HEMT Layers Research Articles

Silicon-based integrated passive device stack for III-V/Si monolithic 3D circuits operating on RF band

In this study, we demonstrated a silicon (Si)-based integrated passive device (IPD) stack to support III-V/Si monolithic 3D (M3D) ICs operating on the radio frequency (RF) band. The IPD stack was fabricated based on an 8-inch CMOS process line and integrated via M3D with an InGaAs HEMT layer. A process condition for a trap rich layer and a buried oxide layer in the IPD was established to simultaneously minimizing both the RF loss and wafer bowing. Through the process condition, the RF loss of the coplanar waveguides was −0.631 dB/mm at 30 GHz, lower than that of the CMOS foundry, and the wafer bowing of the stack was as low as −5.5 μm. The maximum quality factor of the inductors showed good values when compared to those of other CMOS foundry process-based inductors operating on the RF bands reported thus far. To obtain a compressive profile for the IPD stack, which is one of the most important requirements in advancing to wafer-to-wafer-level 3D bonding with the III-V active layer, a process method for the final IMD layer of the IPD was developed, resulting in a change from a tensile profile to a compressive profile for the IPD (corresponding wafer bowing value from −12.6 to + 10.7 μm).

Study of a MHEMT heterostructure with an In0.4Ga0.6As channel MBE-grown on a GaAs substrate using reciprocal space mapping

The crystallographic characteristics of the design elements of a metamorphic high-electron-mobility (MHEMT) heterostructure with an In0.4Ga0.6As channel are determined based on reciprocal space mapping. The heterostructure is grown by molecular beam epitaxy on the vicinal surface of a GaAs substrate with a deviation angle from the (001) plane of 2° and consists of a stepped metamorphic buffer containing six layers including an inverse step, a high-temperature buffer layer with constant composition, and active HEMT layers. The InAs content in the layers of the metamorphic buffer is varied from 0.1 to 0.48. Reciprocal space maps are constructed for the (004) symmetric reflection and (224)+ asymmetric reflection. It is found that the heterostructure layers are characterized both by a tilt angle relative to the plane of the (001) substrate and a rotation angle around the [001] axis. The tilt angle of the layer increases as the InAs concentration in the layer increases. It is shown that a high-temperature buffer layer of constant composition has the largest degree of relaxation compared with all other layers of the heterostructure.

Determination of Ga auto-incorporation in nominal InAlN epilayers grown by MOCVD

We report on the consistent measurement of gallium incorporation in nominal InAlN layers using various complimentary techniques, underpinned by X-ray diffraction. Nominal InAlN layers with similar growth conditions were prepared, and the change in unintended Ga content in the group III sublattice ranged from ∼24% to ∼12% when the total reactor flow rate was increased from 8000 to 24 000 standard cubic centimetres per minute. Ultra-thin InAlN/GaN HEMT layers were grown in a clean reactor to minimize Ga auto-incorporation, and measured using X-ray photoelectron spectroscopy and secondary ion mass spectrometry. The implications of Ga incorporation in InAlN layers within optoelectronic and power devices is discussed.

Windowed growth of AlGaN/GaN heterostructures on Silicon 〈111〉 substrates for future MOS integration

A novel windowed growth technique that allows for the exposure of a smooth silicon surface for MOS circuitry between AlGaN/GaN HEMT layers has been developed. A sacrificial oxide based dielectric stack is used to protect regions of the silicon surface before growth by ammonia-MBE. SIMS analysis shows elevated oxygen impurity incorporation in the stress-relief layer, but controllable levels in the C-doped GaN buffer. A Hall mobility of 1.37 × 103 cm2/Vs is measured in the AlGaN/GaN regions. AFM results show a RMS roughness of 0.12 nm on the silicon surface. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An AlGaN/GaN two‐color photodetector based on an AlGaN/GaN/SiC HEMT layer structure

AbstractThe optoelectronic properties of MSM diodes that are based on an AlGaN/GaN HEMT layer system are investigated as function of the applied bias. For low voltages, the two‐dimensional electron gas acts as a barrier for the depletion layer. Therefore, only the upper AlGaN barrier layer contributes to the photocurrent. In the high voltage regime the depletion region penetrates the GaN buffer that adds the spectral responsivity of GaN to the pure AlGaN behaviour in the low voltage regime. The ratio of the responsivities at 350 nm and 300 nm wavelength can be switched from 0.01 at 2.5 V to 0.8 at 4 V bias. This property makes the MSM‐2DEG a candidate for use as two‐color photodetector. Device fabrication uses standard HEMT processing steps, allowing integration in HEMT circuits without the need of sophisticated growth or etching techniques. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Comparison of laser-wavelength operation for drilling of via holes in AlGaN/GaN HEMTs on SiC substrates

By using a frequency-tripled Nd:YVO4 laser source (355 nm) for drilling through-wafer via holes in SiC substrates, we can reduce the surface contamination and achieve better smoothness inside the via holes compared to use of the more common 1064-nm Nd:YVO4 laser. The sheet and contact resistance of AlGaN/GaN HEMT layers grown on SiC substrates were similar after formation of vias by 355-nm laser drilling to those of the undrilled reference sample. By sharp contrast, 1064-nm laser drilling produces significant redeposition of ablated material around the via and degrades the electrical properties of the HEMT layers.

A Novel Two-Color Photodetector Based on an InAlAs–InGaAs HEMT Layer Structure

The spectral responsivity of an InAlAs-InGaAs metal-semiconductor-metal diode above a two-dimensional electron gas (2DEG) is investigated as a function of the applied bias. At low voltages, only the InAlAs layer above the 2DEG contributes to the photocurrent, while the InGaAs channel layer is activated at higher bias. This results in a voltage-dependent spectral response of the photodetector. The ratio of the responsivities at 1300 and 850 nm changes from 0.03- at 1-V to 0.44- at 1.6-V bias. This property makes the device a candidate suitable to detect and to separate optical information originated both from the GaAs (850 nm) and in the InGaAs (1300, 1550 nm)-based optoelectronic technology.

SURFACE STABILIZATION FOR HIGHER PERFORMANCE ALGAN/GAN HEMT WITH IN-SITU MOVPE SIN

ABSTRACTPassivation of AlGaN/GaN HEMT by SiN has been shown by several authors to improve device performance. In this paper we demonstrate a new method of passivating the top AlGaN layer, that leads to a drastic improvement of the device characteristics. On top of the AlGaN/GaN HEMT layer structures, a thin SiN layer is grown at high temperature in-situ, i.e. in the MOCVD reactor prior to unloading the HEMT epiwafers. The quality of the SiN is shown to be very high. As the AlGaN surface is protected against any air contamination, ohmic contacts deposited directly on the SiN layer exhibit better properties than on uncapped AlGaN layers. Transistor performance increases impressively: the source-drain dc current, for a positive gate voltage of 2 V, increases from 0.5 A/mm to 1.2 A/mm for similar AlGaN/GaN HEMT structures by depositing SiN in-situ. RF characteristics also show some improvement. The positive impact of in-situ SiN layer has different origins, some being intrinsically linked: surface protection against oxide formation, better ohmic contact formation, passivation of the surface states, reduction of AlGaN relaxation. All this leads to an increase of the 2DEG carrier concentration.

X-ray characterization of stacked InP/(InGa)As:C HBT and InP/(InGa)As HEMT layer sequences grown by LP-MOVPE using non-gaseous sources

Stacked layer sequences of InP-based HBT and HEMT are of interest for today's (OC-768) and tomorrows (OC-3072, 100 Gbit Ethernet) communication standards. The better low noise performance of the HEMT can be combined with the driver capability of the HBT while also a PIN-photodiode can be provided by the HBT base-collector-diode. However this results in a complex procedure for MOVPE growth process development. For a reliable fabrication of complex radio frequency (opto-)electronic circuit applications a quantitative InP process technology control is necessary starting at the level of device epilayer stacks. In this report the detailed characterization of stacked InP/(InGa)As:C HBT and InP/(InGa)As HEMT layer sequences is carried out by non-destructive X-ray analysis. Based on X-ray measurements in 004- and 002-reflection a detailed analysis of complex device layer stacks is purposed. As a result, an automatic calculation of layer parameters, e.g. thickness and composition is possible, reducing the turn-around time for the statistical process control (SPC).

AlGaN/GaN varactor diode for integration in HEMTcircuits

Fabrication and characterisation of metal-semiconductor-metal (MSM) diodes above an AlGaN/GaN HEMT layer system for varactor applications are reported. Device fabrication uses standard HEMT processing steps, allowing integration in HEMT circuits without the need of sophisticated growth or etching techniques. Capacitance-voltage measurements exhibit CMAX/CMIN ratios up to 100, tunable by the electrode geometry. These results exceed best values for published heterostructure varactor diodes. Fabrication of AlGaN/GaN HEMTs on the same layer system with identical technology prove the potential for monolithic integration.

AlGaN/GaN HEMT Optimization Using the RoundHEMT Technology

The electrical characterization of epitaxially grown HEMT layer systems for device fabrication is commonly performed by Hall measurements. However, the ultimate characterization of a HEMT layer system is the transistor device itself. The RoundHEMT concept meets the need for a device technology with few fabrication steps that allows a fast feedback to epitaxy while providing an evaluation of important electrical and also processing data. Even though nearly identical Hall data on structures with different thickness and doping concentration of the AlGaN layers suggest similar device properties, the RoundHEMTs resolve remarkable differences in device performance. The best layer structure was used to fabricate HEMTs with I DS = 700 mA/mm, f T = 35 GHz, and f max = 70 GHz for L G = 0.2 μm.

AlGaN/GaN HEMT Optimization Using the RoundHEMT Technology

The electrical characterization of epitaxially grown HEMT layer systems for device fabrication is commonly performed by Hall measurements. However, the ultimate characterization of a HEMT layer system is the transistor device itself. The RoundHEMT concept meets the need for a device technology with few fabrication steps that allows a fast feedback to epitaxy while providing an evaluation of important electrical and also processing data. Even though nearly identical Hall data on structures with different thickness and doping concentration of the AlGaN layers suggest similar device properties, the RoundHEMTs resolve remarkable differences in device performance. The best layer structure was used to fabricate HEMTs with I DS = 700 mA/mm, f T = 35 GHz, and f max = 70 GHz for L G = 0.2 μm.

InP-based mixed device (HEMT/HBT) technology onplanar substrate for high performance mixed-signal and optoelectronic circuits

The authors report the first successful demonstration of an integrated circuit process on planar InP substrates that incorporates a stacked single layer MBE growth of HEMTs and HBTs. In this process, the HBT layers are patterned and selectively wet etched to expose the HEMT layers after the MBE growth is completed. The authors report functional HEMTs and HBTs on the same wafer, with pertinent DC and microwave measurements.

High-speed digital and analog performance of low-noise integrated MSM-HEMT photoreceivers

The digital and analog performance of high-speed, monolithically integrated, low-noise metal-semiconductor-metal MSM-HEMT photoreceivers is reported. The MSM photodetectors were vertically integrated with the HEMT's using a stacked layer structure, with the HEMT layers beneath the MSM layers. The photoreceivers were implemented using a HEMT-based low-noise three-stage transimpedance amplifier. The amplifier transimpedance was 803 /spl Omega/, and the photoreceivers exhibited -3 dB bandwidths up to 7.2 GHz. Analog noise performance was characterized, and average input-referred noise current spectral densities of 6.5 and 7.3 pA/Hz/sup 1/2 / were found for receivers with 1and 1.5-/spl mu/m MSM inter-electrode spacings, respectively. The digital performance of the photoreceivers was examined at bit rates up to 10 Gb/s, and open eye patterns obtained. The measured sensitivity for a bit-error rate of 10/sup -9/ was found to be -16.9 and -13.1 dBm at 5 and 8 Gb/s, respectively, for photoreceivers with a 1.5 /spl mu/m inter-electrode spacing, and -10.7 dBm at 10 Gb/s for photoreceivers with a 1-/spl mu/m inter-electrode spacing. To the authors' knowledge, this is the first report of multigigabit measured sensitivities of MSM-HEMT photoreceivers on InP substrates.

Design, fabrication, and performance of high-speed monolithically integrated InAlAs/InGaAs/InP MSM/HEMT photoreceivers

A detailed study of the performance of monolithically integrated photoreceivers based on metal-semiconductor-metal (MSM) photodetectors (PD's) and HEMT's is undertaken. Two different stacked-layer approaches to integrating MSM-PD's with HEMT's are investigated, and the performance of detectors and HEMT's for each approach is compared. The structure with the MSM layers grown on top of the HEMT layers exhibited the best overall performance. A physics-based MSM model is developed and incorporated into microwave circuit design software; excellent agreement between circuit simulations and measured frequency responses is demonstrated. To evaluate the effects of MSM electrode geometry and detector area on photoreceiver performance, photoreceivers with MSM interelectrode spacings of 1, 1.5, and 2 /spl mu/m were fabricated and characterized. The electrical amplifier used in the photoreceivers is a two-stage, variable-transimpedance amplifier with a common-gate HEMT as the feedback path. By adjusting the DC voltage applied to the gate of this feedback HEMT, transimpedances ranging from 55.8 to 38.1 dB/spl Omega/, with corresponding -3 dB cutoff frequencies from 6.3 to 18.5 GHz, were measured experimentally. Excellent noise performance has been measured, with average input noise current spectral densities of 7.5, 8, and 12 pA/Hz/sup 1/2/ obtained for bandwidths of 6.3, 8, and 13.7 GHz, respectively. A packaged receiver has been tested at 5 Gb/s and an open eye pattern obtained.

Material and device properties of MOCVD grown [formula omitted] HEMTs

A systematic study of MOCVD grown InP based heterostructures and their application in HEMT technology is reported. The heterostructures are realized by Metal-Organic-Chemical-Vapor-Deposition (MOCVD) which is a well suited for production growth technique. The impact of growth temperature and AsH 3 flow rates on the material properties was investigated and optimized. To improve the device performance even further, Fe-doped InAlAs is also studied. By optimizing the growth conditions of InAlAs InGaAs heterostructure, we demonstrates high performance 1 μm gate length InAlAs InGaAs HEMTs which show f t of 60 GHz and f max of 120 GHz. By applying the same submicron technology on MOCVD HEMT layers grown by a commercial vendor as on Molecular-Beam-Epitaxy grown layers, we could demonstrate equivalent f t (180 GHz) and f max (220 GHz) performance.

Investigation of Passivation Effects in InP Hemt Layers

AbstractWe have investigated the influence of the deposition of a passivating silicon nitride layer on the electron transport in InAlAs/InGaAs/InAlAs quantum wells capped with a thin InGaAs layer. This structure is grown by Molecure Beam Epitaxy lattice-matched on InP substrates for the fabrication of High Electron Mobility Transistors. The InGaAs cap layer is added to reduce the ohmic contact resistance. In a first step to illuminate the complicated structural changes involved when passivating fully processed devices, we studied the influence of silicon nitride layer deposition on the virgin epilayers. The electrical transport properties were characterised by resistance, mobility, carrier concentration and magnetoresistance measurements, while Raman spectroscopy was used to probe the vibrational properties which are influenced by the free carriers. Because of the small penetration depth of the laser light mainly the lattice vibrations in the InGaAs ohmic contact layer were observed in the Raman spectra. Before passivation this thin highly doped cap layer is depleted, as confirmed by magnetoresistance measurements, that show a single conducting path in the two-dimensional electron gas formed in the InAlAs/InGaAs/InAlAs QW, and only bulk-like InGaAs phonons are observed in the cap layer. After passivation a shift to lower frequencies of the GaAs-like mode indicates a high electron concentration in the InGaAs cap. The results of the Raman analysis are in good agreement with the electrical transport data that show a considerably lower resistance after passivation. Magnetoresistance measurements on passivated layers confirmed the creation of an additional conducting path in the InGaAs cap layer introduced by the silicon nitride deposition process.

A study of wet chemical gate recess etching of pseudomorphic In0.15GaAs/GaAs HEMTS

During non-selective wet chemical gate recess etching of pseudomorphic In0.15GaAs/GaAs HEMTs, the rate of change of saturated drain current with etch time decreased suddenly to one-tenth of the initial rate when etching began. This reduced rate lasted for around 45 s of etching before the rate increased again. A simple model, based on Poisson's equation with a constant etch rate in all the layers of the HEMT structure, has been developed to explain the effect. This shows that the variation in the rate of change of drain current with etch time is related to the thickness and doping concentration of the layers in the HEMT structure. As will be explained, this has important ramifications for HEMT layer design.

MOCVD-grown InAlAs/InGaAs HEMTs with f T = 200 GHz

The performance of 0.15 mu m gate InAlAs/InGaAs HEMTs grown by MOCVD is reported. The HEMT layer structure exhibited excellent transport properties, indicating the high quality of the heterointerface. The devices fabricated using a T-gate process showed an f/sub T/ of 200 GHz and f/sub max/ of 230 GHz extrapolated from 26 GHz at -6dB/octave. These are the best speed performances of any MOCVD-grown FETs. The results clearly demonstrate that MOCVD is a powerful growth technique for materials for high-speed applications.

High performance MBE of (In) GaAs/AlGaAs heterostructures for HEMTs

High quality (In)GaAs/AlGaAs HEMT layers were grown with a long-term reproducibility of ±1%. The control of growth rates and doping levels is based on calibration layers and the predictable behaviour of the effusion sources. Lateral homogeneity of the heterostructures grown on 3 inch epiready substrates is about ±1% with respect to layer thickness, alloy composition and doping. Defect density is routinely below 100cm -2. Low noise GaAs/AlGaAs HEMTs (gate length 0.3μm) fabricated from these epilayers using optical lithography, selective dry etching and full dielectric passivation reveal excellent device characteristics (noise figures at 12 GHz and 0.60 dB (on wafer: gain 10 dB); 0.75 dB (in commercial ceramic package: gain 11 dB)).