Scaling of InGaAs/InAlAs and InAs/AlSb HEMTs for microwave/mm-wave applications

Abstract: The InGaAs/InAlAs high electron mobility transistor (HEMT) offers the highest maximum frequency of oscillation fmax and the lowest noise performance (NFmin) for microwave/mm-wave receivers. Similar to other device technologies, the performance of the InGaAs/InAlAs HEMT has gradually been improved by device scaling. However, the reduction of gate length in relation to the epitaxial Schottky and channel layers presents large challenges in device optimization. More experimental data is needed to learn about the scaling behaviour of HEMTs to further improve analogue speed and noise performance. This thesis deals with studies of device scaling for two different HEMT structures intended for microwave/mm-wave applications, the traditional pseudomorphic InGaAs/InAlAs and the emerging metamorphic InAs/AlSb. For the InGaAs/InAlAs HEMT, scaling of the gate-to-channel distance d at a fixed gate length of 120 nm has been studied. The optimum d was found to be sufficiently small to ensure good control of the charges in the channel and sufficiently large to suppress extensive gate leakage. An optimized InGaAs/InAlAs HEMT exhibited a maximum fmax of 400 GHz when biased for maximum high-frequency performance at VDS = 1.5 V. For the InAs/AlSb HEMT, scaling of the gate length was studied in the interval 225-335 nm. A decrease in gate length had a larger impact on the cut-off frequency fT than on fmax. This was explained by an increase in gds with reduced gate length, bringing down fmax. A 225 nm gate-length HEMT exhibited a maximum fmax and fT of 115 GHz and 165 GHz, respectively. With a reduction of the DC power consumption by 80% the InAs/AlSb HEMT exhibited an fmax of 110 GHz and fT of 120 GHz. This showed the InAs/AlSb HEMTs potential for low-power microwave/mm-wave applications.