Analysis of current collapse effect in AlGaN/GaN HEMT: Experiments and numerical simulations

In this work, current collapse effects in AlGaN/GaN HEMTs are investigated by means of measurements and two-dimensional physical simulations. According to pulsed measurements, the used devices exhibit a significant gate-lag and a less pronounced drain-lag ascribed to the presence of surface/barrier and buffer traps, respectively. As a matter of fact, two trap levels (0.45 eV and 0.78 eV) were extracted by trapping analysis based on isothermal current transient. On the other hand, 2D physical simulations suggest that the kink effect can be explained by electron trapping into barrier traps and a consequent electron emission after a certain electric-field is reached.     

A 24 W Ku band GaN based power amplifier with 9.1 dB linear gain

A Ku-band power amplifier is successfully developed with a single chip 4.8 mm AlGaN/GaN high electron mobility transistors (HEMTs). The AlGaN/GaN HEMTs device, achieved by E-beam lithography г-gate process, exhibited a gate-drain reverse breakdown voltage of larger than 100 V, a cutoff frequency of f_T=30 GHz and a maximum available gain of 13 dB at 14 GHz. The pulsed condition (100 μs pulse period and 10% duty cycle) was used to test the power characteristic of the power amplifier. At the frequency of 13.9 GHz, the developed GaN HEMTs power amplifier delivers a 43.8 dBm (24 W) saturated output power with 9.1 dB linear gain and 34.6% maximum power-added efficiency (PAE) with a drain voltage of 30 V. To our best knowledge, it is the state-of-the-art result ever reported for internal-matched 4.8 mm single chip GaN HEMTs power amplifier at Ku-band.     

Study on mobility enhancement in MOVPE-grown AlGaN/AlN/GaN HEMT structures using a thin AlN interfacial layer

Al_0.26Ga_0.74N/AlN/GaN high-electron-mobility transistor (HEMT) structures with AlN interfacial layers of various thicknesses were grown on 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy, and their structural and electrical properties were characterized. A sample with an optimum AlN layer thickness of 1.0 nm showed a highly enhanced Hall mobility (μ_Hall) of 1770 cm²/Vs with a low sheet resistance (ρ_s) of 365 Ω/sq. (2DEG density n_s = 1.0 × 10¹³/cm²) at room temperature compared with those of a sample without the AlN interfacial layer (μ_Hall = 1287 cm²/Vs, ρ_s = 539 Ω/sq., and n_s = 0.9 × 10¹³/cm²). Electron transport properties in AlGaN/AlN/GaN structures were theoretically studied, and the calculated results indicated that the insertion of an AlN layer into the AlGaN/GaN heterointerface can significantly enhance the 2DEG mobility due to the reduction of alloy disorder scattering. HEMTs were successfully fabricated and characterized. It was confirmed that AlGaN/AlN/GaN HEMTs with the optimum AlN layer thickness show superior DC properties compared with conventional AlGaN/GaN HEMTs.     

Study of the leakage current suppression for hybrid-Schottky/ohmic drain AlGaN/GaN HEMT

The leakage current suppression mechanism in AlGaN/GaN High Electron Mobility Transistors (HEMTs) is investigated. It is known that leakage current can cause severe reliability problems for HEMT devices and conventional AlGaN/GaN HEMT devices suffer from detrimental off-state drain leakage current issues, especially under high off-state drain bias. Therefore, a leakage current suppression technique featuring hybrid-Schottky/ohmic-drain contact is discussed. Through the 2-zones leakage current suppression mechanism by the hybrid-Schottky/drain metal including the shielding effect of the rough ohmic-drain metal morphology and the drain side electric field modulation, AlGaN/GaN HEMT featuring this novel technique can significantly enhance the leakage current suppression capability and improve the breakdown voltage. An analytical method using loop-voltage-scanning is proposed to illustrate the optimization procedure of the hybrid-Schottky/ohmic drain metallization on leakage current suppression. Through the comparison of the loop leakage current hysteresis of conventional ohmic drain HEMT and hybrid-Schottky/ohmic drain, the leakage current suppression mechanism is verified through the leakage current considering surface acceptor-like trap charging/discharging model. Device featuring the hybrid-Schottky/ohmic drain technique shows an improvement in breakdown voltage from 450 V (with no Schottky drain metal) to 855 V with a total drift region length of 9 μm, indicating enhanced off-state reliability characteristics for the AlGaN/GaN HEMT devices.     

Drain-to-gate field engineering for improved frequency response of GaN-based HEMTs

We report on a novel approach for designing high-frequency AlGaN/GaN HEMTs based on gate-drain field engineering. This approach uses a drain-connected field controlling electrode (FCE). The devices with gate-to-FCE separation of 0.5–0.7 μm exhibit much smaller frequency behavior degradation with drain bias at least up to 30 V and yield RF gain and output power improvement up to ～2 times compared to conventional devices. These results show that the FCE is a powerful technique of improving the high-frequency, high power performance of GaN HEMTs at high drain biases.     

Charge trapping related channel modulation instability in P-GaN gate HEMTs

In this paper, a study of the channel modulation instability of commercial p-GaN gate HEMTs is presented. During the gate-voltage stress test, substantial R_DS(ON) variations up to 78 mΩ (93.8%) were observed. It is found that the p-GaN/AlGaN/GaN gate structure enables the injection of holes and electrons, which can be captured by the donor/acceptor-like traps located in the AlGaN layer. Therefore, the trapped holes and electrons concurrently modulate the channel conductivity, resulting in R_DS(ON) variations. Device simulation was performed to help explain the mechanism from the perspective of energy band. In addition, results reveal that with the recommended working gate-voltage stress V_GS = 7 V, the on-state resistance, the threshold voltage and the off-state drain to source leakage current vary up to 8 mΩ (16.3%), 0.2 V (14.8%) and 12.8 μA (42.66%) within 1 h, respectively, which could raise reliability issues for the power electronics applications of p-GaN gate HEMTs.     

Electron transport in passivated AlGaN/GaN/Si HEMTs

AlGaN/GaN/Si high electron mobility transistors (HEMTs) grown by molecular beam epitaxy are investigated using direct-current and radio-frequency measurements. As has been found, the maximum of drain current achieves 881 mA/mm with an extrinsic current gain cutoff frequency of 37 GHz for a 0.25 µm gate length. Pulsed characteristics also showed a reduction of trapping centers that improves the quality of the epilayers.     

Identification of an RF degradation mechanism in GaN based HEMTs triggered by midgap traps

Quantitative defect spectroscopy was performed on low gate leakage operational S-band GaN HEMTs before and after RF accelerated life testing (ALT) to investigate and quantify potential connections between the evolution of observed traps and RF output power loss in these HEMTs after stressing. Constant drain current deep level transient spectroscopy and deep level optical spectroscopy (CI_D-DLTS and CI_D-DLOS, respectively) were used to interrogate thermally-emitting traps (CI_D-DLTS) and deeper optically-stimulated traps (CI_D-DLOS) so that the entire bandgap can be probed systematically before and after ALT. Using drain-controlled CI_D-DLTS/DLOS, with which traps in the drain access region are resolved, it is found that an increase in the concentration of a broad range of deep states between E_C–1.6 to 3.0 eV, detected by CI_D-DLOS, causes a persistent increase in on-resistance of ~ 0.22 Ω-mm, which is a likely source for the 1.2 dB reduction in RF output power that was observed after stressing. In contrast, the combined effect of the upper bandgap states at E_C–0.57 and E_C–0.72 eV, observed by CI_D-DLTS, is responsible for only ~ 10% of the on-resistance increase. These results demonstrate the importance of discriminating between traps throughout the entire bandgap with regard to the relative roles of individual traps on degradation of GaN HEMTs after ALT.     

Development and characterization of the thermal behavior of packaged cascode GaN HEMTs

This study investigates the heat generation behavior of packaged normally-on multi-finger AlGaN/GaN high electron mobility transistors (HEMTs) that are cascoded with a low-voltage MOSFET (LVMOS) and a SiC Schottky barrier diode (SBD). By foremost carrying out electro-thermal simulation and related thermal measurements with infrared thermography and Raman spectroscopy for basic 5 mm GaN HEMTs, the location of hot spot in operating device can be obtained. Based on the outcome, further packaged cascode GaN HEMT is analyzed. A hybrid integration of the GaN-HEMT, LVMOS, and SiC SBD are assembled on a directly bonded copper (DBC) substrate in the four-pin metal case TO-257 package. The metal plate is used as both the source terminal and heat sink. The analytical results of thermal investigation are confirmed by comparing them with the infrared thermographic measurements and numerical results obtained from a simulation using Ansys Icepak. For a power dissipation of less than 11.8 W, the peak temperature of the GaN HEMTs is 118.7 °C, obtained from thermal measurements.     

Critical design issues for high-power GaN/AlGaN anti-serial Schottky varactor frequency triplers

In this paper the importance of a new design variable for high power anti-serial Schottky varactors, the aluminum composition of the AlGaN barrier layer, is demonstrated. AlGaN/GaN varactors containing either (1) a high-doped/low-doped GaN region or (2) just a low doped GaN region have been compared demonstrating that the selection of the device structure also depends on the amplitude of the input signal being tripled in frequency. Stronger susceptance modulation is exhibited in AlGaN/GaN ASVs made from Ga-face polar material compared to N-face polar material. Results indicate choosing the proper aluminum composition results in 27% conversion efficiency with an input signal of 5 GHz and over 7% conversion efficiency with an input signal of 60 GHz along with optimization trends. With input voltage amplitudes over 10 V an AlGaN/GaN structure with 15% Al provides greater conversion efficiency than one with 5% Al. Power absorbed in the varactor also increases as aluminum percent increases affecting reliability and power transfer. Results of a GaN ASV performing as a frequency tripler for fundamental frequencies up to 110 GHz indicate an advantage to using an AlGaN/GaN epi-structure over only a GaN epi-structure.     

Study of the breakdown failure mechanisms for power AlGaN/GaN HEMTs implemented using a RF compatible process

The breakdown failure mechanisms for a family of power AlGaN/GaN HEMTs were studied. These devices were fabricated using a commercially available MMIC/RF technology with a semi-insulating SiC substrate. After a 10 min thermal annealing at 425 K, the transistors were subjected to temperature dependent electrical characteristics measurement. Breakdown degradation with a negative temperature coefficient of ?0.113 V/K for the devices without field plate was found. The breakdown voltage is also found to be a decreasing function of the gate length. Gate current increases simultaneously with the drain current during the drain-voltage stress test. This suggests that the probability of a direct leakage current path from gate to the 2-DEG region. The leakage current is attributed by a combination of native and generated traps/defects dominated gate tunneling, and hot electrons injected from the gate to channel. Devices with field plate show an improvement in breakdown voltage from ～40 V (with no field plate) to 138 V and with lower negative temperature coefficient. A temperature coefficient of ?0.065 V/K was observed for devices with a field plate length of 1.6 μm.     

Influence of processing and annealing steps on electrical properties of InAlN/GaN high electron mobility transistor with Al2O3 gate insulation and passivation

We report on preparation and electrical characterization of InAlN/AlN/GaN metal–oxide–semiconductor high electron mobility transistors (MOS HEMTs) with Al₂O₃ gate insulation and surface passivation. About 12 nm thin high-κ dielectric film was deposited by MOCVD. Before and after the dielectric deposition, the samples were treated by different processing steps. We monitored and analyzed the steps by sequential device testing. It was found that both intentional (ex situ) and unintentional (in situ before Al₂O₃ growth) InAlN surface oxidation increases the channel sheet resistance and causes a current collapse. Post deposition annealing decreases the sheet resistance of the MOS HEMT devices and effectively suppresses the current collapse. Transistors dimensions were source-to-drain distance 8 μm and gate width 2 μm. A maximum transconductance of 110 mS/mm, a drain current of ～0.6 A/mm (V_GS = 1 V) and a gate leakage current reduction from 4 to 6 orders of magnitude compared to Schottky barrier (SB) HEMTs was achieved for MOS HEMT with 1 h annealing at 700 °C in forming gas ambient. Moreover, InAlN/GaN MOS HEMTs with deposited Al₂O₃ dielectric film were found highly thermally stable by resisting 5 h 700 °C annealing.     

Analysis of traps effect on AlGaN/GaN HEMT by luminescence techniques

The study is carried out on AlGaN/GaN HEMTs presenting current collapse effect at V_ds lower than 6 V. This effect is completely recovered by illuminating the component with light of 710 nm wavelength (1.75 eV). The spectral analysis of the light emission in the visible near infrared spectrum shows a bell-shape with superimposed distinct emission peaks. These features suggest that the electroluminescence (EL) signal is due to the direct intraband of electrons and inelastic intraband transition of electrons due to scattering by charged centres. Photoionisation experiments have been conducted to determine the light wavelengths/energies that separately change the drain current and the gate leakage current.     

Ultraviolet photoresponse of ZnO nanostructured AlGaN/GaN HEMTs

We present the first active visible blind ultraviolet (UV) photodetector based on zinc oxide (ZnO) nanostructured AlGaN/GaN high electron mobility transistors (HEMTs). The ZnO nanorods (NRs) are selectively grown on the gate area by using hydrothermal method. It is shown that ZnO nanorod (NR)-gated UV detectors exhibit much superior performance in terms of response speed and recovery time to those of seed-layer-gated detectors. It is also found that the best response speed (~10 and~190 ms) and responsivity (~1.1×10⁵ A/W) were observed from detectors of the shortest gate length of 2 µm among our NR-gated devices of three different gate dimensions, and this responsivity is about one order higher than the best performance of ZnO NR-based UV detectors reported to date.     

Experimental and simulated dc degradation of GaN HEMTs by means of gate-drain and gate-source reverse bias stress

In this paper, the degradation of AlGaN/GaN high-electron mobility transistors (HEMTs) is investigated by means of dc stresses performed on fresh devices either on the gate-drain junction only (i.e., with the source terminal floating) or on the gate-source junction only (i.e., with the drain terminal floating). In both cases step-stresses were carried out by increasing V_DG and V_SG respectively up to 35 V: the saturated drain current decreased in both cases, and a significant increase in the output conductance was found for the drain-stressed devices, whereas it was negligible for the source-stressed devices. The reason for these different behaviors was believed to be the creation of acceptor traps in the AlGaN layer underneath the stressed side of the gate junction, their influence being different in the two cases because of the high horizontal electric field at the drain end of the gate during on-state operation. We carried out numerical simulations showing that the presence of a defective region with an acceptor trap concentration underneath the gate-drain or gate-source junction fits our hypothesis.     

DC and microwave characteristics of Lg 50 nm T-gate InAlN/AlN/GaN HEMT for future high power RF applications

The DC and microwave characteristics of L_g = 50 nm T-gate InAlN/AlN/GaN High Electron Mobility Transistor (HEMT) on SiC substrate with heavily doped n+ GaN source and drain regions have demonstrated using Synopsys TCAD tool. The proposed device features an AlN spacer layer, AlGaN back-barrier and SiN surface passivation. The proposed HEMT exhibits a maximum drain current density of 1.8 A/mm, peak transconductance (g_m) of 650 mS/mm and ft/fmax of 118/210 GHz. At room temperature, the measured carrier mobility, sheet charge carrier density (n_s) and breakdown voltage are 1195 cm²/Vs, 1.6 × 10¹³ cm⁻² and 18 V respectively. The superlatives of the proposed HEMTs are bewitching competitor for future monolithic microwave integrated circuits (MMIC) applications particularly in W-band (75–110 GHz) high power RF applications.     

La2O3 gate dielectrics for AlGaN/GaN HEMT

The annealing temperature dependent electrical characteristics of La₂O₃ gate dielectrics for W gated AlGaN/GaN high electron mobility transistors (HEMTs) have been characterized. The threshold voltage (V_th) has been found to shift to positive direction with higher temperature annealing, exceeding those of Schottky HEMTs, presumably attributed to the presence of negative fixed charges at the interface between La₂O₃ and AlGaN layers. At a high temperature annealing over 500 °C, a high dielectric constant (k-value) of 27 has been achieved with poly-crystallization of the La₂O₃ film, which is useful to limit the reduction in gate capacitance. A high k-value for La₂O₃ gate dielectrics and the presence of negative charges at the interface are attractive for AlGaN/GaN HEMTs with low gate leakage and normally-off operation.     

Determination of the average channel temperature of GaN MOSHFETs under continuous wave and periodic-pulsed RF operational conditions

We present a method to determine the average device channel temperature of AlGaN/GaN metal–oxide–semiconductor heterostructure field effect transistors (MOSHFETs) in the time domain under continuous wave (CW) and periodic-pulsed RF (radiation frequency) operational conditions. The temporal profiles of microwave output power densities of GaN MOSHFETs were measured at 2 GHz under such conditions and used for determination of the average channel temperature. The measurement technique in this work is also being utilized to determine the thermal time constant of the devices. Analytical temporal solutions of temperature profile in MOSHFETs are provided to support the method. The analytical solutions can also apply to generic field effect transistors (FETs) with an arbitrary form of time-dependent heat input at the top surface of the wafer. It is found that the average channel temperature of GaN MOSHFETs on a 300 μm sapphire substrate with the output power of 10 W/mm can be over 400 °C in the CW mode while the average channel temperature of GaN MOSHFETs on a SiC substrate with the same thickness only reaches 50 °C under the same condition. The highest average channel temperature in a pulsed RF mode will vary with respect to the duty cycle of the pulse and type of the substrate.     

High temperature induced failure in Ti/Al/Ni/Au Ohmic contacts on AlGaN/GaN heterostructure

Ti/Al/Ni/Au (200/1200/500/2000 Å) Ohmic contact on AlGaN/GaN was prepared and it was subjected to thermal aging experiments. Thermal processing at 400 and 500 °C did not change the contact resistance significantly, while high temperature storage at 600 °C resulted in a surge in the contact resistance. The Al–Au alloy in the contact metal is believed to re-melt because its lowest melting temperature is 525 °C. The liquid of Al–Au alloy is observed to diffuse to the AlGaN surface and consume some AlGaN layer. In addition, voids are found to be produced during thermal process, which can reduce the effective contact area and thus lead to higher contact resistance. The TEM and EDX results of Ohmic contact’s cross sectional images provide evidence for this proposed mechanism.     

Reliability of 100 nm AlGaN/GaN HEMTs for mm-wave applications

The effect of gate metallization and gate shape on the reliability and RF performance of 100 nm AlGaN/GaN HEMTs on SiC substrate for mm-wave applications has been investigated under on-state DC-stress tests. By replacing the gate metallization from NiPtAu to PtAu the median time to failure at T_ch = 209 °C can be improved from 10 h to more than 1000 h. Replacing the PtAu T-gate by a spacer gate further reduces the degradation rate under on-state stress, but decreases the current-gain cut-off frequency from 75 GHz to 50 GHz. Physical failure analysis using electroluminescence and TEM cross-section revealed pit and Ni void formation at the gate foot as the main degradation mechanisms of devices with NiPtAu T-gate. High resolution EDX mapping of stressed devices indicates that the formation of pits is caused by a local aluminium oxidation process. Simulation of the stress induced changes of the input characteristics of devices with NiPtAu gate further proves the formation of pits and Ni voids.