1:30pm - 2:00pmInvited
Demonstration of ultra-thin AlN-based HEMTs grown on silicon substrates
1Université Côte d'Azur, CRHEA, CNRS, France; 2C2N, CNRS, Univ. Paris-Sud, Univ. Paris-Saclay, France; 3IEMN/CNRS, France
We, at CRHEA, have a long experience in the epitaxial growth of AlGaN/GaN HEMTs on silicon substrates. Some efforts have been also dedicated toward integration with silicon and results are promising. However, the growth of AlGaN/GaN structures on silicon needs the development of complicated epitaxial stacks and typically, several micrometer thick epilayers
are needed. Simplification of structures and development of thinner epilayers would help to move forward toward integration. Recently, the concept of AlN-based HEMT has been demonstrated by D. Jena et al.. AlN-based HEMTs have a very simple structure and are potentially very thin. So far, AlN-based HEMTs, have been demonstrated on sapphire, silicon carbide and AlN bulk substrates. In this presentation, using NH3-MBE, we demonstrate for the first time AlN-based HEMTs grown on Si(111) substrates. Heterostructures typically consist of a fully relaxed 200 nm-thick AlN buffer layer grown on silicon, followed by a compressively-strained 20-30 nm-thick GaN channel, and an almost strain-free 3-10 nm-thick AlN barrier. The total epilayer thickness does not exceed 300 nm. Such an ultra-thin epilayer is of great interest regarding strain and bow issues which are key features for integration with silicon. Using an AlN barrier material, heterostructures having very high sheet carrier concentrations (2DEG density) are expected. The 2DEG density is measured as a function of the AlN barrier thickness and a value as high as 2.7x1013 cm-2 is measured before passivation. Mobility values above 600 cm²/Vs are measured. Improvements are on-going to increase mobility values and lower sheet resistances as required for transistors. Also, preliminary results about in-situ SiN surface passivation using NH3-MBE will be presented. Lastly, the future development of AlN-based HEMTs on Si(100) will be discussed.
2:00pm - 2:15pmOral
Fin-like Nanowire-channel InAlN/GaN HEMTs on Si with Improved gm and fT Linearity
1Nanyang Technological University, Singapore; 2Singapore-MIT Alliance for Research and Technology, Singapore; 3Massachusetts Institute of Technology, United States
As compared to AlGaN/GaN HEMTs, InxAl1-xN/GaN HEMT (x~17%) on Si structure is an excellent candidate for RF and power application due to its outstanding physical properties. As compared to AlGaN/GaN HEMTs, InxAl1-xN/GaN HEMT (x~17%) structure is excellent candidate for RF and power application due to its outstanding physical properties. This structure can provide more than two times higher Two Dimensional-Electron-Gas (2DEG) sheet carrier density (ns up to 3×1013cm-2) thus can extend GaN based HEMTs to millimeter-wave frequency applications due to improved DC and RF performance. Also, InAlN/GaN HEMT structure suffers from less strain induced reliability issues compared with traditional AlGaN/GaN HEMTs. However, the linearity of GaN transistors ultimately limits these devices in many applications.
In this work, we have demonstrated a Fin-like nanowire-channel InAlN/GaN HEMT with improved linearity. Due to the reduction of the increase of source resistance Rs at a high channel current benefitting from the nanowire-channel structure, the gm of Fin-like nanowire-channel device remains not dropped even at Vg=2 V, while the gm of a conventional device starts to drop after reach its peak at Vg=-2 V. The corresponding values of the gate-voltage-swing (GVS), which is defined here as the range of gate voltage that gm or fT remains not smaller than 80% of their peak values, of the Fin-like nanowire HEMTs is higher than 5.3 V (from -3.3 to >2 V), which is much wider than that of 3.3 V (from -3.5 to -0.2 V) for the conventional HEMT. For RF performance, the current-gain cutoff frequency (fT) up to 105 GHz and maximum oscillation frequency (fmax) of 43 GHz of the fin-like nanowire device were measured. The corresponding GVS for Fin-like nanowire device is 3 V (from -3.4 to -0.4 V), which is 30% wider than that of 2.3 V (from -3.6 to -1.3 V) for conventional device, respectively. The Fin-like nanowire-channel GaN HEMT shows much improved linearity in gm and fT
2:15pm - 2:30pmOral
AlGaN/GaN Circular High Electron Mobility Transistors on Si (111) Substrates for Sensing Applications
1Department of Physics, Indian Institute of Technology Delhi, India; 2Solid State Physics Laboratory, Delhi, India; 3Centre for Nano Science and Engineering, Indian Institute of Science, India
In this study, we have investigated AlGaN/GaN circular high electron mobility transistors on Si (111) substrates for sensing applications. For such measurements the ohmic contacts consisting of Ti/Al/Ni/Au metal stack were deposited on the AlGaN/GaN epilayers by electron beam evaporation followed by lift off process. The sample was then rapidly thermally annealed at 8500C for 60s. The DC characteristics of the fabricated gateless circular high electron mobility transistors were then investigated. The source drain current showed a linear relationship for source drain voltage varying from -1.0 V to +1.0 V which indicated that the contacts are ohmic in nature. The AlGaN/GaN transistors were then used for the detection of ultraviolet light. The source drain current between the two ohmic contacts was measured as a function of time for the sensing measurements. AlGaN/GaN circular high electron mobility transistors showed a response (δIDS/IDS) of about 0.694% at source drain voltage of 100 mV along with a slow recovery time of 2394s to ultraviolet light at room temperature with excellent repeatability. The sensing experiments indicate that AlGaN/GaN circular high electron mobility transistors on Si(111) substrates show an excellent sensitivity to ultraviolet light. The slow recovery time can be attributed to persistent photoconductivity effect which became more predominant with decrease in temperature. It is observed that with decrease in temperature the recovery time of the source drain current increases. Our study suggests that AlGaN/GaN circular high electron mobility transistors on Si (111) substrates are potential candidates for the detection of ultraviolet light which finds applications in sensors, bragg reflectors and ultraviolet photodetectors.
2:30pm - 2:45pmOral
Study of Substrate Thermal Conductivity Effects on the AlGaN/GaN HEMTs Performance
1Centre for Micro-/Nano-electronics (NOVITAS), School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore; 2Temasek Laboratories @NTU, Nanyang Technological University, Singapore
Self-heating is a major deteriorating issue for the GaN-based High-Electron Mobility Transistor (HEMT) device performance. The thickness of the epilayers in nitride heterostructures is in micron-scale, hence lateral dissipation of heat is insignificant, and majority of heat dissipation takes place in vertical direction. The substrate thus acts as the heat sink for the device and plays a vital role in heat dissipation. The substrate thermal conductivity effects on the self-heating in AlGaN/GaN HEMTs (on different substrates: Sapphire, Si, GaN, AlN and diamond) were studied in this work using TCAD simulations on ATLAS by Silvaco. Simulator was calibrated with AlGaN/GaN HEMT grown on sapphire substrate, making simulations experimentally verified. The devices simulated are identical in terms of 2DEG carrier density, carrier mobility, models for calculations, polarization scale and epistructure except for the substrate material. Hence, the simulations performed for this study, exclusively compare the effect of substrate thermal properties on the device performance. Probing the region below gate, where a hotspot is developed during device operations, for thermal measurements remains a great challenge. Simulation results show that the hotspot temperature at Vg=0V; Vd=20V reduces significantly with increasing thermal conductivity of the substrate. The rise in hotspot temperature is 165K, 109K, 108K, 86K and 63K for Sapphire, Si, GaN, AlN and Diamond substrates, respectively. The Idmax values show a similar trend with an approximately 49%, 37.5%, 33%, 26% and 9% drop in its value due to thermal degradation (at Vg=0V; Vd=20V) for Sapphire, Si, GaN, AlN and Diamond substrates, respectively. The transconductance peaks (gm) also show a correspondence to the substrate thermal conductivities with values of 152mS/mm 186mS/mm, 178mS/mm, 191mS/mm and 200mS/mm for Sapphire, Si, GaN, AlN and Diamond substrates, respectively. These results quantitatively express the profound impact of the substrate thermal conductivity effect on the device performances of AlGaN/GaN based HEMTs.
2:45pm - 3:00pmOral
Sputtered TiN Schottky Barrier Diodes on AlGaN/GaN Heterostructures
1School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore; 2Temasek Laboratories@NTU, Nanyang Technological University, Singapore; 3Singapore-MIT Alliance for Research and Technology, Singapore
AlGaN/GaN high-electron-mobility transistors (HEMTs) have exhibited great potential for high-power applications owing to the excellent material properties. In order to make it commercially viable, it is essential to have low-cost non-gold CMOS-compatible GaN device processes utilizing the matured Si wafer lines. Recently, TiN has received interests for Schottky gates due to its advantages over other materials [1, 2]. Although AlGaN/GaN HEMTs with TiN gate has been reported [3, 4], TiN Schottky barrier diodes (SBDs) properties on AlGaN/GaN heterostructures (HSs) by Schottky Barrier Height (SBH) measurements and its conduction mechanisms were not thoroughly studied. In this work, we have systematically investigated and analyzed the electrical properties of AlGaN/GaN SBDs using sputtered TiN.
The TiN SBDs were fabricated [1, 2] and characterized using current-Voltage-Temperature (I-V-T) and capacitance-voltage (C-V) measurements. The extracted SBH of TiN SBDs by I-V-T method (~1.098 eV) is in good agreement with the value obtained by modified Norde’s function (~1.105 eV) . The C-V measurement yields an even larger SBH ~1.45 eV . The improved SBH is due to the incorporation of oxygen in the sputtered TiN  as verified by XPS (~24%), RBS (~11%)  and EDX (~15%) . Due to the improved SBH, the reverse leakage current (IR) is also reduced with respect to Ni/Au SBDs. With the increase of reverse bias (VR) from 0 to -3.2 V, the exponentially increased IR is dominated by modified Poole-Frenkel emission through an interface state of 0.53 eV. When -20 < VR < -3.2 V, IR is dominated by the trap-assisted tunneling through an interface state at ~0.115 eV . The characteristics of the HEMTs with TiN gates will also be discussed in the conference. In summary, the sputtered TiN is a good potential candidate for non-gold Schottky gate for AlGaN/GaN devices on Si.