The 1.6-kV AlGaN/GaN HFETs

The breakdown voltages in unpassivated nonfield-plated AlGaN/GaN HFETs on sapphire substrates were studied. These studies reveal that the breakdown is limited by the surface flashover rather than by the AlGaN/GaN channel. After elimination of the surface flashover in air, the breakdown voltage scaled linearly with the gate–drain spacing reaching 1.6 kV at 20$muhboxm$. The corresponding static ON-resistance was as low as 3.4$hboxmOmega cdot hboxcm^2$. This translates to a power device figure-of-merit$V_ BR^2/R_ ON = hbox7.5times hbox10^8 hboxV^2 cdot Omega^-1 hboxcm^-2$, which, to date, is among the best reported values for an AlGaN/GaN HFET.     

High-frequency noise in AlGaN/GaN HFETs

We present in this letter the benefits of GaN-based electronic devices for low-noise MMICs. A temperature-dependent two-temperature noise model for AlGaN/GaN HFETs is implemented on a wide range of bias conditions. This study enables to access the device high-frequency noise parameters, and allow a comparison of the noise performances with SiC and GaAs technologies.     

High-temperature performance of AlGaN/GaN HFETs and MOSHFETs

Performance of AlGaN/GaN HFETs and Al₂O₃/AlGaN/GaN MOSHFETs at the elevated temperatures up to 425 °C was investigated. Static output and transfer characteristics were measured and the saturation drain current, the peak transconductance and the series conductance as a function of temperature were evaluated. All these characteristic features of HFETs and MOSHFETs decreased with increased temperature. At 425 °C the devices exhibited ∼30% of their saturation drain current, peak transconductance and series conductance evaluated at room temperature. The device performance at elevated temperatures follows exactly the T^x dependence with a power x = −1.5. This indicates that the temperature dependence of the mobility of channel electrons due to phonon scattering is the predominant effect describing high-temperature performance of AlGaN/GaN HFETs and MOSHFETs.     

Nonlinear source resistance in high-voltage microwave AlGaN/GaN HFETs

Wide bandgap semiconductors are used to fabricate field-effect transistors with significantly improved RF output power compared to GaAs- and InP-based devices. Nitride-based heterostructure field-effect transistors can be biased at high drain voltages, up to and exceeding 100 V, which results in high RF output power. However, the operation of these devices at high drain bias introduces physical phenomena within the device that affect both dc and RF performance. In this study, the existence of a nonlinear source resistance due to space-charge limited current conditions is demonstrated and verified. Inclusion of the nonlinear source resistance in a physics-based device simulator produces excellent agreement between simulated and measured data. The nonlinear source resistance degrades RF performance and limits amplifier linearity.     

Improved RF modeling techniques for enhanced AlGaN/GaN HFETs

Modeling procedures of an AlGaN/GaN HFET that incorporate the effects of both a GaN cap layer and an AlN sub-buffer layer are presented. A single off-state measurement method to extract all eight parasitic elements of an enhanced HFET has been successfully applied. In addition, procedures to model the nonlinear drain-to-source current characteristics featuring a kink are described.     

Hot electron induced degradation of undoped AlGaN/GaN HFETs

This work presents the effects of hot electron stress on the degradation of undoped Al_0.3GaN_0.7/GaN power HFET’s with SiN passivation. Typical degradation characteristics consist of a decrease in the drain current and maximum transconductance, an increase in the drain series resistance, gate leakage and a subthreshold current. Degradation mechanism has been investigated by means of gate lag measurements (pulsed I-V) and current-mode deep level transient spectroscopy (DLTS). Stressed devices suffered from aggravated drain current slump (DC to RF dispersion) which indicates possible changes in surface charge profiles occurred during hot electron stress test. The DLTS was used to identify the trap creation by hot electron stress. The DLTS spectra of stressed device revealed the evidence of trap creation due to hot electron stress.     

High-temperature performance of AlGaN/GaN HFETs on SiC substrates

The performance results AlGaN-GaN Heterostructure Field Effect Transistors (HFETs) grown on SiC substrates are reported. The maximum transconductance of these devices was 142 mS/mm and the source-drain current was as high as 0.95 A/mm. The maximum dissipated DC power at room temperature was 0.6 MW/cm², which is more than three times higher than that in similar devices grown on sapphire. This high thermal breakdown threshold was achieved primarily due to the effective heat sink through the SiC substrate. These devices demonstrated stable performance at elevated temperatures up to 250°C. The source-drain current saturation was observed up to 300°C. The leakage current in the below threshold regime was temperature-activated with an activation energy of 0.38 eV     

Silicon Dioxide-Encapsulated High-Voltage AlGaN/GaN HFETs for Power-Switching Applications

In this letter, new approach in achieving high breakdown voltages in AlGaN/GaN heterostructure field-effect transistors (HFETs) by suppressing surface flashover using solid encapsulation material is presented. Surface flashover in III-Nitride-based HFETs limits the operating voltages at levels well below breakdown voltages of GaN. This premature gate-drain breakdown can be suppressed by immersing devices in high-dielectric-strength liquids (e.g., Fluorinert); however, such a technique is not practical. In this letter, AlGaN/GaN HFETs encapsulated with PECVD-deposited SiO₂ films demonstrated breakdown voltage of 900 V, very similar to that of devices immersed in Fluorinert liquid. Simultaneously, low dynamic ON-resistance of 2.43 mOmega ldr cm² has been achieved, making the developed AlGaN/GaN HFETs practical high-voltage high-power switches for power-electronics applications.     

Delta-doped AlGaN/AlN/GaN microwave HFETs grown by metalorganicchemical vapour deposition

The performance of an innovative delta-doped AlGaN/AlN/GaN heterojunction field-effect transistor (HFET) structure is reported. The epitaxial heterostructures were grown on semi-insulating SiC substrates by low-pressure metalorganic chemical vapour deposition. These structures exhibit a maximum carrier mobility of 1058 cm²/V s and a sheet carrier density of 2.35×10¹³ cm^-2 at room temperature, corresponding to a large n_s μ_n product of 2.49×10¹⁶ V s. HFET devices with 0.25 μm gate length were fabricated and exhibited a maximum current density as high as 1.5 A/mm (at V_G=+1 V) and a peak transconductance of g_m=240 mS/mm. High-frequency device measurements yielded a cutoff frequency of f_t≃50 GHz and maximum oscillation frequency f_max≃130 GHz     

Surface leakage currents in SiN/sub x/ passivated AlGaN/GaN HFETs

A novel guarded surface leakage test structure is used to isolate the surface and bulk leakage contributions to gate current in AlGaN/GaN HFETs. Passivation with various recipes of SiN/sub x/ always resulted in the commonly observed increase in gate leakage, which was found to be dominated by bulk leakage through the AlGaN. However, high temperature deposited SiN/sub x/ recipes gave a 1-2 orders reduction in surface leakage, whereas low temperature deposition gave an increase. Gate lag measurements were found to correlate closely with the surface leakage component, giving direct evidence that the key device problem of current slump is associated with current flow at the AlGaN surface.     

Recessed-gate structure approach toward normally off high-Voltage AlGaN/GaN HEMT for power electronics applications

A recessed-gate structure has been studied with a view to realizing normally off operation of high-voltage AlGaN/GaN high-electron mobility transistors (HEMTs) for power electronics applications. The recessed-gate structure is very attractive for realizing normally off high-voltage AlGaN/GaN HEMTs because the gate threshold voltage can be controlled by the etching depth of the recess without significant increase in on-resistance characteristics. With this structure the threshold voltage can be increased with the reduction of two-dimensional electron gas (2DEG) density only under the gate electrode without reduction of 2DEG density in the other channel regions such as the channel between drain and gate. The threshold-voltage increase was experimentally demonstrated. The threshold voltage of fabricated recessed-gate device increased to -0.14 V while the threshold voltage without the recessed-gate structure was about -4 V. The specific on-resistance of the device was maintained as low as 4 m/spl Omega//spl middot/cm/sup 2/ and the breakdown voltage was 435 V. The on-resistance and the breakdown voltage tradeoff characteristics were the same as those of normally on devices. From the viewpoint of device design, the on-resistance for the normally off device was modeled using the relationship between the AlGaN layer thickness under the gate electrode and the 2DEG density. It is found that the MIS gate structure and the recess etching without the offset region between recess edge and gate electrode will further improve the on-resistance. The simulation results show the possibility of the on-resistance below 1 m/spl Omega//spl middot/cm/sup 2/ for normally off AlGaN/GaN HEMTs operating at several hundred volts with threshold voltage up to +1 V.     

Comparison of electrical characteristics between AlGaN/GaN and lattice-matched InAlN/GaN heterostructure Schottky barrier diodes

Lattice-matched Pt/Au–In_0.17Al_0.83N/GaN hetreojunction Schottky barrier diodes (SBDs) with circular planar structure have been fabricated. The electrical characteristics of InAlN/GaN SBD, such as two-dimensional electron gas (2DEG) density, turn-on voltage, Schottky barrier height, reverse breakdown voltage and the forward current-transport mechanisms, are investigated and compared with those of a conventional AlGaN/GaN SBD. The results show that, despite the higher Schottky barrier height, more dislocations in InAlN layer causes a larger leakage current and lower reverse breakdown voltage than the AlGaN/GaN SBD. The emission microscopy images of past-breakdown device suggest that a horizontal premature breakdown behavior attributed to the large leakage current happens in the InAlN/GaN SBD, differing from the vertical breakdown in the AlGaN/GaN SBD.     

Submicrometer Copper T-Gate AlGaN/GaN HFETs: The Gate Metal Stack Effect

We demonstrate the realization of $sim$ $hbox{0.2}hbox{-}muhbox{m}$ T-shaped copper gate AlGaN/GaN heterojunction field-effect transistors (HFETs) and compare the performance achieved with Cu, Cu/Au, Ni/Cu, and Ni/Au gate metal stack configurations in otherwise nominally identical T-gate transistors that are fabricated on the same chip. Surprisingly, simply replacing the Ni/Au gate stack by a Cu metallization increases the $f_{T} times L_{G}$ product by up to 40%, yielding a current-gain cutoff frequency as high as 82 GHz without further modifications to the process. Cu-based metallizations reduce the gate leakage current by one to two orders of magnitude and also result in broader transconductance characteristics. The results call for a detailed investigation of gate metallurgy effects in the AlGaN/GaN-based HFETs.      

Origin of improved RF performance of AlGaN/GaN MOSHFETs compared to HFETs

In this paper, the influence of a 10-nm-thick silicon-dioxide layer, as a passivation or as a gate insulation, on the performance of heterojunction field-effect transistors (HFETs) and metal-oxide-semiconductor HFETs (MOSHFETs), based on an undoped AlGaN/GaN heterostructure on a SiC substrate, was investigated. Channel-conductivity results yield a nearly 50% increase of mobility in the MOSHFET samples compared to the unpassivated HFETs. This increase of the transport properties of the MOSHFET channel is confirmed by a similar 45% increase of the cutoff frequency, from 16.5 to 24 GHz. Hall measurements, however, show a 10% decrease of the mobility in the heterostructure with a SiO/sub 2/ top layer. In this paper, the superior performance of the MOSHFET transistor, in contradiction to the Hall results, is attributed to the screening of the Coulomb scattering of the charged surface defects by the gate-metallization layer.     

Extraction of Transport Dynamics in AlGaN/GaN HFETs Through Free Carrier Absorption

The importance of AlGaN/GaN heterostructure field-effect transistor (HFETs) in high-power high-frequency applications is now well established. However, detailed information on high-field mobilities, velocity–field relations, carrier temperature, and momentum and energy relaxation times are not available. In this paper we carry out theoretical simulations based on Monte Carlo techniques to show that transport dynamics can be effectively extracted through free carrier absorption. Using short pulses of infrared radiation, it is possible to obtain the velocity–field curve by fitting the absorption spectrum without heating the device. We show this by solving the classical transport equation and then verify the results through Monte Carlo simulations. With the model presented it would be possible to extract carrier dynamics from experimentally measured results. Our work suggests that free carrier absorption experiments on AlGaN/GaN HFETs would provide important transport information, which would be very useful in device design and modeling.     

Channel Temperature Determination in High-Power AlGaN/GaN HFETs Using Electrical Methods and Raman Spectroscopy

Self-heating in AlGaN/GaN HFETs was investigated using electrical analysis and micro-Raman thermography. Two typically employed electrical methods were assessed to provide a simple means of extracting average channel temperatures in devices. To quantify the accuracy of these electrical temperature measurements, micro-Raman thermography was used to provide submicron resolution temperature information in the source-drain opening of the devices. We find that electrical methods significantly underestimate peak channel temperatures, due to the fact that electrical techniques measure an average temperature over the entire active device area. These results show that, although electrical techniques can be used to provide qualitative comparisons between different devices, they have challenges for the accurate estimation of peak channel temperatures. This needs to be taken into account for lifetime testing and reliability studies based on electrical temperature measurements.     

Synthesis and characterization of p-type NiO films suitable for normally-off AlGaN/GaN HFETs application

p-type NiO films were prepared at different oxidizing temperatures in O₂ ambient for normally-off AlGaN/GaN HFETs application. The crystalline structure, electrical properties and band gap of NiO films are dependent upon temperatures. Compared with the conventional Ni-gated HFETs, NiO-gated HFETs present positively shifted threshold voltage and smaller gate leakage current, while the drain current density shows slightly degradation. Combining the recess structure and NiO gate, normally-off GaN HFETs was achieved with a threshold voltage of approximately 0.5 V. The band diagram of the NiO/AlGaN/GaN structure demonstrates that the p-type conductivity and large conduction band offset between NiO and GaN cause the lift-up potential, which result in 2DEG depletion and positive threshold voltage shift.     

Power Stability of AlGaN/GaN HFETs at 20 W/mm in the Pinched-Off Operation Mode

High power-added efficiency (PAE) (ap74%) and rf-power (20 W/mm) operation of Schottky and insulated-gate AlGaN/GaN heterostructure field-effect transistors (HFETs) is reported at 2 GHz. In the pinched-off mode of operation, the PAE increases from a value of 55% to 74% when the drain bias is changed from 35 to 60 V. While both the Schottky and the insulated HFETs show high powers and PAE values, only the insulated-gate devices are stable at 20-W/mm output powers during a 60-h continuous wave rf-stress test. Their power drop of less than 0.1 dB is much smaller than the 0.8-dB drop for identical geometry Schottky-gate HFETs. The superior stability of the insulated-gate HFETs is attributed to the low forward gate currents     

Analysis of DC–RF Dispersion in AlGaN/GaN HFETs Using RF Waveform Engineering

This paper describes how dc–radio-frequency (RF) dispersion manifests itself in AlGaN/GaN heterojunction field-effect transistors when the devices are driven into different RF load impedances. The localized nature of the dispersion in the $I$– $V$ plane, which is confined to the “knee” region, is observed in both RF waveform and pulsed $I$–$V$ measurements. The effect is fully reproduced using 2-D physical modeling. The difference in dispersive behaviors has been attributed to the geometry of a trap-induced virtual-gate region and the resulting carrier velocity saturation being overcome by punchthrough effects under high electric fields.      

Growth of AlGaN epitaxial layers and AlGaN/GaN superlattices by metal-organic chemical vapor deposition

Special features of metal-organic chemical vapor deposition of AlGaN epitaxial layers and AlGaN/GaN superlattices either in an Epiquip VP-50 RP research and development reactor (for a single wafer 2 in. in diameter) or in an AIX2000HT production-scale reactor (for up to six wafers 2 in. in diameter) are stud-ied. It is found that the dependence of the aluminum content in the solid phase on the trimethylaluminum (TMA) flux in a reactor levels off; this effect hinders the growth of the layers with a high aluminum content in both types of reactors and is more pronounced in the larger reactor (AIX2000HT). Presumably, this effect is a consequence of spurious reactions in the vapor phase and depends on the partial pressure of TMA in the reactor. The aluminum content in the layers can be increased not only by reducing the total pressure in the reactor but also by increasing the total gas flow through the reactor and reducing the trimethylgallium flux. The approaches described above were used to grow layers with a mole fraction of AlN as large as 20% in the AIX2000HT production-scale reactor at a pressure of 400 mbar (this fraction was as large as 40% at 200 mbar). AlGaN layers with the entire range of composition were grown in the Epiquip VP-50 RP reactor.