Andrei Vescan

Investigation of trapping effects in AlGaN/GaN/Si field-effect transistors by frequency dependent capacitance and conductance analysis

We report on frequency dependent capacitance and conductance analysis of the AlGaN/GaN/Si heterostructure field-effect transistors (HFETs) and Al2O3/AlGaN/GaN/Si metal-oxide-semiconductor HFETs (MOSHFETs) in order to investigate the trap effects in these devices. The capacitance of the HFETs exhibits significantly higher frequency dispersion than that of the MOSHFETs. Two different types of traps were found from the conductance analysis on both types of devices, fast with the time constant τ≅(0.1–1) μs and slow with τ=8 ms. The density of trap states evaluated on the HFETs was DT≅2.5×1012 and up to 1013 cm−2 eV−1 for the fast and slow traps, respectively. Analysis of the MOSHFETs yielded only slightly lower DT of the fast traps (≅1.5×1012 cm−2 eV−1), but nearly two orders of magnitude lower density of slow traps (≤4×1011 cm−2 eV−1) than those of the HFETs. This indicates an effective passivation of slow surface related traps but less influence on fast (probably bulk related) trapping states by applying an...

Recessed-Gate Enhancement-Mode AlGaN/GaN Heterostructure Field-Effect Transistors on Si with Record DC Performance

Enhancement-mode devices are in the centre of current research on group-III nitride transistors. The realisation of high-performance enhancement-mode transistors via gate recessing requires damage-free processing. We report on enhancement-mode AlGaN/GaN-on-Si heterostructure field-effect transistors (HFETs) fabricated with a damage-free digital etch technique. The threshold voltage (Vth) achieved is as high as +0.5 V. For AlGaN/GaN-on-Si HFETs, a record extrinsic transconductance (gm) of 420 mS/mm and a record maximum drain current Idmax of 500 mA/mm have been demonstrated. Furthermore, proper turn-off characteristics have been realised. Pulsed I–V characteristics reveal nearly no current collapse.

Study on quaternary AlInGaN/GaN HFETs grown on sapphire substrates

We report on AlInGaN/GaN heterostructure field effect transistors (HFETs) and the effect of different barrier material compositions. The analytical model for the interface charge in quaternary nitride heterostructures is described in detail and is applied in the calculation of the expected sheet carrier density. Experimental results from different lattice-matched AlInGaN/GaN heterostructures are presented and compared with the analytical predictions. Three heterostructures with AlInGaN barriers grown on sapphire substrates were processed and have been investigated. Each barrier layer was lattice-matched to GaN and the gallium content was 0.1, 0.15 and 0.2 at a barrier thickness of 13.5, 12.8 and 11.3 nm, respectively. Additionally, from these experiments, the basic trends for quaternary nitride Schottky barrier contacts are discussed. Finally, comprehensive dc characterizations have been performed. All devices had a gate length of 1 µm and exhibited a good transconductance of around 260 mS mm−1 at nearly the same current density level. An increase in threshold voltage as well as a decrease in gate leakage current for increasing GaN content has been observed. The nearly constant electron mobility in the range of 1700 cm2 V−1 s−1 at room temperature is within the best reported so far for HFETs with InN-containing barriers.

Dielectric function and optical properties of quaternary AlInGaN alloys

The optical properties of quaternary Alx Iny Ga1-x-yN alloy films with 0.16< x<0.64 and 0.02< y<0.13 are presented. The (0001)-oriented AlInGaN layers were grown by metal-organic vapor phase epitaxy on thick GaN/sapphire templates. High-resolution x-ray diffraction measurements revealed the pseudomorphic growth of the AlInGaN films on the GaN buffer. Rutherford backscattering and wavelength-dispersive x-ray spectroscopy analysis were used in order to determine the composition of the alloys. The ordinary dielectric function (DF) of the AlInGaN samples was determined in the range of 1–10 eV by spectroscopic ellipsometry (SE) at room temperature (synchrotron radiation: BESSY II). The sharp onset of the imaginary part of the DF defines the direct absorption edge of the alloys. At higher photon energies, pronounced peaks are observed in the DF indicating a promising optical quality of the material. These features are correlated to the critical points of the band structure (van Hove singularities). An analytica...

p-Channel Enhancement and Depletion Mode GaN-Based HFETs With Quaternary Backbarriers

Within the last years, III-nitride-based devices have been demonstrated with exceptional performance. There is, however, a severe lack of knowledge when it comes fabrication of p-channel devices. p-Channel heterostructure field-effect transistors (HFETs) could open the way for nitride-based complementary logic. Here, a comprehensive study of enhancement and depletion mode p-channel GaN/AlInGaN HFETs is performed. The influence of a highly p-doped GaN cap layer on device performance is investigated. Gate recessing and changes in composition of the backbarrier are analyzed. ON/OFF ratios of up to 108 and subthreshold swings of about 75 mV/decade are achieved.

First polarization-engineered compressively strained AlInGaN barrier enhancement-mode MISHFET

One current focus of research is the realization of GaN-based enhancement-mode devices. A novel approach for the realization of enhancement-mode behaviour is the utilization of polarization matching between the barrier and the GaN buffer. Yet, the utilization of a quaternary barrier combining polarization engineering together with a large conduction band offset has not been demonstrated so far. Here, epitaxially grown, compressively strained AlInGaN is applied as a nearly polarization-matched barrier layer on GaN resulting in enhancement-mode operation. The insulated-gate devices are fabricated gate-first with Al2O3 as gate dielectric. Passivated metal insulator semiconductor heterostructure field effect transistors yielded threshold voltages (Vth) of up to +1 V. The devices withstand negative and positive gate-biased stress and a positive Vth is maintained even after long-time negative bias stress.

The effect of the inversion channel at the AlN/Si interface on the vertical breakdown characteristics of GaN-based devices

GaN-on-Si transistors attract increasing interest for power applications. However, the breakdown behavior of such devices remains below theoretical expectations, for which the Si substrate is typically made responsible. In this work, the effect of the thickness of an aluminum nitride buffer layer on the vertical breakdown voltage, measured relative to a grounded silicon substrate, has been investigated. A voltage-polarity-dependent breakdown mechanism has been observed. It has been found that the breakdown in the positive bias voltage regime is initiated by carrier injection, for which the carriers originate from an inversion channel formed between the epitaxial layers and the p-silicon substrate. TCAD simulations have confirmed the proposed explanations, and suggest that appropriate modification of the electronic structure at the AlN/silicon interface could significantly improve the vertical breakdown voltage.

InAlN/GaN HEMTs on Sapphire Substrate With 2.9-W/mm Output Power Density at 18 GHz

In this letter, small- and large-signal measurements of an In_0.15Al_0.82N/AlN/GaN high-electron-mobility transistor (HEMT) grown on a sapphire substrate with a 225-nm T-shaped gate are described. A maximum dc current density of 1.2 A/mm and a peak extrinsic transconductance of 460 mS/mm are obtained. The device exhibits a current gain cutoff frequency (<i>F</i>_T) and a power gain cutoff frequency (<i>F</i>_MAX) of 52 and 120 GHz, respectively. At <i>V</i>_DS = 15 V, a continuous-wave output power density of 2.9 W/mm was achieved at 18 GHz with an associated power-added efficiency of 28% and a power gain of 15 dB. It is the best value ever reported from InAlN/GaN HEMTs grown on a sapphire substrate.

Threshold Voltage Engineering in GaN-Based HFETs: A Systematic Study With the Threshold Voltage Reaching More Than 2 V

One of the key challenges for the adoption of gallium nitride (GaN)-based heterostructure field effect transistors (HFETs) in power-switching applications is obtaining enhancement mode behavior. A large variety of methods have been applied to shift the threshold voltage Vth of HFETs. However, most of the time, approaches were discussed individually, neglecting the effects of combinations. Hence, in this paper, a comprehensive study of four different approaches to shift Vth well into the positive range is presented. We show the effects of different gate metallizations, of a backbarrier, of a gate oxide, and of a gate recess. Each approach is discussed individually, and special focus is on the insulator/semiconductor interface, which is apparently different with and without gate recess. The final device exhibits a Vth of +2.3 V, which is shown to be stable when applying OFF-state stress during dynamic characterization.

Fabrication of p-channel heterostructure field effect transistors with polarization-induced two-dimensional hole gases at metal-polar GaN/AlInGaN interfaces

Novel nitride-based heterostructures have been fabricated demonstrating two-dimensional hole gases as the basis for p-channel transistors. The carrier density in the 2DHG is adjusted between very high values of 2 × 1013 cm−2 and low values of 6 × 1011 cm−2 by the polarization difference, ΔP, between quaternary AlInGaN backbarriers and a GaN channel on top. Record mobilities for holes in GaN of 43 cm2 V−1 s−1 (median 30 cm2 V−1 s−1) are observed for a moderate 2DHG density of 1.3 × 1012 cm−2 (median 2.2 × 1012 cm−2).Heterostructures with different backbarrier compositions are processed to field effect transistors and show a systematic threshold voltage shift from positive to negative values according to the corresponding 2DHG density. It is shown for the first time that by appropriate polarization-engineering through changing the AlInGaN composition, both depletion and enhancement mode behaviour can be achieved for p-channel devices.Drain current densities |Id| above 40 mA mm−1 at a drain source voltage Vds of −10 V are achieved for heterostructures with high polarization differences, ΔP, between AlInGaN backbarrier and GaN channel. Reducing ΔP leads to decreasing on-state drain currents |Id| with a simultaneous reduction in off-state current. This results in very large on/off ratios of up to 108 for enhancement mode devices, demonstrating record performances and great potential for future applications.