Heterojunction Field-effect Transistor Structures Research Articles

Alloy disorder scattering limited mobility of two-dimensional electron gas in the quaternary AlInGaN/GaN heterojunctions

Nitride heterojunction field effect transistors (HFETs) with quaternary AlInGaN barrier layers have achieved remarkable successes in recent years based on highly improved mobility of the two-dimensional electron gases (2DEGs) and greatly changed AlInGaN compositions. To investigate the influence of the AlInGaN composition on the 2DEG mobility, the quaternary alloy disorder (ADO) scattering to 2DEGs in AlInGaN/GaN heterojunctions is modeled using virtual crystal approximation. The calculated mobility as a function of AlInGaN alloy composition is shown to be a triangular-scarf-like curved surface for both cases of fixed thickness of AlInGaN layer and fixed 2DEG density. Though the two mobility surfaces are quite different in shape, both of them manifest the smooth transition of the strength of ADO scattering from quaternary AlInGaN to ternary AlGaN or AlInN. Some useful principles to estimate the mobility change with the Al(In,Ga)N composition in Al(In,Ga)N/GaN heterojunctions with a fixed 2DEG density are given. The comparison between some highest Hall mobility data reported for AlxGa1−xN/GaN heterojunctions (x=0.06~0.2) at very low temperature (0.3~13K) and the calculated 2DEG mobility considering ADO scattering and interface roughness scattering verifies the influence of ADO scattering. Moreover, the room temperature Hall mobility data of Al(In,Ga)N/AlN/GaN heterojunctions with ADO scattering eliminated are summarized from literatures. The data show continuous dependence on Hall electron density but independence of the Al(In,Ga)N composition, which also supports our theoretical results. The feasibility of quaternary AlInGaN barrier layer in high conductivity nitride HFET structures is demonstrated.

GaN Power Transistors on Si Substrates for Switching Applications

In this paper, GaN power transistors on Si substrates for power switching application are reported. GaN heterojunction field-effect transistor (HFET) structure on Si is an important configuration in order to realize a low loss and high power devices as well as one of the cost-effective solutions. Current collapse phenomena are discussed for GaN-HFETs on Si substrate, resulting in suppression of the current collapse due to using the conducting Si substrate. Furthermore, attempts for normally off GaN-FETs were examined. A hybrid metal-oxide-semiconductor HFET structure is a promising candidate for obtaining devices with a lower on-resistance (Ron) and a high breakdown voltage (Vb).

Effect of lattice mismatch on gate lag in high quality InAlN/AlN/GaN HFET structures

AbstractGrown lattice matched to GaN, InAlN‐based heterojunction field effect transistors (HFETs) are promising due to the relatively large band discontinuity at the interface and lack of misfit strain. Despite the recent progress in the growth, there still exists some questions as to the true lattice matching condition of InAlN to GaN due to discrepancies in the value of the lattice parameters of the InN binary, as well as the literature value of the InAlN bowing parameters. In order to address this, we used the gate lag as a supplementary measurement to verify lattice matching to the underlying GaN, as strain‐free layers would not have piezoelectric charge at the surface, which would be one source of lag. We observe very low lag for a nearly lattice matched barrier, and a marked increase as the composition deviates from the lattice matched condition. Additionally, FETs fabricated on a nearly matched layer boast a maximum drain current of over 1.5 and ∼2.0 A/mm and transconductances of ∼275 and ∼300 mS/mm at DC and in pulsed modes, respectively, and a cutoff frequency of 15.9 GHz (an fT*LG product of 10.3) for a gate length of 0.65 µm.

Non‐polar cubic AlGaN/GaN HFETs grown by MBE on Ar+ implanted 3C‐SiC (001)

AbstractThe growth of cubic group III‐nitrides is a direct way to eliminate polarization effects, which inherently limit the fabrication of normally‐off heterojunction field‐effect transistors (HFETs) in GaN technology. HFET structures were fabricated of non‐polar cubic AlGaN/GaN hetero layers grown by plasma assisted molecular beam epitaxy (MBE) on free standing 3C‐SiC (001). The electrical insulation of 3C‐SiC was realised by Ar+ implantation before c‐AlGaN/GaN MBE. The structural properties of the epilayers were studied by high‐resolution x‐ray diffraction (HRXRD). HFETs with normally‐off and normally‐on characteristics were fabricated of cubic AlGaN/GaN. Capacitance‐voltage (CV) characteristics of the gate contact were performed to detect the electron channel at the c‐AlGaN/GaN hetero interface. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Proposal and simulated results of a normally off AlGaN/GaN HFET structure with a charged floating gate

AbstractA normally off AlGaN/GaN Hetero‐junction Field Effect Transistor (HFET) structure is proposed. Two dimensional electron gas (2DEG) of more than 1 × 1013 cm–2, which a AlGaN/GaN HFET generally has, can be compensated completely by the negative charge in a floating gate or in a SiO2 layer on the AlGaN. Computer simulation showed that the threshold voltage was shifted from ‐3 V to ‐15 V with 20 nm SiO2 layer between the control gate and the AlGaN layer, but it was also shifted to +5V with 5 × 1013 cm–2 electrons (negative charge) in the middle of the SiO2 layer (a floating gate) or minus ions in the SiO2 layer itself. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

High-sensitivity UV phototransistor with GaN/AlGaN/GaN gate epi-structure

A highly sensitive UV phototransistor was developed using AlGaN/GaN heterojunction field effect transistor (HFET) technology. The phototransistor consisted of a thin undoped GaN layer added on top of a conventional AlGaN/GaN HFET structure. In such a structure, holes generated in the surface i-GaN layer are accumulated at the GaN/AlGaN interface, and they modulate the electrons in the FET channel. Due to this charge modulation effect, high photocurrent can be obtained. In the structure under study, hole life time was so long that a hole-evacuating refresh operation was required for fast response and linear sensitivity. When irradiated with UV light of 360 nm, a device with 6 × 10–4 mm2 active area produced two hundred times higher photocurrent than a commercial Si UV diode with a much larger area. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Interfacial chemistry and energy band line-up of pentacene with the GaN (0 0 0 1) surface

The chemical nature of the GaN surface combined with the bulk and surface electronic structure opens new potential application areas for this material. The nature of specific organic-GaN was developed for two cases in which the surface electronic structure of such interfaces was developed and utilized. The band alignment between n-GaN and pentacene was determined for surfaces prepared through reaction with HCl. The energy band offset was estimated through combined X-ray photoelectron spectroscopy and ultraviolet photoemission measurements. XPS measurements indicated that there was no interfacial chemical reaction. The measured valence band offset between the n-GaN and the vapor-deposited pentacene was estimated to be greater than 2 eV providing a favorable band-offset for hole-injection from the GaN layer into pentacene. The surface of a AlGaN/GaN heterojunction field effect transistor (HFET) structures was also functionalized by an adsorbed hemin layer and was shown to be sensitive to the presence of reactive species such as NO. The HFET structure provides enhanced sensitivity to changes in the surface electric field, altered through the adsorption of chemically active species either directly on the surface or through the interactions with surface functionalization.

Complex dielectric function of AlGaN/GaN/InGaN heterojunction structures

Theoretical calculations of the complex dielectric function of AlGaN/GaN heterojunction field-effect transistor (HFETs) structures and InGaN/GaN single quantum wells (SQWs) are presented. The model is based on a self-consistent calculation of the band structure, taking into account the spontaneous and piezoelectric polarizations and the presence of a two-dimensional electron gas (2DEG) at the interface. The calculated results show that the large built-in electric field at the heterojunction smears out any fine structures in the dielectric function that would ordinarily be expected from transitions between the valence band edge and the confined electron states. For HFET structures, two broad features are seen in the calculated spectra, one due to the GaN layer and the other to the AlGaN barrier. There are no features due to the confined states in the 2DEG. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimization of GaN Channel Conductivity in AlGaN/GaN HFET Structures Grown by MOVPE

ABSTRACTOptimization of GaN channel conductivity in AlGaN/GaN Heterojunction Field Effect Transistor (HFET) structures was performed using High Resistivity (HR) GaN templates grown by Metal-organic Vapor Phase Epitaxy (MOVPE). The GaN sheet resistance was tuned using final nucleation layer (NL) annealing temperature. Using an annealing temperature of 1033°C, GaN with sheet resistance of 10 Ω/sq was achieved, comparable to that of Fe-doped GaN. X-Ray Diffraction (XRD) and Photoluminescence (PL) analysis show that the high resistance GaN is achieved due to compensating acceptor levels introduced through edge-type threading dislocations. XRD analysis also shows optimization of annealing temperature provided a means to maximize GaN sheet resistance without significantly degrading material quality. AlGaN/GaN HFET layers grown using HR GaN templates gave surface and interface roughness of 14 and 7 Å, respectively. The 2DEG Hall mobility and sheet charge of HFETs grown using HR GaN templates was comparable to similar layers grown using unintentionally doped (UID) GaN templates.

AlGaN/GaN microwave HFET including a thin AlN carrier exclusion layer

A study of the effect of the insertion of a thin AlN exclusion layer between the AlGaN and GaN buffer layer in microwave hetero-junction field effect transistor structures grown by MOVPE on sapphire and SI-SiC substrates is presented. A dramatic improvement in carrier drift mobility is observed and we present evidence from CV analysis that this improvement is associated with reduced penetration of the 2D electron gas into the AlGaN. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Phonon modes of GaN/AlN heterojunction field-effect transistor structures grown on Si(111) substrates

Phonon modes of GaN/AlN heterojunction field-effect transistor (HFET) structures were investigated using Fourier-transform infrared spectroscopy. The HFET structure was grown on Si(111) substrate with AlN-based buffer layers. The phonon modes were also investigated in structure without AlGaN cap layer. The phonon mode spectra were obtained at the normal incident, waveguide, and Brewster’s angle configurations. Several vibrational frequencies were observed and found to be strongly dependent on the angle of the incident light. In particular, a phonon frequency of ∼734 cm−1 was observed only in the spectra when the samples are in the Brewster’s angle configuration. Moreover, a phonon mode was observed at 880 cm−1 in both waveguide and Brewster’s angle configurations. Additionally, a phonon absorption band is observed around 576 cm−1 , which appears to be composed of two modes, was redshifted to ∼550 cm−1 when the spectra were recorded in the waveguide configuration.

Carrier-removal rate and mobility degradation in heterojunction field-effect transistor structures

In this paper, we report experimental results and theoretical investigations of neutron irradiation-induced carrier-removal rate and mobility degradation in AlGaAs/GaAs heterojunction field-effect transistor structures. The measured two-dimensional (2-D) carrier removal rate of /spl sim/6.0 /spl times/ 10/sup -3/ is found to be consistent with a net (volume) introduction rate of /spl sim/20 cm/sup -1/ acceptor-like defects in the GaAs layer. Radiation-induced acceptors in the GaAs layer have the most significant effect on the 2-D electron concentration, whereas the acceptors in AlGaAs layer have negligible effect for neutron fluence up to 5 /spl times/ 10/sup 14/ cm/sup -2/. The measured 77-K mobility degradation, which is very sensitive to ionized impurity scattering, however, suggests that the introduction rate of the combined donor-like and acceptor-like defects is almost an order of magnitude higher (200 cm/sup -1/). The 300-K mobility, which is dominated by polar-optic phonon scattering, shows only marginal degradation.

Study of contact formation in AlGaN/GaN heterostructures

The contact formation of Ti/Al and Ti metallization on AlGaN/GaN heterojunction field effect transistors (HFET) was investigated. It was found that ohmic contact formation is related to the low work function of the Ti contacting layer and the formation of a TiN phase at the Ti/nitride interface. Contact resistance as low as 1 Ω mm or less can be obtained on HFET samples with a nsμ product of ∼0.8×1016/V s and on n-GaN with a carrier concentration of 1.5×1018/cm3. Ti/Al bilayer contact scheme is superior to Ti-only contact due to a surface Al3Ti layer in the bilayer contact, which may reduce the oxidation problem when annealed in N2 at high temperatures. Preannealing the HFET samples at 850 °C for 1 h in N2 appears to improve the ohmic contact in general, but not always observed. Our results indicate that Ti/Al contact scheme yields sufficiently low contact resistance on HFET structures for microwave applications.

Theoretical study of tunneling current in the access region of various heterojunction field-effect transistor structures

The source resistance of a heterojunction field-effect transistor (HFET), whose reduction is mandatory for high-performance devices, consists of an ohmic contact resistance and an access resistance. The access region is located between the geometrical source and the geometrical source side of the gate contact. By means of a quantum-mechanical modeling program, the effect of changes in layer structure in the access region of a HFET is studied. A new heterojunction structure using a Si planar doped layer is designed to improve the linearity and reduce the access resistance by more than ten times for a specific transistor layout. Thanks to the higher sensitivity of the modeling program to structural information, the contribution of the tunneling current and the change of equilibrium as a function of temperature is investigated.

Selective Implantation Patterning and MBE Regrowth for Integration of Mm-Wave Application Heterojunction Devices

ABSTRACTA technology based on a combination of selective implantation for lateral patterning and a following large area regrowth by molecular beam epitaxy (MBE) is introduced. The technology allows the monolithic integration of devices with different vertical layer sequences on s.i. GaAs substrates in a quasi planar way, e.g. a heterojunction field-effect-transistor (HFET) with a Schottky diode or a MESFET with a Schottky diode. Selective high resistive (>109 Ω/sq) and highly conducting (<20Ω/sq) buried layers are fabricated by high temperature stable oxygen and silicon implantation, respectively. The following MBE regrowth of epitaxial layers yields to an excellent surface morphology. HFET structures with pseudomorphic AlGaAs/InGaAs/GaAs heterostructures grown on implanted substrates exhibit a strong photoluminescence response. Carrier densities up to 2.0×1012cm−2 (1.6×1012cm−2) and Hall mobilities of 6150 cm2/Vs (17700 cm2/Vs) at 300 K (80 K) in the dark are achieved. The material quality is drastically improved by substrate preparation and in-situ cleaning prior to MBE regrowth.

Demonstration of high performance heterojunction field effect transistor in InAlAs/InGaAs/InAlGaAs/InP material system

The heterojunction field effect transistor (HFET) is demonstrated for the first time in InAlAs/InGaAs/InAlGaAs/InP material system using molecular beam epitaxy (MBE). A tungsten gate selfaligned HFET structure was made by ion implantation and rapid thermal annealing. The 1.0 μm self-aligned gate HFET exhibited room temperature transconductance of 490 mS/mm with cutoff frequency of 9 GHz.