Antimonide Based HEMTs for Ultra-Low-Power and High-Frequency Applications
Bollaert, sb¹; Desplanque L., LD¹; Malmkvist M., MM²; Lefebvre E., EL²; Olivier A., AO¹; Shchepetov A, AS¹; Roelens Y., YR¹; Wallart X., XW¹; Cappy A., AC¹; Grahn J., JG²; Dambrine G., GD^1
1IEMN; ²Chalmers University

High frequency devices with ultra low power consumption properties for mixed-mode circuits, are investigated in numerous research projects [1-2]. Narrow bandgap materials, based on antimonide, are attractive candidates to reduce power consumption. The high electron mobility in the channel offers the possibility of reducing supply voltage while keeping high performance. Although immature, the AlSb/InAs technology has shown a potential for high frequency, low power dissipation and low-noise performance [3,4].

Our preliminary study concerns AlSb/InAs HEMTs. The AlSb/InAs HEMT structure was grown on InP substrate by molecular beam epitaxy (MBE) in a Riber 21 TM chamber. It consists on: a thick AlSb metamorphic buffer, a 15nm InAs channel, a 5nm AlSb spacer, a 4.5x1012/cm2 delta-doped, a 12nm AlSb Schottky barrier layer, a 3nm InAlAs hole barrier and finally a 5nm InAs cap layer. The delta doped is defined by a thin InAs layer with Silicon donor. AlSb metamorphic buffer was used to accommodate the large lattice mismatched between InP and the 6.1Å materials. The hole barrier was inserted on top of the Schottky layer to reduce gate leakage current. Finally, an AlGaSb etch-stopped layer is added in the buffer, acting as a stable mesa floor thus avoiding AlSb buffer oxidation. Indeed the AlSb material is extremely reactive with air exposure (even during the process) and this will affect the reliability performance. Hall measurements at room temperature indicate an electron mobility of 21,500cm2/Vs with sheet carrier density of 2.31012/cm2. Similar results are obtained with any AlGaSb in the buffer. Moreover adjusting spacer thickness and delta-doping level, a high value of 34,000cm2/Vs (with 1.31012/cm2) is obtained, indicating the excellent quality of the material. HEMT was fabricated using mesa isolation (shallow dry or deep wet etching), ohmic contact with Pd/Pt/Au metals, and Ti/Pt/Au T-gate definition. DC-characteristics exhibited a maximum drain current of 1A/mm and extrinsic transconductance of 1.5S/mm at Vds=0.4V. Microwave characterizations of 130nm Sb-HEMT at Vds=0.4V give fT and fmax of 170GHz and 160GHz respectively. For 70nm gate length, fT is 200GHz. At very low Vds down to 100mV, fT is even higher than 100GHz. This technology offers the opportunity of weak power dissipation (lower than 10μW/μm) in comparison with value of 100μW/μm obtained with standard technologies, while keeping high frequency capability.
Future trend of the work is the study of type I heterostructure. Even the insertion of InAlAs hole barrier, the gate hole current is not totally suppressed. Holes can also accumulate in the AlSb buffer. Moreover, we propose to suppress pure AlSb material, which is not well suited for reliability.