Parametric investigation of InGaAs/InAlAs HEMTs grown by CBE

Abstract

The InAlAs/InGaAs high electron mobility transistor offers excellent high frequency, low noise operation for amplifiers. While this material system has been grown primarily by conventional MBE, other growth techniques have been examined for improved throughput. The flexibility of chemical beam epitaxy offers semi-infinite sources, good source stability, efficient phosphorus utilization, and extended uptime (more than 560 growth runs over 1.5 years). However, CBE has only recently been shown to produce excellent quality InAlAs suitable for the growth of InAlAs/InGaAs HEMTs [1]. This is the first parametric investigation of the properties of InAlAs/InGaAs HEMTs grown by CBE. A series of lattice matched, pulse doped HEMTs have been grown in which the dopant dose, spacer layer, and channel thickness were systematically varied. Low field 300 K Hall mobilities as high as 8700 cm2/V[middle dot]s for a sheet carrier concentration of 3x1012 cm-2have been measured. This mobility is somewhat lower than uniformly doped HEMTs, which have shown mobilities over 10,000 cm 2/V[middle dot]s at room temperature. A figure of merit, the low field conductivity, has been correlated among the device structure, gateless saturation currents, and DC and microwave device performance. Its applicability as a rough predictor of device performance will be discussed. For a given spacer thickness, the mobility improves as the pulse dose is decreased up to a mobility somewhat below that for uniformly doped structures. As the dopant to channel thickness is increased, this saturated mobility also increases. Secondary ion mass spectroscopy has shown no increase in carbon or oxygen levels at the dopant pulse. This has led to speculation that interface scattering at the top InAlAs/InGaAs interface may be important; however, initial SIMS results do not conclusively show intermixing of the Group III elements at this interface. It is possible that a reduction in the substrate temperature during growth may improve any interface roughness. Results of this modification in growth conditions shall be reported. Self-aligned 0.15 [mu]m HEMTs fabricated from these layers have shown external DC transconductances over 1000mS/mm, unity current gain cutoff frequencies as high as 190 GHz and unity power gain frequencies above 300 GHz. These results and those of more conventional 0.1 [mu]m gate length HEMTs demonstrate the potential of InAlAs/InGaAs HEMTs grown by CBE.