Planar High Electron Mobility Transistor Research Articles

Strain relaxation and self-heating effects of fin AlGaN/GaN HEMTs

In this paper, we present a methodology of 3D electro-thermo-mechanical simulation to analyze the strain relaxation and self-heating effects of fin AlGaN/GaN high electron mobility transistors (HEMTs). The free boundaries of narrow fins cause strain relaxation of the AlGaN barrier and a non-uniform strain distribution near the AlGaN/GaN interface. The strain relaxation not only reduces the surface piezoelectric polarization charges (PPCs), but also introduces space PPCs in AlGaN/GaN, leading to a reduction of two-dimensional electron gas density and a positive shift of threshold voltage (V th). The simulated V th shift with fin width agrees well with experimental results from literature. In addition, the inter-fin trenches facilitate more efficient lateral heat spreading and suppress the self-heating effect compared with the planar HEMTs with the same effective gate width.

Influence of Fin-Like Configuration on Small-Signal Performance of AlGaN/GaN HEMTs

In this work, a new small-signal model (SSM), which is called the distributes intrinsic subdevice model (DISDM), is established for AlGaN/gallium nitride (GaN) Fin-like high-electron mobility transistors (HEMTs) according to the device structure feature. Perfect agreements between the measured and simulated scattering parameters (S-parameters) illustrated the validity of the DISDM and the accuracy of parameters extracted from the new model. Then, the characteristics of Fin-like HEMTs are analyzed using the DISDM theoretically. Different from the traditional model, the DISDM can accurately show the characteristics of the Fin-like HEMTs, especially the “double-peak” of the intrinsic transconductance. Compared with the planar HEMTs, the Fin-like HEMTs exhibit a higher linearity in direct current and small-signal parameters. For Fin-like HEMTs with different recess depths and duty ratios, the variation of the small-signal parameters and the linearity of devices were characterized to evaluate the performance of devices and optimize the process in the end.

Modeling Electrostatics and Low-Field Electron Mobility of GaN FinFETs

Gallium nitride (GaN) high electron mobility transistors (HEMTs) are currently being used for RF applications due to the intrinsically high saturation velocity and high mobility of GaN compared to both Silicon and SiC. However, GaN HEMTs suffer from a variety of issues, such as a lack of E-Mode operation and non-linearity, which impacts their widespread adoption in the RF and low-voltage switching devices. Recent advances in material processing, high aspect ratio epitaxial growth, and etching methods has led to an increased interest in 3-D GaN nanostructures such as AlGaN/GaN MIS FinFET wherein a layer of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> surrounds the AlGaN/GaN fin. Theoretical calculations of transport properties of AlGaN/GaN FinFETs are scarce compared to those of their planar HEMT counterparts. This work employs for the first time self-consistent solution of the coupled 1-D Boltzmann – 2-D Schrödinger – 3-D Poisson problem to yield the channel electrostatics and the transport characteristics of AlGaN/GaN MIS FinFETs. The low field electron mobility in the quasi-1-D region is calculated, using 1-D ensemble Monte Carlo method, as a function of temperature and fin width. Acoustic, piezoelectric and polar optical phonon scattering, and interface roughness scattering at the AlGaN/GaN interface, are incorporated in the theoretical model. Our simulations suggest that E-mode FinFETs can be achieved in ultranarrow fin channels. As suggested experimentally, we confirm via simulation experiments that strain relaxation increases the sheet resistance of the channel.

DC and RF performance of lateral AlGaN/GaN FinFET with ultrathin gate dielectric

In this study, an enhancement-mode (E-mode) GaN high electron mobility transistor (HEMT) with lateral tri-gate structure field effect transistor (FinFET) is proposed. To passivate the fin width, while keeping the normally-off performance of the FinFET intact, an ultrathin aluminium-oxide/sapphire (Al2O3) gate dielectric is proposed (in a basic single-finger 0.125 mm device). Later, the DC and radio frequency (RF) performances of the proposed FinFET designs (with optimized fin width and Al2O3 thickness) are compared with that of conventional planar HEMT. DC and RF measurements are performed using power transistors in ten-fingers configuration, with a total gate periphery of 2.5 mm. The effect of Fin structure and Al2O3 thickness on the electrical performance of HEMTs, including threshold voltage (V th) shift, transconductance (g m) linearity, small-signal gain, cut off frequency (f t), output power (P out), and power-added efficiency (PAE) are investigated. Based on our findings, FinFET configuration imposes normally-off functionality with a V, while the planar architecture has a V. Originating from passivation property of the alumina layer, the FinFET design exhibits two orders of magnitude smaller drain and gate leakage currents compared to the planar case. Moreover, large signal RF measurements reveals an improved P out density by over 50% compared to planar device, attributed to reduced thermal resistance in FinFETs stemming from additional lateral heat spreading of sidewall gates. Owing to its superior DC and RF performance, the proposed FinFET design with ultrathin gate dielectric could bear the potential of reliable operating for microwave power applications, by further scaling of the gate length.

True Fractional Dimensional Nature of Semiconductor Nanostructures: Removing the Anomaly in the Estimation of Quantum Mechanical Properties

Here, we have investigated the fractional dimensional nature of the experimental semiconductor quantum mechanical systems to understand the effect on the accurate prediction of electronic and optoelectronic properties. The formalism presented in this paper, to estimate the actual dimensionality of the system, lifts the anomaly of using the conventional integer density of states (DOS), although the carrier confinement isn't infinite. The effective bound state widths estimated by considering the carrier wavefunction leakage is the basis of this formalism. We have experimentally verified the formalism in AlGaN/GaN high electron mobility transistors (HEMTs). The dimensionality varies over a wide range from 3 to 2.38, as the gate bias sweeps the channel from depletion to strong inversion. However, the manifestation of fractional dimension on Capacitance-Voltage characteristics is insignificant for planar HEMTs. The effect becomes noticeable with the increasing confining dimensions of the heterostructure, which is experimentally verified in AlGaN/GaN nanofins. The effect is manifested as a shift in the threshold voltage and the rate of conduction-band state filling. This efficient and accurate formalism can estimate the dimension of any quantum mechanical system, thereby enhancing the accuracy of the models.

Influence of Fin-Like Configuration Parameters on the Linearity of AlGaN/GaN HEMTs

In this letter, we explore the impact of configuration parameters for Fin-like high-electron-mobility transistors (HEMTs) formed by partially etching barrier under the gate on improving transconductance (Gm) and cutoff frequency ( f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) linearity. It is found that the Gm profile for Fin-like HEMTs can be optimized by choosing appropriate device parameters, including the etching depth ( H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> ) and width ( W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> ) of recess region, as well as the duty ratio ( α) of the planar elements in a periodic unit along the gate width. In general, not only does W <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> affect the gate voltage swing (GVS) but also H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</sub> and α have an important role in Gm profile flatness. In addition, the fabricated Fin-like HEMTs shows a GVS of the transconductance plateau larger than 5.6 V and a constant f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> /f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> of 45 GHz/65 GHz over a wide gate voltage range. Furthermore, the proposed architecture also features an exceptional linearity performance at 8 GHz with an output third-order intercept point (OIP3) of 38.5 dBm, whereas that of the planar HEMT is 31 dBm. The device demonstrated in this article has great potential to be a new paradigm for future wireless communication systems where high linearity is essential.

AlGaN/GaN-Based Laterally Gated High-Electron-Mobility Transistors With Optimized Linearity

In this work, highly linear AlGaN/GaN laterally gated (or buried gate) high-electron-mobility transistors (HEMTs) are reported. The effect of gate dimensions on source-access resistance and the linearity of laterally gated devices are investigated experimentally in detail for the first time. Transistors with different gate dimensions and conventional planar devices are fabricated using two-step electron beam lithography (EBL). Current–voltage, source-access resistance, small-signal, and two-tone measurements are performed to evaluate the linearity of devices. Contrary to conventional planar HEMTs, the intrinsic transconductance of laterally gated devices monotonically increases with increasing gate voltage, showing a similar behavior as junction field-effect transistors (FETs). The source-access resistance shows a polynomial increase with the drain current, which can be reduced by decreasing the filling ratio of the buried gates. Through the optimization of these two competing factors, i.e., intrinsic transconductance and the source-access resistance, flat transconductance with high linearity is achieved experimentally. The laterally gated structure shows flat transconductance and small-signal power gain over a larger span of gate voltage that is 2.5 times higher than a planar device. Moreover, 6.9-dB improvement in output intercept point (OIP3)/ ${P}_{\text {DC}}$ is achieved. This approach can be used to improve the linearity of AlGaN/GaN HEMTs at the device level.

Selective Area Epitaxial Growth of Stretchable Geometry AlGaN-GaN Heterostructures

We report on the selective area epitaxy (SAE) of AlGaN/GaN microstructures in stretchable geometric patterns. We have investigated dependence of Al incorporation, lateral/sidewall growth profile and electrical properties of SAE heterostructures on Al/Ga source ratio and mask material. We observe higher Al incorporation along with improvement in heterointerface between the SAE layers with an increase in Al to Ga ratio. AlGaN composition, crystal quality and electrical properties of SAE AlGaN/GaN microstructures are shown to be affected by the choice of mask material. Low resistivity in the range of 0.12–0.35 mΩ cm was measured in these heterostructures. Two terminal current–voltage measurements showed 3–5× higher current density in SAE AlGaN/GaN microstructure devices compared to conventional planar High electron mobility transistor (HEMT). A detailed secondary ion mass spectroscopy (SIMS) profiling on SAE structures showed dependency of unintentional n-type dopants on the mask material.

Blood Based Biomarker Detection Using FET Biosensor: Towards Self-Health Management

Diseases that are the leading causes of mortality worldwide such as cardiovascular diseases (CVD), cancer and infectious diseases can be better managed and cured with early and prompt diagnosis and prognosis. Molecular biomarkers in whole blood (blood serum proteins) have predictive, diagnostic and prognostic capacity and semiconductor based electronic biosensors can be used to deliver reliable and affordable point-of-care disease monitoring. GaN High Electron Mobility Transistor (HEMT) based sensors have previously been used in a variety of applications such as ion, gas and biomolecule sensing. However, the limitations of FET biosensors set by the charge screening effect in high ionic strength solutions required additional sample processing steps, making them practically unusable in terms of point-of-care testing. With a novel biosensing technique enabling direct detection of target proteins in high salt concentration medium using GaN HEMT and innovative biosensor packaging methodology, a portable biosensor system has been designed and fabricated which can provide quantitative blood biomarker analysis in 5 minutes with less than 5 µl sample. Antibody or aptamer functionalized reference gate electrode and transistor channel form an electrical double layer (EDL) FET structure that can detect binding of target proteins with very high sensitivity. The sensing and packaging techniques have been specifically optimized to meet the requirements and convenience of a mobile diagnostic system that can be used by anyone, at all times. HEMT chip is fabricated in a 4-5 mask process: mesa isolation, ohmic metal deposition and annealing, interconnect metal deposition and device passivation. A simple and robust two step epoxy based planar HEMT chip packaging allows for easy integration with commercially available reader sockets, much like the micro-SD card. A single step surface functionalization strategy is followed for immobilizing receptors which allows for high sensitivity with a wide dynamic range of detection. Pulsed gate voltage modulates the channel conductivity which behaves as a function of the charge distribution within the EDL structure. Thus, without complex sample pre-treatments and extremely low sampling requirements, quantitative detection of protein biomarkers can be achieved. The structural design of EDL FET brings the potential to investigate the binding events that were previously undetectable in traditional FET based biosensors. Results from testing purified proteins and clinical serum samples elucidate the potential of the sensing technique to overcome the charge screening effect and offer sensing beyond Debye length in physiological salt environment, with high sensitivity and ultra-low detection limit. This diagnostic technique can be applied to all types of FET based sensors that uses immunological reactions to detect the target. Owing to standard semiconductor fabrication processes and minimal or no sampling requirements, the portable sensing system can be affordable and accessible to all. Enabled with cloud management, this diagnostic system houses the potential to be a pioneering IoT biosensor. Figure 1

Enhancement- and depletion-mode AlGaN/GaN HEMTs on 3C-SiC(111)/Si(111) pseudosubstrates

The realization of depletion-mode planar and both enhancement- and depletion-mode tri-gate high electron mobility transistors (HEMTs) based on Al0.2Ga0.8N/AlN/GaN heterostructures grown on silicon substrates using an ultrathin 3C-SiC transition layer is presented. This substrate configuration simplifies heterostructure growth compared to SiC and thick 3C-SiC substrates. The threshold voltage of the tri-gate devices strongly depends on the AlGaN/GaN body (fin) width. A transition from depletion-mode to enhancement-mode operation occurred at 110 nm body width for tri-gate devices without Si3N4 passivation, and is expected to occur at 75 nm for devices with Si3N4 passivation. Threshold voltages of −0.25/0.35 V were achieved at fin widths of 82/100 nm with/without Si3N4 passivation, respectively, for the tri-gate HEMTs in comparison to −3.5 V for the planar HEMTs. Cutoff frequencies fT of 45 GHz and maximum frequencies of oscillation fmax of 50 GHz have been measured for our best 100-nm tri-gate enhancement-mode HEMTs, while depletion-mode planar HEMTs with the same gate length showed 110 GHz for both fT and fmax. This demonstrates the applicability of the developed substrate configuration for high-performance device applications.

High-Electron-Mobility Transistors Based on InAlN/GaN Nanoribbons

We demonstrate high-electron-mobility transistors (HEMTs) based on InAlN/GaN nanoribbon (NR) structures. These devices, with NR widths d in the 50-90 nm range, are fabricated through a top-down technology on planar InAlN/GaN samples grown on a SiC substrate. The electrical properties of the InAlN/GaN NRs have been characterized by transmission-line model measurements and in HEMT structures, and compared with similar planar devices fabricated in close proximity. External mechanical stress and adequate surface passivation have a great impact on the NRs' performance. For example, when a thin Al2O3 dielectric is used to passivate the surface and to introduce tensile stress in the NR devices, the sheet resistance in the NRs becomes up to ~46% lower than that in passivated planar structures. These enhanced transport properties resulted in NR HEMTs with ~20-45% higher raw current than that in planar HEMTs. The NR technology combined with strain-passivation engineering allows the fabrication of NR HEMTs with record current densities (~2.9 A/mm) in the InAlN/GaN material system.

Oxygen Ion Implantation Isolation Planar Process for AlGaN/GaN HEMTs

A multienergy oxygen ion implantation process was demonstrated to be compatible with the processing of high- power microwave AlGaN/GaN high electron mobility transistors (HEMTs). HEMTs that are isolated by this process exhibited gate-lag- and drain-lag-free operation. A maximum output power density of 5.3 W/mm at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gs</sub> = -4 V and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> = 50 V and a maximum power added efficiency of 51.5% at V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gs</sub> = -4 V and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ds</sub> = 30 V at 3 GHz were demonstrated on HEMTs without field plates on sapphire substrate. This isolation process results in planar HEMTs, circumventing potential problems with enhanced gate leakage due to the gate contacting the 2-D electron gas at the mesa sidewall.