AlGaN/GaN high electron mobility transistors with InGaN back-barriers

A GaN/ultrathin InGaN/GaN heterojunction has been used to provide a back-barrier to the electrons in an AlGaN/GaN high-electron mobility transistor (HEMT). The polarization-induced electric fields in the InGaN layer raise the conduction band in the GaN buffer with respect to the GaN channel, increasing the confinement of the two-dimensional electron gas under high electric field conditions. The enhanced confinement is especially useful in deep-submicrometer devices where an important improvement in the pinchoff and 50% increase in the output resistance have been observed. These devices also showed excellent high-frequency performance, with a current gain cut-off frequency (f/sub T/) of 153 GHz and power gain cut-off frequency (f/sub max/) of 198 GHz for a gate length of 100 nm. At a different bias, a record f/sub max/ of 230 GHz was obtained.     

Thermal effects in AlGaN/GaN/Si high electron mobility transistors

Group III-nitride compounds are of increasing interest for designing high power and high temperature transistors. A considerable progress in the growth and process technology of these devices has been achieved. However, there are still limitations concerning particularly the lack of native substrates. Comparison of the AlGaN/GaN high electron mobility transistors investigated favours the SiC substrate. Recently, encouraging results have been reported for AlGaN/GaN/Si. The crucial problem found in AlGaN/GaN transistors operating at high biases is the self-heating induced by high power dissipation in the active zone. The present work reports on a study of the self-heating in AlGaN/GaN HEMTs grown on Si(1 1 1). The electron-band parameters of the heterostructures have been calculated self-consistently by taking into account the piezoelectric and spontaneous polarizations. As an experiment support, direct-current characteristics of AlGaN/GaN/Si HEMTs have been used to derive the drain voltage-dependent temperature rise in the conductive channel. As has been found, the self-heating is relatively weak. An improvement in the electron transport is achieved by optimizing the epilayers and adjusting the electrode sizes at output of the transistors investigated.     

Erosion defect formation in Ni-gate AlGaN/GaN high electron mobility transistors

High electron mobility transistors based on Aluminum Gallium Nitride/Gallium Nitride heterostructures are poised to become the technology of choice for a wide variety of high frequency and high power applications. Their reliability in the field, particularly the reliability of the gate electrode under high reverse bias, remains an ongoing concern, however. Rapid increases in gate leakage current have been observed in devices which have undergone off-state stressing. Scanning Electron Microscopy, scanning probe microscopy, and Transmission Electron Microscopy have been used to evaluate physical changes to the structure of Ni-gated devices as the gate leakage current begins its initial increase. This evaluation indicates the formation of an interfacial defect similar to erosion under the gate observed by other authors. Defect formation appears to be dependent upon electrical field as well as temperature. Transmission Electron Microscopy has been used to demonstrate that a chemical change to the interfacial oxynitride layer present between the semiconductor and gate metal appears to occur during the formation of this defect. The interfacial layer under the gate contact transitions from a mixed oxynitride comprised of gallium and aluminum to an aluminum oxide.     

Study of proton irradiation effects on AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors (HEMTs) were exposed to 3 MeV protons at fluences of 6 × 10¹³, 4 × 10¹⁴ and 1 × 10¹⁵ protons/cm². The drain saturation currents decreased by 20% and the maximum transconductance decreased by 5% at the highest fluence. As the fluence increased, the threshold voltage shifted more positive values. After proton irradiation, the gate leakage current increased. The Schottky barrier height changed from 0.63 eV to 0.46 eV, and the ideality factor from 2.55 to 3.98 at the highest fluence. The degradations of electrical characteristics of AlGaN/GaN HEMTs are caused by displacement damages induced by proton irradiation. The density of vacancies at different proton fluence can be calculated from SRIM. Being an acceptor-like defect, the Ga vacancy acts as a compensation center. While N vacancy acts as a donor. Adding the vacancies model into Slivaco device simulator, simulation results match well with the trends of experimental data. Hall measurement results also indicate the concentration and mobility of 2DEG decrease after proton irradiation. It is concluded that the Ga vacancies introduced maybe the primary reason for the degradation of AlGaN/GaN HEMTs performance.     

AlGaN/GaN high electron mobility transistors on Si(111) substrates

AlGaN/GaN high electron mobility transistors (HEMTs) on silicon substrates have for the first time been realized using organometallic vapor phase epitaxy (OMVPE). Using 1 Ω-cm p-Si(111), these devices exhibited static output characteristics with low output conductance and isolation approaching 80 V. Under microwave rf operation, the substrate charge becomes capacitively coupled and parasitically loads these devices thereby limiting their performance. As a result, typical 0.3 μm gate length devices show a 25 GHz cutoff frequency, with near unity f_max/f_T ratio and 0.55 W/mm output power. A small-signal equivalent circuit incorporating elements representing the parasitic substrate loading accurately models the measured S-parameters. Removal of the conductive substrate is one way to effectively eliminate this parasitic loading. Through backside processing, freestanding 0.4-mm HEMT membranes with no thermal management were demonstrated and exhibited a significant improvement in their f_max/f_T ratio up to 2.5 at the cost of lower f_T and f_max along with an almost four-fold reduction of I_dss     

Unpassivated GaN/AlGaN/GaN power high electron mobility transistors with dispersion controlled by epitaxial layer design

In this paper, a novel GaN/AlGaN/GaN high electron mobility transistor (HEMT) is discussed. The device uses a thick GaN-cap layer (∼250 nm) to reduce the effect of surface potential fluctuations on device performance. Devices without Si₃N₄ passivation showed no dispersion with 200-ns-pulse-width gate-lag measurements. Saturated output-power density of 3.4 W/mm and peak power-added efficiency (PAE) of 32% at 10 GHz (V_DS=+15 V) were achieved from unpassivated devices on sapphire substrates. Large gate-leakage current and low breakdown voltage prevented higher drain-bias operation and are currently under investigation.     

MOCVD-grown AlGaN/GaN heterostructures with high electron mobility

Specific features of MOCVD growth of AlGaN/GaN heterostructures have been studied. In the structures obtained, the 2D electron gas in the channel had a density of 1.2×10¹³ cm^?2 and a mobility of 1290 cm²/(V s) at room temperature. The effect of the purity of starting components on the properties of the structure is studied.     

Implantation angle periphery effects on non-alloyed Si-implanted ohmic contacts for AlGaN/GaN high electron mobility transistors

We report on the effect of implantation angle on contact resistance of non-alloyed ohmic contacts to selectively implanted source/drain regions in AlGaN/GaN high electron mobility transistor (HEMT) heterostructures. Three different components of contact resistance are observed for such contacts: (i) contact resistance between the metal and the semiconductor, (ii) resistance of the implanted region and (iii) an additional resistance attributed to a transition region between implanted and non-implanted region. This third component varies strongly with implantation angle. The variation with implantation angle shows that the ratio of lateral implantation damage to penetration depth is critical for implantation of AlGaN/GaN HEMT source/drain contact regions. Our results also show that increasing the implantation angle in combination with reducing the implantation width can reduce contact resistance.     

Influence of layer structure on performance of AlGaN/GaN high electron mobility transistors before and after passivation

Intentionally undoped and three different, doped layer structures are used to investigate properties of AlGaN/GaN high electron mobility transistors (HEMTs) before and after SiN passivation. For unpassivated devices, the drain current, transconductance, cutoff frequency, and microwave output-power increase with increased doping level, in spite of an increase in the gate-leakage current. After passivation, an overall performance improvement of all devices occurs. The passivation-induced sheet charge decreases from 2×10¹² cm⁻² in undoped structures to ∼0.7×10¹² cm⁻² in higher doped structures and performance improvement with passivation is less pronounced for higher doped devices. However, the output power of unpassivated and passivated devices on higher doped structures is much higher than that on the undoped-passivated counter-part. These results underline an advantage of the doped layer structure for the preparation of high-performance AlGaN/GaN HEMTs.     

Device reliability study of AlGaN/GaN high electron mobility transistors under high gate and channel electric fields via low frequency noise spectroscopy

A set of different short term stress conditions are applied to AlGaN/GaN high electron mobility transistors and changes in the electronic behaviour of the gate stack and channel region are investigated by simultaneous gate and drain current low frequency noise measurements. Permanent degradation of gate current noise is observed during high gate reverse bias stress which is linked to defect creation in the gate edges. In the channel region a permanent degradation of drain noise is observed after a relatively high drain voltage stress in the ON-state. This is attributed to an increase in the trap density at the AlGaN/GaN interface under the gated part of the channel. It was found that self-heating alone does not cause any permanent degradation to the channel or gate stack. OFF-state stress also does not affect the gate stack or the channel.     

Nanocrack formation in AlGaN/GaN high electron mobility transistors utilizing Ti/Al/Ni/Au ohmic contacts

AlGaN/GaN HEMTs are poised to become the technology of choice in RF and power electronics applications where high operating frequencies and high breakdown voltages are required. The alloyed contacting scheme utilized in the formation of the source and drain contacts of these devices affects the conduction of electrons through the 2DEG from the moment of ohmic contact formation onward to operation in the field. Analysis of the ohmic contacts of as-fabricated and electrically stressed AlGaN/GaN HEMTs, via chemical deprocessing and Scanning Electron Microscopy, indicates the presence of cracks oriented along the [11-20] directions, which nucleate at metal inclusions present under the alloyed ohmic source/drain contact metal. Cracks which form at the edges of these contact regions can extend into the channel region. It appears that electrical biasing induces additional growth in the longest cracks present within the channel regions of these devices.     

Photo-ionization spectroscopy of traps in AlGaN/GaN high-electron mobility transistors

Four different layer structures are used to study deep-level traps in AlGaN/GaN high-electron mobility transistors (HEMTs) by photo-ionization spectroscopy. The structures grown on sapphire substrates by metal-organic chemical vapor deposition show nearly identical Hall data. However, the direct current (DC) performance of HEMTs with identical geometry is found to differ strongly. In all structures investigated, two distinct defect levels, namely, at 2.84–2.94 eV and 3.24–3.28 eV, were found from the fits of the photo-ionization cross-sectional data. Additionally, different trap concentrations can be deduced. These are in good correlation with the different transconductance and drain current measured. It is assumed that the defect levels observed are related to the AlGaN surface.     

High-energy proton irradiation effects on AlGaN/GaN high-electron mobility transistors

AlGaN/GaN high-electron mobility transistors (HEMTs) show decreases in extrinsic transconductance, drain-source current threshold voltage, and gate current as a result of irradiation with 40 MeV protons at doses equivalent to decades in low-earth orbit. The data are consistent with the protons creating deep electron traps that increase the HEMT channel resistance. Postirradiation annealing at 300°C was able to restore ∼70% of the initial g_m and I_DS values in HEMTs receiving proton doses of 5×10¹⁰ cm⁻².     

Hot electrons induced degradation in lattice-matched InAlN/GaN high electron mobility transistors

In this paper, lattice-matched Pt/Au-In_0.17Al_0.83N/GaN high electron mobility transistors (HEMTs) were fabricated, and the degradation characteristics of the gate leakage current were investigated by drain-to-source voltage (V_DS) step-stress measurements under the ON, semi-ON, and OFF stress conditions and at different temperatures, respectively. It is found that, (1) there exists a critical value of V_DS, beyond which the gate leakage current begins to increase significantly; and (2) the degradation of gate leakage current has a positive temperature coefficient, indicating that high temperature can accelerate the degradation. A hot electron model is used to explain the experimental results, emphasizing that the hot electrons from the channel can induce additional negatively charged defects at the InAlN/GaN interface, which can increase the local electrical field and introduce a thinner surface barrier and finally enhance the vertical leakage current component, leading to the current degradation.     

Correlation of device performance and defects in AlGaN/GaN high-electron mobility transistors

Device performance and defects in AlGaN/GaN high-electron mobility transistors (HEMTs) have been correlated. Surface depressions and threading dislocations, revealed by optical-defect mapping and atomic force microscopy (AFM), compromised the effectiveness of the SiN_x surface-passivation effect as evidenced by the gate-lag measurements. The residual carriers in the GaN-buffer layer observed from the capacitance-voltage depth profile have been attributed to the point defects and threading dislocations either acting as donors or causing local charge accumulations. Deep-level transient-spectroscopy measurements showed the existence of several traps corresponding to surface states and bulk-dislocation defects. The formation of electron-accumulation regions on the surface or (and) in the GaN-buffer layer was confirmed by currentvoltage measurements. This second, virtual gate formed by electron accumulations can deplete the channel and cause a large-signal gain collapse leading to degraded output power. A good correlation was established between the device performance and defects in AlGaN/GaN HEMT structure.     

Liquid crystal electrography: Electric field mapping and detection of peak electric field strength in AlGaN/GaN high electron mobility transistors

The liquid crystal mixture E7, based on cyanobiphenyl, has been successfully employed to map electric field strength and distribution in AlGaN/GaN high electron mobility transistors. Using a transmitted light image through crossed polarizers the optical response of the liquid crystal deposited onto the surface of the devices was recorded as a function of source–drain bias, V_ds. At a critical voltage of 4 V the preferred direction of orientation of the long axes of the liquid crystal molecules in the drain access region aligned with one of the polarizers resulting in reduced transmitted light intensity. This indicates that at this electric field strength molecule orientation in most of the liquid crystal film is dominated by the electric field effect rather than the influence of surface anchoring. The experimental results were compared to device simulations. Electric field strength above the surface at V_ds = 4 V was simulated to reach or exceed 0.006 MV/cm. This electric field is consistent with the field expected for E7 to overcome internal elastic energy. This result illustrates the usefulness of liquid crystals to directly determine and map electric fields in electronic devices, including small electric field strengths.     

Terahertz detection by GaN/AlGaN transistors

Detection of subterahertz and terahertz radiation by high electron mobility GaN/AlGaN transistors in the 0.2-2.5 THz frequency range (much higher than the cutoff frequency of the transistors) is reported. Experiments were performed in the temperature range 4-300 K. For the lowest temperatures, a resonant response was observed. The resonances were interpreted as plasma wave excitations in gated two-dimensional electron gas. Non-resonant detection was observed at temperatures above 100 K. Estimates for noise equivalent power show that these transistors can be used as efficient detectors of terahertz radiation at cryogenic and room temperatures     

Reliability study of AlGaN/GaN HEMT under electromagnetic,RF and DC stress

In the context of studying the reliability of RF High Power Amplifiers (HPA) in their real environment, a study of the behavior of AlGaN/GaN HEMTs performances under electromagnetic stress is presented in this paper. The DUT has undergone several stress combinations (electromagnetic stress, electromagnetic and RF stress, electromagnetic and DC stress, …). The near field setup is used to disturb with electromagnetic field the device under test (DUT). Degradations in DC and power characteristics are observed for all stress types. This could be associated with electron trapping within the AlGaN barrier and AlGaN surface leading depletion of the 2-DEG.     

Multiple-channel GaAs/AlGaAs high electron mobility transistors

Multiple-channel high electron mobility transistors (HEMT's) have been designed and fabricated on GaAs/AlGaAs heterostructural material grown by molecular beam epitaxy (MBE). The sheet carrier density of the two-dimensional electron gas (2-DEG) measured at 77 K was linearly proportional to the number of high mobility electron channels, and reached 5.3 × 10¹²cm^-2for six-channel HEMT structures. Depletion-mode devices of the double-heterojunction HEMT were operated between negative pinchoff voltage and forward-biased gate voltage without any transconductance degradation. A peak extrinsic transconductance of 360 mS/mm at 300 K and 550 mS/mm at 77 K has been measured for a 1-µm gate-length double-heterojunction enhancement-mode device. An extremely high drain current of 800 mA/mm with a gate-to-drain avalanche breakdown voltage of 9 V was measured on six-channel devices.     

Millimeter-wave low-noise high electron mobility transistors

High electron mobility transistors (HEMT's) have been fabricated which demonstrate excellent millimeter-wave performance. A maximum extrinsic transconductance as high as 430 mS/mm, corresponding to an intrinsic transconductance of 580 mS/mm, was observed in these transistors. A unity current gain cutoff frequency fTas high as 80 GHz and a maximum frequency of oscillationf_{max}of 120 GHz were projected for these HEMT's. At 40 GHz, a minimum noise figure of 2.1 dB with an associated gain of 7.0 dB has also been measured. These are the highestf_{T}, f_{max}, and the best noise performance reported to date. The results clearly demonstrate the potential of HEMT's for millimeter-wave low-noise applications.