Papers by Keyword: HEMT

Abstract: A breakthrough high throughput WBG semiconductor dopant monitoring method has recently been introduced based on the novel concept of sweeping the electrical bias by near UV illumination-induced photoneutralization of corona charge. As originally discovered for 4H-SiC, the doping determination can be realized using the value of the photoneutralization time constant. In the present work this procedure is tested for β-Ga₂O₃ with a larger energy gap of 4.8eV, using a correspondingly deeper UV range. Such deep UV application to the AlGaN/GaN HEMT structure resulted in the development of a new measurement principle capable of increasing the HEMT wafer measurement throughput 10 times compared to previous corona noncontact C-V metrology. The new principle involves a linear illumination-induced corona charge bias sweep. Combined with surface voltage monitoring, it provides a means for fast and precise determination of the pinch-off voltage, V_P, the AlGaN electrical thickness, and the 2D electron gas density.

Abstract: GaN power HEMTs enable the design of power electronic systems with highest efficiencies and reduced size. Despite strong advancements in device reliability, charge carrier trapping is still an important challenge. The applied methodology allows to characterize defects that cause the dynamic R_DS,on in GaN power devices at product level with flexibility in duty cycle, number of pulses and mission profile. A pronounced trapping is observed for lateral GaN-on-Si HEMTs with Schottky p-GaN gate structure at low drain bias and long off-state pulses (> 100 ms). The effect is investigated by fast determination of the on-state resistance R_DS,on under different trap capturing conditions: a) different drain bias b) off-state time and number of cycles c) variation of temperature. The trapping and detrapping effects are characterized and the activation energy is extracted from time constants. An elevated on-state resistance was present for up to 3 hours. The threshold voltage modification due to high drain bias does explain the significant R_DS,on increase.

Abstract: Metal organic chemical vapor deposition (MOCVD) was used to grow AlGaN/GaN HEMT on a sapphire substrate with a 3.0 nm GaN cap and a sample without a GaN cap. High resolution X-ray diffraction (HRXRD) was utilized to investigate the structural characteristics of the materials. The relationship between the electrical properties and two-dimensional electron gas (2DEG) I-V and Hall Effect measurement. The I-V measurement was used to investigate the resistance properties of AlGaN/GaN heterostructures. Hall Effect measurement was used to quantify electron mobility and sheet carrier concentration in both samples. The sample with a 3.0 nm GaN cap exhibited excellent electrical properties with 436.8 Ω/sq sheet resistivity and possessed a high value of sheet carrier concentration 3.46E+14 per cm².

Abstract: Different material thickness with medium and high dielectric constant can impact the performance and reliability of high electron mobility transistor device. With varying the thickness of the passivation layer, the effect of it towards the device performance is still unclear. Two different insulator layers with a medium dielectric and a high dielectric constant namely Aluminium Nitride and Hafnium Oxide are used as passivation layer in AlGaN/GaN HEMT. Both material performance was simulated via COMSOL software by varying the thickness and the drain current output were compared. The passivation layer thickness of 10nm at V_ds=6 V and V_gs=5 V, HfO₂ outperforms AlN with the output drain current of 39 mA compared to 35 mA respectively. It was observed that HfO₂ can attain higher threshold voltage, V_th as compared to the AlN because of the influence of its material properties that shows a direct proportional relationship between V_th and dielectric constant. Using high dielectric constant material like HfO₂, we observe the ON-voltage gradually decreases as the thickness of the passivation layer increased. Out of all the thickness simulated for HfO₂ and AlN, 10nm produced the highest drain current output instead of layer thickness of 20nm.

Abstract: Advanced power electronic application normally requires high-speed semiconductor switches in a compact design that are capable to transform electrical energy between the sources and the loads with high efficiency. In electronics, inefficiency is a waste that also translated into unnecessarily high costs and limits the device performance. As the number of connected devices increases in modern applications, more efficient power conversion is necessary especially for advanced power electronic systems. Therefore, in this research, on-chip thermal management is designed to improve the power conversion efficiency of Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT). Since the inefficiency in the electronic component is always referred to as losses in the form of heat, proper thermal management is needed to improve the device performance. As nanotechnology promise to be the foundation of the next industrial revolution, the research towards nanoenhanced semiconductor devices has aroused widespread attention from researchers, scientists and engineers. In this research, two-dimensional nanomaterials (2DNMs) are used as heat spreaders to reduce the localized hot spot temperature in GaN HEMT for higher device efficiency. The fabrication process flow, process issues, process characterization, material characterization and thermal performance of the nanomaterial heat spreader are the main topics to be discussed in this paper. Based on the experiment the monolayer graphene can improve the thermal resistance by at least 0.5 K/W. This may help to improve the GaN HEMT device efficiency especially when the device is operated under high power density.

Abstract: With the development of high-voltage switches and high-speed RF circuits, the enhancement mode(E-mode) AlGaN/GaN HEMTs have become a hot topic in those fields. The E-mode GaN-based HEMTs have channel current at the positive gate voltage, greatly expanding the device in low power digital circuit applications. The main methods to realize E-mode AlGaN/GaN HEMT power devices are p-GaN gate technology, recessed gate structure, fluoride ion implantation technology and Cascode structure (Cascode). In this paper, the advantage and main realizable methods of E-mode AlGaN/GaN HEMT are briefly described. The research status and problems of E-mode AlGaN/GaN HEMT devices fabricated by p-GaN gate technology are summarized. The advances of p-GaN gate technology, and focuses on how these research results can improve the power characteristics and reliability of E-mode AlGaN/GaN HEMT by optimizing device structure and improving process technology, are discussed.

Abstract: Gallium nitride (GaN) and its AlGaN/GaN heterostructures grown on large area Si substrates are promising systems to fabricate power devices inside the existing Si CMOS lines. For this purpose, however, Au-free metallizations are required to avoid cross contaminations. In this paper, the mechanisms of current transport in Au-free metallization on AlGaN/GaN heterostructures are studied, with a focus on non-recessed Ti/Al/Ti Ohmic contacts. In particular, an Ohmic behavior of Ti/Al/Ti stacks was observed after an annealing at moderate temperature (600°C). The values of the specific contact resistance ρ_c decreased from 1.6×10⁴ Ω^.cm² to 7×10⁵ Ω^.cm² with increasing the annealing time from 60 to 180s. The temperature dependence of ρ_c indicated that the current flow is ruled by a thermionic field emission (TFE) mechanism, with barrier height values of 0.58 eV and 0.52 eV, respectively. Finally, preliminary results on the forward and reverse bias characterization of Au-free tungsten carbide (WC) Schottky contacts are presented. This contact exhibited a barrier height value of 0.82 eV.

Abstract: In this work, a charge storage based enhancement mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMT) is proposed and studied. A stacked gate dielectrics, consisting of a tunnel oxide, a charge trap layer and a blocking oxide are applied in the HEMT structure. The E-mode can be realized by negative charge storage within the charge trap layer during the programming process. The impact of the programming condition and the thickness of the dielectrics on the threshold voltage (V_th) are simulated systematically. It is found that the V_th increases with the increasing programming voltage and time due to the increase of the storage charge. Under proper programming condition, the V_th can be increased to more than 2 V. Moreover, It is also found that the V_th increases with the decrease of the thickness of the dielectrics. In addition, it is found that the breakdown voltage of such HEMT can be adjusted by varying the gate dielectric stacks.

Abstract: We show the influence of Fe-doping upon bowing and cracking in GaN-on-Si based high-electron-mobility transistors (HEMTs) and report how to prevent from bowing and cracking. In-situ reflectance measurements revealed that stress relaxation occurred during the growth of GaN:Fe on Al_0.25Ga_0.75N, resulting in the wafer bowing and cracking. In-situ measurements and transmission electron microscope images showed that the relaxation was caused by the 3D growth of GaN:Fe and the propagation of threading dislocations. To suppress the relaxation, a 100 nm-thick un-doped GaN interlayer was inserted between GaN:Fe and Al_0.25Ga_0.75N. As a result, a crack-free low-bow surface was obtained for GaN-on-Si HEMTs with GaN:Fe.

Abstract: AlGaN/GaN heterostructures are important materials for the fabrication of high power and high frequency devices. However, the mechanisms of Ohmic contacts formation on these systems are continuously under scientific debate. In this paper, a structural and electrical investigation of Ti/Al/Ni/Au Ohmic contacts to AlGaN/GaN heterostructures is reported. In particular, the behavior of Ti/Al/Ni/Au multilayers was monitored at different annealing temperatures. The contacts became Ohmic after annealing at 750°C and showed a decreasing temperature behavior of the specific contact resistance R_C, described by a thermionic field emission mechanism. On the other hand, annealing at 850°C led to a further reduction of R_C , with a slightly increasing dependence of R_C on the measurement temperature (here regarded as a “metal-like” behavior). The microstructural analysis of the interfacial region allowed to explain the results with the formation of metallic intrusions contacting directly the two dimensional electron gas.