The development of an active retrodirective antenna involving frequency conversion stages is presented. This retrodirective array of printed dipoles and regular commercial components is designed to operate at the GSM1900 band. Experimental measurements of monoestatic and biestatic responses are given, showing very good performance for a small retrodirective array.

We have made AlGaN∕GaN high electron mobility transistors with a Si3N4 passivation layer that was deposited in situ in our metal-organic chemical-vapor deposition reactor in the same growth sequence as the rest of the layer stack. The Si3N4 is shown to be of high quality and stoichiometric in composition. It reduces the relaxation, cracking, and surface roughness of the AlGaN layer. It also neutralizes the charges at the top AlGaN interface, which leads to a higher two-dimensional electron-gas density. Moreover, it protects the surface during processing and improves the Ohmic source and drain contacts. This leads to devices with greatly improved characteristics.

In this work, we report on the growth by metalorganic vapor phase epitaxy (MOVPE) of GaN layers on AlN/Si(111) templates with step-graded AlGaN intermediate layers. First, we will discuss the optimization of the AlN/Si(111) templates and then we will discuss the incorporation of step-graded AlGaN intermediate layers. It is found that the growth stress in GaN on high-temperature (HT) AlN/Si(111) templates is compressive, although, due to relaxation, the stress we have measured is much lower than the theoretical value. In order to prevent the stress relaxation, step-graded AlGaN layers are introduced and a crack-free GaN epitaxial layer of thickness .1 mm is demonstrated. Under optimized growth conditions, the total layer stack, exceeding 2 mm in total, is kept under compressive stress, and the radius of the convex wafer bowing is as large as 119 m. The crystalline quality of the GaN layers is examined by highresolution x-ray diffraction (HR-XRD), and the full-width-at-half maximums (FWHMs) of the x-ray rocking curve (0002) v-scan and (ÿ1015) v-scan are 790 arc sec and 730 arc sec, respectively. It is found by cross-sectional transmission electron microscopy (TEM) that the step-graded AlGaN layers terminate or bend the dislocations at the interfaces.

A standard metallization scheme for the formation of Ohmic contacts on n-type GaN does exist. It has the following multilayer structure: Ti∕Al∕metal∕Au. Ti is known to extract N out of the GaN. This leaves a high density of N vacancies (donors) near the interface pinning the Fermi level. The created tunnel junction is responsible for an Ohmic contact behavior. Au is deposited as the final metal layer to exclude oxidation of the contact and the metal should limit the diffusion of Au into the layers below and vice versa. Al in the metallization scheme is known to improve the contact resistance, but the reason why has not been reported yet. We studied Ti and Ti∕Al contacts on GaN and AlGaN∕GaN as a function of annealing temperature by transmission electron microscopy. The role of Al in the metal multilayer, and of Al in the AlGaN on the Ohmic contact formation, has been determined. The latter result indicates that the standard metallization scheme for GaN cannot be simply transferred to AlGaN∕GaN structures.

High quality GaN layers with dislocation density of (3.0±0.5)×108∕cm2 have been grown on silicon(111) substrates using a combination of AlGaN intermediate layers and a SixNy interlayer. A smooth and fully coalesced layer was obtained by virtue of a high temperature growth process which accelerates coalescence and improves at the same time the crystalline quality. This was confirmed by high resolution x-ray diffraction showing a full width of half maximum of 415arcsec for the asymmetric (−2201) rocking curve.

AlGaN/GaN/AlGaN double heterostructure field-effect transistors (DHFET) with high breakdown voltage and low on-resistance were fabricated on silicon substrates. A linear dependency of the breakdown voltage on the buffer thickness and on the buffer Aluminium concentration was found. A breakdown voltage as high as 830 V and an on-resistance as low as 6.2 Ω·mm were obtained in devices processed on 3.7 µm buffer thickness. The gate–drain spacing was 8 µm and the devices did not have any field plates.

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AlGaN/GaN high electron mobility transistors (HEMTs) grown on 150 mm Si(111) substrates are reported in this work. The sheet resistance of the AlGaN/GaN HEMT structure is as low as 260 AE 3:4 /Ã. The electron mobility is in the range of 1560 -1650 cm 2 V À1 s À1 . The crack-free mirror-like wafers were obtained by using a simple AlGaN/AlN buffer. The mechanism for dislocation reduction in GaN above the AlGaN/AlN buffer is presented in this work. The dislocation density is around ð1:5 { 2:5Þ Â 10 9 /cm 2 . Some of the wafers were processed and a current density close to 1 A/mm was achieved. The maximum transconductance was 270 mS/mm and the on-state resistance was as low as 2.6 mm.