Surface Of High Electron Mobility Transistors Research Articles

A novel Si-rich SiN bilayer passivation with thin-barrier AlGaN/GaN HEMTs for high performance millimeter-wave applications

We demonstrate a novel Si-rich SiN bilayer passivation technology for AlGaN/GaN high electron mobility transistors (HEMTs) with thin-barrier to minimize surface leakage current to enhance the breakdown voltage. The bilayer SiN with 20-nm Si-rich SiN and 100-nm Si3N4 was deposited by plasma-enhanced chemical vapor deposition (PECVD) after removing 20-nm SiO2 pre-deposition layer. Compared to traditional Si3N4 passivation for thin-barrier AlGaN/GaN HEMTs, Si-rich SiN bilayer passivation can suppress the current collapse ratio from 18.54% to 8.40%. However, Si-rich bilayer passivation leads to a severer surface leakage current, so that it has a low breakdown voltage. The 20-nm SiO2 pre-deposition layer can protect the surface of HEMTs in fabrication process and decrease Ga–O bonds, resulting in a lower surface leakage current. In contrast to passivating Si-rich SiN directly, devices with the novel Si-rich SiN bilayer passivation increase the breakdown voltage from 29 V to 85 V. Radio frequency (RF) small-signal characteristics show that HEMTs with the novel bilayer SiN passivation leads to f T/f max of 68 GHz/102 GHz. At 30 GHz and V DS = 20 V, devices achieve a maximum P out of 5.2 W/mm and a peak power-added efficiency (PAE) of 42.2%. These results indicate that HEMTs with the novel bilayer SiN passivation can have potential applications in the millimeter-wave range.

Growth of microcrystalline diamond films after fabrication of GaN high-electron-mobility transistors for effective heat dissipation

Diamond films were grown on GaN high-electron mobility transistors (HEMTs) to improve thermal dissipation. We observed the temperature reduction in GaN HEMTs using Raman spectroscopy. The large-grain-size (over 1 μm) diamond films grown under the 〈110〉 preferential growth condition exhibited a high thermal conductivity of over 200 W m−1 K−1. These diamond films, which were grown at a high temperature of 700 °C, could be directly applied onto the GaN HEMT surface using a thermally stable SiN x metal insulator semiconductor gate structure. The maximum temperature and thermal resistance of the GaN HEMTs decreased by 100 K and from 12.7 mm K W−1 to 7.4 mm K W−1, respectively, when microcrystalline diamond films were applied onto the HEMT surfaces. As a result, the current density and trans-conductance of the GaN HEMTs were improved after diamond-film growth.

Boron Nitride as a Passivation Capping Layer for AlGaN/GaN High Electron Mobility Transistors.

We report on the electrical characteristics of AlGaN/GaN high-electron mobility transistors (HEMTs) with hexagonal boron nitride (h-BN) as a passivation capping layer. The HEMTs with h-BN layers showed an increase in current drainage and 103-times reduction in the gate-leakage current compared with those of conventional unpassivated HEMTs. Moreover, the extrinsic transconductance and the pulse responses were improved due to the reduced charge-trapping effect at the surface of HEMTs. From our observations, the h-BN can be used as a passivation capping layer for high-power electronic devices.

Advanced Characterization Techniques and Analysis of Thermal Properties of AlGaN/GaN Multifinger Power HEMTs on SiC Substrate Supported by Three-Dimensional Simulation

In this paper, several methods suitable for real time on-chip temperature measurements of power AlGaN/GaN-based high-electron mobility transistor (HEMT) grown on a SiC substrate are presented. The measurement of temperature distribution on HEMT surface using Raman spectroscopy is presented. The second approach utilizes electrical I–V characteristics of the Schottky diode neighboring to the heat source of the active transistor under different dissipated power for temperature measurement. These methods are further verified by measurements with microthermistors. The features and limitations of the proposed methods are discussed. The thermal parameters of materials used in the device are extracted from the temperature distribution in the structure with the support of three-dimensional thermal simulation of the device. Thermal analysis of the multifinger power HEMT is performed. The effects of the structure design and fabrication processes from semiconductor layers, metallization, and packaging up to cooling solutions are investigated. The influence of individual layer properties on the thermal performance of different HEMT structures under different operating conditions is presented. The results show that the proposed experimental methods supported by simulation have a potential for the design, analysis, and thermal management of HEMT.

Advances in AlGaN/GaN HEMT Surface Passivation

The high efficiency and power density of AlGaN/GaN high electron mobility transistors (HEMTs) can be limited by parasitic charge trapping at sites on the semiconductor surface. Trapped electrons at the surface deplete the two-dimensional electron gas (2DEG), resulting in effects known as current collapse, increased dynamic on-resistance, DC-RF dispersion, among others. A SiN layer is conventionally implemented as surface passivation for GaN HEMTs, however the properties of this SiN layer and the impact of this layer on current collapse depend drastically on the deposition conditions of this film. Although passivation of GaN HEMTs has been studied extensively, the specific details behind the SiN passivation deposition are not frequently provided in literature. In this work, we investigate several passivation techniques and study their performance under dynamic switching conditions, under off-state stress, and implement hyperspectral electroluminescence imaging to identify electric field nonunifornities. We investigate plasma enhanced chemical vapor deposition (PECVD) SiN, a common GaN HEMT passivation method, and demonstrate how to optimize the deposition conditions to effectively passivate the HEMT. Generally, a mixed frequency PECVD SiN process is used, where a combination of low frequency (LF) e.g. 100-360 kHz and high frequency (HF) 13.56 MHz plasmas are used to create the SiN film. However, the LF step could create undesirable damage to the surface of the AlGaN barrier from N+ ion bombardment, degrading device performance. We developed a bilayer PECVD process, where a 10 nm thick layer is deposited using HF plasma only in order to protect the AlGaN surface, followed by a HF/LF mixed frequency SiN deposition. Annealing the 10 nm thick HF layer densified the film and resulted in a HEMT with significantly improved current collapse. On the other hand, SiN layers grown in situ during MOCVD growth of the GaN/AlGaN layers have also been demonstrated to serve as an effective protective layer for subsequent HF/LF PECVD SiN deposition, yielding HEMTs with low current collapse. Atomic layer epitaxy (ALE) deposition of a thin (10 nm) AlN layer is evaluated, and is shown to serve well as a passivation layer. This is particularly advantageous, since the thin layer is beneficial for reducing gate capacitance for high frequency operation. Finally, a nanocrystalline diamond (NCD) layer is incorporated on the surface of the GaN HEMT, primarily to improve thermal management, but this also is effective as a surface passivation layer, mitigating surface charge trapping. Effective passivation of the AlGaN/GaN HEMT surface is essential for mitigating current collapse, and we demonstrate recent advancements in passivation technology.

Advances in AlGaN/GaN HEMT Surface Passivation

The high efficiency and power density of AlGaN/GaN high electron mobility transistors (HEMTs) can be limited by parasitic charge trapping at sites on the semiconductor surface. Trapped electrons at the surface deplete the two-dimensional electron gas (2DEG), resulting in effects known as current collapse, increased dynamic on-resistance, DC-RF dispersion, among others. A plasma-enhanced chemical vapor deposition (PECVD) SiN layer is conventionally implemented as surface passivation for GaN HEMTs, however the properties of this SiN layer and the impact of this layer on current collapse depend drastically on the deposition conditions of this film. Although passivation of GaN HEMTs has been studied extensively, the specific details behind the SiN passivation deposition are not commonly provided within literature. In this work, we investigate several passivation PECVD SiN passivation schemes and study their performance under dynamic switching conditions, under off-state stress, and implement electroluminescence imaging to identify electric field nonunifornities.

Early Detection of Lung Cancer Using High Electron Mobility Transistors

Lung cancer is the leading cause of cancer deaths in the world, among both men and women. It claims more lives each year than do colon, prostate, ovarian and breast cancers combined. Currently, diagnosis of lung cancer in stages I and II is very difficult and hence prognosis is delayed, leading to increased mortality rate. Carcinoembryonic antigen (CEA), is a common cancer biomarker and the expression of CEA above 5 ng/mL is suggestive of the presence of cancer. In this study, we utilize immunology, which is antibody/antigen reaction, with the combination of High Electron Mobility Transistor (HEMT) as the transducer, to develop a biosensor for detecting lung cancer biomarkers, electronically, as opposed to traditional ELISA techniques. HEMTs have already been demonstrated in gas, ion and biomolecule sensing purpose. When analytes accumulate on the gate area, the net charge on the HEMT surface changes. This electrical detection technique is simple, fast, and convenient. The detecting signal from the gate is amplified through the drain-source current, making the sensor very sensitive. Figure 1 (a) and (b) show the schematic of AlGaN/GaN HEMT and the top-view of the device. To create a high quality HEMT device, a 3 μm-thick undoped GaN buffer layer and a 150 Å-thick undoped Al0.25Ga0.75N layer were deposited by metal-organic chemical vapor deposition (MOCVD) on the substrate. Mesa isolation was formed by Inductively Coupled Plasma (ICP) etching system for 28 seconds. Ti, Al, Ni and Au were deposited by Electron Beam Evaporation and were annealed at 850 °C for 45 seconds under N2 flowing to form ohmic contacts. Passivation was provided by encapsulating the device with photoresist, with only the gate region open for antibody immobilization using photolithography. After sensor fabrication, surface functionalization is carried out by thiol binding on gold. We mixed bi-functional thio-carboxylate with the CEA antibody and let the antibody bind with the carboxyl functional group by EDC-sulfo NHS coupling reaction. Then the device is incubated in this solution for 12 hours at 4 ºC. BSA is used for surface blocking and as a protein dilution buffer. For the sensor measurement, source-drain bias is fixed at 0.5 V and the current change induced by binding of CEA was measured by applying a 0.5 V gate voltage. Figure 1 (c) shows the total charge accumulated for various concentrations of CEA. This can be used as an index to quantify the amount of CEA present in sample as it can distinguish various protein concentrations even in the physiological blood/serum concentration (1x PBS). The change in total charge with varying concentration of CEA can thus represent sensor calibration curve. Comparing to the conventional methods, the electronic detection of CEA using HEMT has the advantages of high sensitivity and low response time. We believe this method has the potential to further the research in the field of electronic biosensors. Figure 1

High-frequency detection of cell activity of Physarum polycephalum by a planar open gate AlGaN/GaN HEMT

The dynamics of cells of the slime mould Physarum polycephalum are investigated with a planar AlGaN/GaN high electron mobility transistor (HEMT) without any gate metallization. The source–drain contacts are used in a two-electrode arrangement whereas the free gate surface area is occupied by the Physarum cell. In order to understand the measured signals, basic properties of the interface between the cell and the HEMT surface were analysed by impedance spectroscopy. At high frequencies the interface impedance is governed by the conductance of the cell due to a direct current through the HEMT/cell interface. The locomotive dynamics of Physarum were recorded by the source–drain impedance at 10 kHz in combination with simultaneous video imaging that monitored the degree of occupancy of the HEMT surface by the cell. A precise correlation was found between the impedance and the coverage of the HEMT surface by the cell. It is observed that the entire region between the contacts is sensitive to the cell activity.Well-resolved cellular oscillations were observed for all measured parameters. Their periods corresponded to the typical periods of the intracellular shuttle streaming of protoplasma in Physarum. This demonstrates that high-frequency impedance measurements with AlGaN/GaN HEMT structures are well suited for the analysis of both the static parts of single Physarum cells as well as of their dynamic behaviour, such as their expansion and motility.

Gate current analysis of AlGaN/GaN on silicon heterojunction transistors at the nanoscale

The gate leakage current of AlGaN/GaN (on silicon) high electron mobility transistor (HEMT) is investigated at the micro and nanoscale. The gate current dependence (25–310 °C) on the temperature is used to identify the potential conduction mechanisms, as trap assisted tunneling or field emission. The conductive atomic force microscopy investigation of the HEMT surface has revealed some correlation between the topography and the leakage current, which is analyzed in detail. The effect of introducing a thin dielectric in the gate is also discussed in the micro and the nanoscale.

Post-process thermal treatment for microwave power improvement of AlGaN/GaN HEMTs

The effects of post-process rapid thermal annealing (RTA) treatment after device fabrication on direct current, microwave and power performances of AlGaN/GaN high electron mobility transistors (HEMTs) with a gate-length of 0.2 μm were fully investigated. By 3 min post-process RTA treatment at 350 °C under N 2 atmosphere, the direct current (DC), radio frequency (RF) small signal and power performances of AlGaN/GaN HEMTs have been much improved. The output power, power gain and power added efficiency (PAE) of GaN HEMT device with gate wide of 1 mm increase from 37.09 dBm, 6.09 dB and 42.79% to 38.22 dBm, 7.22 dB and 67.3%. The post-process RTA after device fabrication has two merits. On the one hand, it improves passivation effect of SiN x dielectric layer on AlGaN/GaN HEMT surface, suppressing RF current dispersion. On the other hand, it helps recover dry-etch damage at the Schottky metal/AlGaN interface, leading to reduction of reverse Schottky leakage current.

Electrical detection of biomaterials using AlGaN/GaN high electron mobility transistors

Chemical sensors can be used to analyze a wide variety of environmental and biological gases and liquids and may need to be able to selectively detect a target analyte. Different methods, including gas chromatography, chemiluminescence, selected ion flow tube, and mass spectroscopy, have been used to measure biomarkers. These methods show variable results in terms of sensitivity for some applications and may not meet the requirements for a handheld biosensor. A promising sensing technology utilizes AlGaN/GaN high electron mobility transistors (HEMTs). HEMT structures have been developed for use in microwave power amplifiers due to their high two dimensional electron gas (2DEG) mobility and saturation velocity. The conducting 2DEG channel of AlGaN/GaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes. HEMT sensors can be used for detecting gases, ions, pH values, proteins, and DNA. In this paper we review recent progress on functionalizing the surface of HEMTs for specific detection of glucose, kidney marker injury molecules, prostate cancer, and other common substances of interest in the biomedical field.

Edge Effects on Gate Tunneling Current in HEMTs

We elucidate five considerations for accurate estimation of electron tunneling from the gate edges of high-electron mobility transistors (HEMTs). These considerations are listed as follows: 1) edge roughness; 2) net charge at the AlGaN/passivation interface; 3) dielectric constant of the medium above the HEMT surface; 4) nontriangular potential barrier; and 5) negligible angular tunneling from the gate edge. Using these considerations, we calculate the reverse gate current IG of AlGaN/GaN HEMTs based on thermionic trap-assisted tunneling (TTT) and direct tunneling (DT) mechanisms. These calculations establish that the observed rise in IG for a gate voltage beyond the threshold is due to tunneling from the gate edges. The calculations also show that the TTT mechanism can predict the measured IG of AlGaN/GaN HEMTs over a wide range of gate voltages and temperatures and point to the possibility of a rapid rise in IG at high gate voltages due to the DT mechanism.

Trapping Effects in the Transient Response of AlGaN/GaN HEMT Devices

In this paper, the transient analysis of an AlGaN/GaN high-electron mobility transistor (HEMT) device is presented. Drain-current dispersion effects are investigated when gate or drain voltages are pulsed. Gate-lag and drain-lag turn-on measurements are analyzed, revealing clear mechanisms of current collapse and related dispersion effects. Numerical 2-D transient simulations considering surface traps effects in a physical HEMT model have also been carried out. A comparison between experimental and theoretical results is shown. The presence of donor-type traps acting as hole traps, due to their low energy level of 0.25 eV relative to the valence band, with densities >1e20 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> (>5e12 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> ), uniformly distributed at the HEMT surface, and interacting with the free holes that accumulated at the top surface due to piezoelectric fields, accounts for the experimentally observed effects. Time constants next to 10 ms are deduced. Some additional features in the measured transient currents, with faster time constants, could not be associated with surface states