The influence of a substrate voltage on the dc characteristics of an AlGaN/GaN high electron mobility transistor ͑HEMT͒ on silicon ͑111͒ substrate is profited to investigate traps that are located between the substrate and the two-dimensional electron gas channel. The transient of the drain current after applying a negative substrate voltage is evaluated in the temperature range from 30 to 100°C. With this method, known as backgating current deep level transient spectroscopy, majority carrier traps with activation energy of 200 meV as well as minority carrier traps at 370 meV are identified. The experiments are performed on completed HEMTs, allowing the investigation of the influence of device fabrication technology.Sapphire and SiC are commonly used as substrate materials for GaN based electronic and optoelectronic devices. Recently Si has been found as a useful alternative because of its low cost and good thermal conductivity. AlGaN/GaN/Si high electron mobility transistors ͑HEMT͒ with unity current gain frequencies comparable to those known for devices using sapphire or SiC substrates have been reported. 1 In contrast to sapphire the electrical conductivity of the Si substrate allows the application of a voltage at the backside of the devices through a substrate contact. This provides the control of backgating to study the physical properties of the device and of the material ͑defect states, trapping effects͒. The backgating effect is well known from GaAs based metalsemiconductor field effect transistors ͑MESFET͒ and HEMTs. 2 The junction between substrate and buffer creates a depletion layer that controls the transistor channel in the form of a gate from the backside. This effect can be used to control the drain current by a substrate voltage. The backgating effect is also one origin of the very high optoelectronic responsivity of MESFETs and HEMTs. 3 For HEMTs the backgating depletion region and therefore the influence of the substrate voltage is limited to the region below the twodimensional electron gas ͑2DEG͒. Therefore, all related effects have their origin in the bulk without contribution of surface effects. Uren et al. 4 have profited from the influence of a backgating voltage on the pinch-off voltage of an AlGaN/GaN HEMT on a conducting SiC substrate to investigate the electrically active centers in the GaN buffer. They have shown that the backgating depletion layer of the AlGaN/GaN HEMTs extends from the 2DEG into the buffer and is not located between substrate and buffer. Therefore, the observed effects do not originate from traps in the lower, more defective part of the buffer, but they are due to traps near the heterointerface. In this work we use backgating current deep level transient spectroscopy ͑DLTS͒ 5 to investigate the activation energy of traps in the buffer of an AlGaN/GaN HEMT on a Si substrate. Figure 1 shows the schematic band diagram of the substrate biased device.The device was fabricated using an AlGaN/GaN heterostructure grown by metalorganic vapor phase epitaxy on a 2 in. ͑111͒-o...

AlGaN/GaN HEMTs on silicon substrates have been fabricated and their static and small-signal RF characteristics investigated. The AlGaN/GaN material structures were grown on (111) p-Si by LP-MOVPE. Devices exhibit a saturation current of 0.91 A/mm, a good pinchoff and a peak extrinsic transconductance of 122 mS/mm. A unity current gain frequency of 12.5 GHz and max =0.83wereobtained.Thehighestsaturationcurrentreported so far, static output characteristics of up to 20 V and breakdown voltage at pinchoff higher than 40 V demonstrate that the devices are capable of handling 16 W/mm static heat dissipation.

We report on fabrication and measurement of photomixers based on nitrogen-ion-implanted GaAs. We used energies of 500 keV, 700 keV, and 880 keV to implant N + ions into GaAs substrates with an ion concentration of ϳ3 ϫ 10 12 cm −2 . The resulting material exhibited 110 fs carrier lifetime due to implantation-induced defects. Our photomixers were fabricated as metal-semiconductor-metal devices, placed at the feed point of a broadband antenna. Optoelectronic measurements were performed in the wavelength range between 350 nm and 950 nm. In comparison to their counterparts ͑photomixers fabricated on low-temperature-grown GaAs͒ the N + -implanted GaAs photomixers exhibit improvements on both the output power and responsivity. A maximal responsivity of above 100 mA/ W was achieved and we did not observe any dependence of the mixer cut-off frequency on the bias voltage. These characteristics make N + -implanted GaAs the material of choice for efficient optoelectronic photomixers.

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PACS: 71.55.Eq; 73.20.Hb; 85.30.De Deep-level defects and surface states are supposed to be responsible for the limitation of AlGaN/GaN high electron mobility transistor (HEMT) performance. In order to investigate the influence of these traps, photoionization spectroscopy was used to study doped and undoped HEMTs grown on sapphire in different metalorganic vapour-phase epitaxy reactors. This measurement technique is based on the optical reversion of the current collapse and it allows one to determine photoionization cross-sections of the participating traps. For doped and undoped HEMTs nearly the same two defect levels with excitation energies of 3.2 eV and 2.9 eV were determined. By varying the source-gate voltage it was found that the photoionization cross-section is reduced for positive gate bias, i.e. the virtual gate on the gate-drain access region is partially neutralized due to the removal of trapped electrons from surface states.

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