Global Si substrate removal (GSSR)-to-Local Si removal between S-to-D contacts (LRSD)-to- Si trench around drain contact (STAD) have been developed. After Si removal, a linear increase in the breakdown voltage with the gate-drain distance is measured compared to a saturated breakdown voltage on Si. A breakdown voltage of over ~ 2000 V is measured for a gate-drain distance of 20 μm and a buffer thickness of only 2 μm. The main thrust from GSSR-to-LRSD-to-STAD is driven by enhancing device thermal performance. In case of a STAD device (compared to a GSSR or a LRSD device), the Si substrate is still under the gate-region where the maximum heat is generated which helps to dissipate the generated heat; confirmed by electrical measurements and thermal simulations. It is presented that after Si removal, devices have un-altered threshold voltages (VTH); confirming no change in 2-DEG channel properties. Other advantages of Si substrate removal include: (1) high voltage operation under high ambient temperature is possible compared to the devices with Si and (2) the device breakdown voltage is independent of the buffer thickness leading to ~ 2000 V breakdown for a sub-micron thin (600 nm) AlGaN buffer layer.

Therefore optimization of thin buffers followed by Si substrate removal is one of the most attractive solutions towards obtaining low cost, high yield and high manufacturability of GaN power devices fabricated on Si substrates.