Advanced Heterostructure Designs and Recessed Ohmic Contacts for III-Nitride-Based HEMTs

Licentiate thesis, 2015

Modern III-Nitride (III-N) heterostructures offer high mobility, high electron density, large breakdown voltages and good thermal capabilities. High electron mobility transistors (HEMT) based on III-Ns are therefore ideal for high frequency, high power amplification. The intended applications are within radar and mobile communication. Commercial devices are available although some issues remain unsolved. This thesis deals with two of these issues: the lack of a reliable, low resistive ohmic contact and problems with dispersion, which can severely impede output power. Recessed ohmic contacts were developed to an Indium Aluminium Nitride / Aluminium Nitride / Gallium Nitride (InAlN/AlN/GaN) heterostructure. The lowest contact resistance (0.14 Ωmm) was found after annealing at 550 °C for samples where the recess was almost through the barrier. Recessing through the whole barrier also gave low resistive contacts but required slightly higher anneal temperatures. The results indicate the viability of a reliable recessed ohmic contact process. Furthermore, two different aspects of heterostructure development has been investigated; buffer doping and the AlGaN/GaN interface. An optimized carbon doping profile in the buffer has been evaluated by fabricating HEMTs and characterizing isolation and dispersion. The optimized buffer showed to minimize dispersive effects while providing good isolation. Low values of drain induced barrier lowering was measured (1.2 mV/V). A common way to increase electron mobility in AlGaN/GaN heterostructures is to include an AlN-exclusion layer. Unfortunately this approach makes it more difficult to form ohmic contacts. An optimized AlGaN/GaN heterostructure, with a sharp transition from AlGaN to GaN, has been investigated. HEMTs on the optimized sample showed less electron penetration into the barrier layer as compared to an un-optimized sample resulting in an increased mobility (1800 compared to 1700 cm2/Vs). HEMTs with a gate-length of 200 nm exhibited transconductance of 400 mS/mm and fT/fmax of 40/156 GHz using the optimized interface, compared to 390 mS/mm and 37/146 GHz for the standard interface.