GaN-based high electron mobility transistors with high Al-content barriers

Gallium nitride based High Electron Mobility Transistors (HEMT) with ultra-thin AlN barriers are developed to realize high-frequency transistors and power amplifiers for millimeter-wave (mm-wave) applications. Epitaxy and fabrication of the AlN/GaN heterostructure at the beginning of the process chain form the basis for the performance of the devices. By using highly-strained, binary AlN as a barrier material, in principal short gate-to-channel distances with simultaneously high sheet carrier densities can be realized as it is required for fast switching of the transistor. Growth characterization and processing of the corresponding HEMT structures is challenging due to the high strain and extremely thin layers. Two different methods were used for growth of the structures, MBE and MOCVD. A suitable HEMT layer sequence was developed with the MBE research machine and further optimized. In addition suitable growth parameters were determined for the AlN/GaN HEMT structures. Depending on the layer thicknesses, thegrown HEMT structures revealed sheet carrier densities from 0.7 · 1013 up to 3.3 · 1013 cm−2 and mobilities from 1040 to 1580 cm2/Vs allowing to control the electrical properties of the structures as necessary. Results of the MBE investigations and the developed layer structure were subsequently transfered to MOCVD and adjusted for this technique, being the more suitable methods for industrial applications.Moreover, a systematical comparison between MBE and MOCVD grown AlN/GaN HEMTstructures was performed for the first time. For this, nominally equal layer structures grownby MBE and MOCVD were investigated by various characterization methods. Consequently, structural and electrical differences resulting from the substantially different epitaxial techniques could be identified and analyzed in detail. Furthermore, band diagram simulations were performed for the investigated structures in order to theoretically describe the observed differences.Process development was performed for the epitaxially optimized AlN/GaN HEMT structures. Successful fabrication of devices with ultra-thin barrier layers could be demonstrated. Realizationof short gate-to-channel distances led to high transconductances of about 600mS/mm.A reproducible output power of 3W/mm with PAE above 55% have been demonstratedfor the developed HEMTs. In addition, MMICs (Monolithic Microwave Integrated Circuit) could be processed on the developed epitaxy structures realizing high output power densities for frequencies in the range of 70 - 100GHz. Combination of vertical and lateral scalingusing 100 nm gate length technology revealed significant advances: A current-gain cut-of frequency of fT = 110GHz and a maximum oscillation frequency of fmax = 260GHz could be achieved demonstrating excellent RF performance of HEMTs. Thus, the newly developed epitaxy structures could significantly contribute to the improvement of the high frequency performance of the fabricated HEMTs and MMIC amplifiers.