arXiv subjects

Chandan Joishi

Publications and source records attributed to Chandan Joishi.

16 recordsLinked to original sources

arXiv2026

Vertical Gallium Oxide Isolated Source Electrode Field Effect Transistors (ISEFET) Without Planarization or Mid-Gap Acceptor Blocking Layers

We propose and demonstrate the first vertical Gallium oxide device architecture without the use of planarization etch back processes or mid-gap acceptor regions. The Isolated Source Electrode Field Effect Transistor (ISEFET) incorporates a dielectric blocking layer to access an isolated source pad extending from the top fin metal. Scaled multi-fin channels were formed by electron beam lithography with a width of 200 nm along with the source pads and then etched to a trench depth of ~1.2 um. The fabricated devices showed enhancement mode operation with threshold voltage of 2 V and on-off ratio > 1e7 with excellent gate modulation characteristics. The resulting device proved to be comparable to existing vertical transistors and suitable for high-throughput prototyping and large-scale manufacturing of future Gallium oxide and other wide bandgap semiconductor devices.

physics.app-ph↗

arXiv2026

A Promising Ohmic Contacts Approach for High-Al AlxGa1-xN (x>0.6) Channel HEMTs with AlN/GaN Digital Alloy Channel

In this paper we report a novel ohmic contact formation scheme for Extreme Bandgap (EBG) AlxGa1-xN (x>0.6) channel HEMTs with undoped barrier layers. Our approach consists of using a new low temperature (LT) pulsed metal-organic chemical vapor deposition (PMOCVD) doping scheme for the n++-GaN regrown contacts and an AlxGa1-xN digital alloy (DA) channel layer comprising short period super lattices (SPSL) of AlN and GaN. Pulsed growth and doping yield a sheet resistivity which is a factor of 3-5 lower than that of conventional doped n++-GaN layers grown under identical conditions. Moreover, the regrown n++-GaN layer has no hetero-barrier with the GaN layers of the AlxGa1-xN DA channel. These innovations led to MOCVD regrown linear ohmic contacts and a record-low contact resistance Rc ~6.5 Ω-mm to the Al0.62Ga0.38N DA channel layer of a HEMT with AlN barrier layer.

physics.app-ph↗

arXiv2025

Scaled Ultra-Wide Bandgap AlGaN Polarization-Graded FET with Ultra-thin Buffer Layer

We report on the design and demonstration of ultra-wide bandgap AlGaN polarization-graded field effect transistors with ultra-thin channels to enable excellent current density and high-frequency performance while significantly reducing thermal resistance. We use polarization-graded AlGaN layers and ultra-thin pseudomorphic AlGaN buffer layers to enable low thermal resistance and excellent structural quality. The polarization-graded field effect transistors (PolFETs) demonstrated here show Imax over 800mA/mm and current/power gain cutoff frequency (fT/fmax) of 26/28 GHz. Small signal modeling and analysis were used to determine parasitic/transit delays, and gate-resistance thermometry was implemented to thermally characterize AlGaN PolFET and benchmark against state-of-the-art AlGaN HEMTs. The ultra-thin AlGaN PolFET showed thermal resistance of 12 K.mm/W, representing a significant reduction from typical AlGaN transistors. These results show state-of-art combination of high current density, excellent fT-LG product for ultra-wide bandgap AlGaN transistors, and superior thermal performance, and highlight the promise of AlGaN transistors for future RF and mm-wave applications.

physics.app-ph↗

arXiv2025

High Breakdown Electric Field (> 5 MV/cm) in UWBG AlGaN Transistors

We report on the design and demonstration of ultra-wide bandgap (UWBG) AlGaN-channel metal-insulator heterostructure field effect transistors (HEFTs) for high-power, high-frequency applications. We find that the integration of gate dielectrics and field plates greatly improves the breakdown field in these devices, with state-of-art average breakdown field of 5.3 MV/cm (breakdown voltage > 260 V) with an associated maximum current density of 342 mA/mm, and cut-off frequency of 9.1 GHz. Furthermore, low trap-related impact was observed from minimal gate and drain lag estimated from pulsed I-V characteristics. The reported results provide the potential of UWBG AlGaN HEFTs for the next generation high-power radio frequency applications.

cond-mat.mtrl-sci↗

arXiv2025

Energy Bands and Breakdown Characteristics in Al2O3/UWBG AlGaN Heterostructures

We report on energy bands and breakdown characteristics of Al2O3 dielectrics on ultra-wide bandgap (UWBG) AlGaN heterostructures. Metal-dielectric-semiconductor structures are important to sustain high fields needed for future high-performance UWBG transistors. Using systematic experiments, we determined the fixed charge density (> 1013 cm-2), the dielectric/interface, and electric fields in the oxide of under flat-band conditions in the semiconductor. Low gate-to-drain leakage current of up to 5 x 10-7 A/cm2 were obtained in the metal-oxide-semiconductor structures. In lateral metal-semiconductor-insulator test structures, breakdown voltage exceeding 1 kV was obtained with a channel sheet charge density of 1.27 x 1013 cm-2. The effective peak electric field and average breakdown field were estimated to be > 4.27 MV/cm and 1.99 MV/cm, respectively. These findings demonstrate the potential of Al2O2 integration for enhancing the breakdown performance of UWBG AlGaN HEMTs.

cond-mat.mtrl-sci↗

arXiv2024

Heterostructure and Interfacial Engineering for Low-Resistance Contacts to Ultra-Wide Bandgap AlGaN

We report on the heterostructure and interfacial engineering of metalorganic chemical vapor deposition (MOCVD) grown reverse-graded contacts to ultra-wide bandgap AlGaN. A record low contact resistivity of 1.4 x 10-6 Ohm.cm2 was reported on an Al0.82Ga0.18N metal semiconductor field effect transistor (MESFET) by compositionally grading the contact layer from Al0.85Ga0.15N to Al0.14Ga0.86N with degenerate doping and proper interfacial engineering considering bandgap-narrowing-induced band offset between channel and contact layer. This represents orders-of-magnitude of lower contact resistivity than that obtained in similar MOCVD-grown structures. A detailed, layer-by-layer analysis of the reverse graded contact and TCAD simulation of the bandgap narrowing effect highlighted that the reverse graded contact layer itself is extremely conductive and interfacial resistance due to bandgap-narrowing-induced barrier between contact and channel dominates the contact resistance.

cond-mat.mtrl-sci↗

arXiv2024

Selective-injection GaN Heterojunction Bipolar Transistors with 275 kA/cm$^2$ Current Density

We design and demonstrate selective injection GaN heterojunction bipolar transistors that utilize a patterned base for selective injection of electrons from the emitter. The design maneuvers minority carrier injection through a thin p-GaN base region, while the majority carrier holes for base current are injected from thick p-GaN regions adjacent to the thin p-GaN base. The design is realized using a regrowth emitter approach with SiO$_2$ as a spacer between the emitter layer and the thick p-GaN base contact regions. The fabricated device demonstrated state-of-art output current density (I$_{C, max}$) ~275 kA/cm$^2$ with a current gain ($β$) of 9, and 17 for the planar HBT design (I$_{C, max}$ =150 kA/cm$^2$). The reported results highlight the potential of the selective injection design to overcome the persistent GaN HBT design tradeoff between base resistance and current gain, paving the way for next-generation radio frequency and mm-Wave applications.

physics.app-ph↗

arXiv2024

Tunnel Junction-Enabled Monolithically Integrated GaN Micro-Light Emitting Transistor

GaN/InGaN microLEDs are a very promising technology for next generation displays. Switching control transistors and their integration are key components in achieving high-performance, efficient displays. Monolithic integration of microLEDs with GaN switching devices provides an opportunity to control microLED output power with capacitive (voltage) control rather than current controlled schemes. This approach can greatly reduce system complexity for the driver circuit arrays while maintaining device opto-electronic performance. In this work, we demonstrate a 3-terminal GaN micro-light emitting transistor that combines a GaN/InGaN blue tunneling-based microLED with a GaN n-channel FET. The integrated device exhibits excellent gate control, drain current control and optical emission control. This work provides a promising pathway for future monolithic integration of GaN FETs with microLED to enable fast switching high efficiency microLED display and communication systems.

physics.app-ph↗

arXiv2024

Design and Simulation of a III-Nitride Light Emitting Transistor

This paper describes the design and characteristics of monolithically integrated three-terminal gated III-Nitride light emitting diodes (LEDs) devices. The impact of channel doping and thickness on the voltage penalty of the transistor-LED hybrid device is analyzed, and it is shown that with appropriate design, low voltage drop can be realized across integrated gated LED structures. The impact of device design on the switching charge is investigated, and it is shown that the adoption of an integrated LED/transistor structure can reduce the switching charge necessary for operation of a switched LED display device by an order of magnitude when compared with stand-alone light-emitting diodes.

physics.app-ph↗

arXiv2023

Selective injection AlGaN/GaN heterojunction bipolar transistors with patterned regrown base contacts

We demonstrate graded AlGaN/GaN heterojunction bipolar transistors (HBTs) with selective injection of minority carriers across a p-GaN base and patterned regrown base contacts. The selective injection design regulates minority carrier transport under emitter-base forward bias through a thin base region, while thick and highly doped p$^+$ GaN regrown layers patterned alongside the thin base regions are utilized to lower the base contact resistance. With SiO$_2$ employed as a spacer between the emitter and the p$^+$ regrown layers, the device with an interdigitated emitter/base-contact stripe design displayed a maximum collector current density (I$_C$) of 101 kA/cm$^2$, a maximum current gain ($β$) of 70 at I$_C$ $\sim$ 1 kA/cm$^2$ and $\sim$ 11 for I$_C$ $>$ 50 kA/cm$^2$. The reported results demonstrate the potential of the selective injection approach to break the long-existing HBT design tradeoff between base resistance and current gain for next-generation radio frequency and mm-Wave applications.

physics.app-ph↗

arXiv2023

$β$-Ga$_2$O$_3$ Trench Schottky Diodes by Novel Low-Damage Ga-Flux Etching

$β$-Ga$_2$O$_3$ trench Schottky barrier diodes fabricated through a Gallium atomic beam etching technique, with excellent field strength and power device figure of merit, are demonstrated. Trench formation was accomplished by a low-damage Ga flux etch that enables near-ideal forward operating characteristics that are independent of fin orientation. The reverse breakdown field strength of greater than 5.10 MV/cm is demonstrated at breakdown voltage as of 1.45 kV. This result demonstrates the potential for Ga atomic beam etching and high-quality dielectric layers for improved performance in $β$-Ga$_2$O$_3$ vertical power devices.

physics.app-ph↗

arXiv2021

Nb$_{2}$O$_{5}$ high-k dielectric enabled electric field engineering of $β$-Ga$_{2}$O$_{3}$ metal-insulator-semiconductor (MIS) diode

We demonstrate an Nb$_{2}$O$_{5}$/$β$-Ga$_{2}$O$_{3}$ metal-insulator-semiconductor (MIS) hetero-junction diode with Nb$_{2}$O$_{5}$ as the high-k dielectric insulator for more efficient electric field management resulting in enhanced breakdown characteristics compared to a $β$-Ga$_{2}$O$_{3}$ Schottky barrier diode. The Nb$_{2}$O$_{5}$ dielectric films were grown using atomic layer deposition and exhibited a high dielectric constant of 50. The high dielectric constant resulted in a 5$\times$ lower electric field at the metal/dielectric interface in the MIS diode compared to the metal/$β$-Ga$_{2}$O$_{3}$ interface in the Schottky barrier diode. With good electron conduction in forward bias enabled by the negative conduction band offset of Nb$_{2}$O$_{5}$ w.r.t $β$-Ga$_{2}$O$_{3}$, the MIS design led to a 3$\times$ improvement in the reverse blocking voltage with a slight trade-off in the specific on-resistance. Overall, a 3.3$\times$ increase in the power figure of merit was observed (3.25 MW/cm$^2$ for the Schottky diode and 10.8 MW/cm$^2$ for the MIS diode). A detailed analysis of the energy band line-up, and the forward and reverse current transport mechanisms are also presented using analytical modeling and 2-D TCAD simulations.

physics.app-ph↗

arXiv2020

Charge trap layer enabled positive tunable V$_{fb}$ in $β$-Ga$_{2}$O$_{3}$ gate stacks for enhancement mode transistors

$β$-Ga$_{2}$O$_{3}$ based enhancement mode transistor designs are critical for the realization of low loss, high efficiency next generation power devices with rudimentary driving circuits. A novel approach towards attaining a high positive flat band voltage (V$_{fb}$) of 10.6 V in $β$-Ga$_{2}$O$_{3}$ metal-oxide-semiconductor capacitors (MOSCAPs), with the ability to fine tune it between 3.5 V to 10.6 V, using a polycrystalline AlN charge trap layer has been demonstrated. This can enable enhancement mode operation over a wide doping range. Excellent V$_{fb}$ retention of ${\sim}$97% for 10$^{4}$ s at 55 $^{\circ}$C was exhibited by the gate stacks after charge trapping, hence reducing the requirement of frequent charge injection cycles. In addition, low gate leakage current density (J$_{g}$) for high negative gate voltages (V$_{g}$${\sim}$-60 V) indicates the potential of this gate stack to enable superior breakdown characteristics in enhancement mode transistors.

physics.app-ph↗

arXiv2020

Deep-recessed $β$-Ga$_2$O$_3$ delta-doped field effect transistors with in situ epitaxial passivation

We introduce a deep-recessed gate architecture in $β$-Ga$_2$O$_3$ delta-doped field effect transistors for improvement in DC-RF dispersion and breakdown properties. The device design incorporates an unintentionally doped $β$-Ga$_2$O$_3$ layer as the passivation dielectric. To fabricate the device, the deep-recess geometry was developed using BCl$_3$ plasma based etching at ~5 W RIE to ensure minimal plasma damage. Etch damage incurred with plasma etching was mitigated by annealing in vacuum at temperatures above 600 $°$C. A gate-connected field-plate edge termination was implemented for efficient field management. Negligible surface dispersion with lower knee-walkout at high V$_\mathrm{DS}$, and better breakdown characteristics compared to their unpassivated counterparts were achieved. A three terminal off-state breakdown voltage of 315 V, corresponding to an average breakdown field of 2.3 MV/cm was measured. The device breakdown was limited by the field-plate/passivation edge and presents scope for further improvement. This demonstration of epitaxially passivated field effect transistors is a significant step for $β$-Ga$_2$O$_3$ technology since the structure simultaneously provides control of surface-related dispersion and excellent field management.

cond-mat.mtrl-sci↗

arXiv2018

High Mobility 2DEG in modulation-doped \b{eta}-(AlxGa1-x)2O3/Ga2O3 heterostructures

Beta-phase Ga2O3 has emerged as a promising candidate for a wide range of device applications, including power electronic devices, radio-frequency devices and solar-blind photodetectors. The wide bandgap energy and the predicted high breakdown field, together with the availability of low-cost native substrates, make \b{eta}-Ga2O3 a promising material compared to other conventional wide bandgap materials, such as GaN and SiC. Alloying of Al with \b{eta}-Ga2O3 could enable even larger band gap materials, and provide more flexibility for electronic and optoelectronic device design. In this work, we demonstrate a high mobility two-dimensional electron gas (2DEG) formed at the \b{eta}-(AlxGa1-x)2O3/Ga2O3 interface through modulation doping. Shubnikov-de Haas oscillation was observed for the first time in the modulation-doped \b{eta}-(AlxGa1-x)2O3/Ga2O3 structure, indicating a high-quality channel formed at the heterojunction interface. The formation of the 2DEG channel was further confirmed by a weak temperature-dependence of the carrier density, and the peak low temperature mobility was found to be 2790 cm2/Vs, which is significantly higher than can be achieved in bulk-doped \b{eta}-Ga2O3. The demonstrated modulation-doped \b{eta}-(AlxGa1-x)2O3/Ga2O3 structure lays the foundation for future exploration of quantum physical phenomena as well as new semiconductor device technologies based on the \b{eta}-Ga2O3 material system.

cond-mat.mtrl-sci↗

arXiv2017

Modulation-doped beta-(Al0.2Ga0.8)2O3/Ga2O3 Field-Effect Transistor

Modulation-doped heterostructures are a key enabler for realizing high mobility and better scaling properties for high performance transistors. We report the realization of modulation-doped two-dimensional electron gas (2DEG) at beta(Al0.2Ga0.8)2O3/ Ga2O3 heterojunction using silicon delta doping. The formation of a 2DEG was confirmed using capacitance voltage measurements. A modulation-doped 2DEG channel was used to realize a modulation-doped field-effect transistor. The demonstration of modulation doping in the beta-(Al0.2Ga0.8)2O3/ Ga2O3 material system could enable heterojunction devices for high performance electronics.

cond-mat.mes-hall↗