In this work, we report on recent results in understanding and addressing the issue of interface smearing in high-aluminum content AlGaN/AlGaN heterostructures. On the one hand, the growth of high-crystal quality AlGaN by metal-organic vapor phase epitaxy requires the use of high temperatures, but, on the other hand, this may lead to alloy intermixing between barrier and channel layers, which smoothens out the polarization contrast and severely degrades or even completely destroys the 2-dimensional electron gas (2DEG). We show that x-ray diffraction analysis can be used as a nondestructive way to assess the sharpness of the interface, and that improved growth schemes can be successfully used to achieve high-quality 2DEG, as confirmed by contactless resistivity measurements. In particular, sheet resistivities around 2500 Omega/square were demonstrated for AlN/Al0.75Ga0.25N, consistent with the best-reported values in the literature.

Despite considerable advancements, high electron mobility transistors (HEMTs) based on gallium nitride (GaN) channels remain largely limited to power applications below 650 V. For higher power demands, the ultra-wide bandgap semiconductor alloy aluminum gallium nitride, (Al,Ga)N, has emerged as a key contender for next-generation HEMTs. In this simulation study, we show that Al-rich AlxGa1−xN-channel HEMTs (with x ≥ 0.5) outperform the GaN-channel counterparts at and above room temperature, across all Al compositions, x. This contrasts with recent theory reports, which suggest that only AlxGa1−xN HEMTs with high Al content (x ≥ 0.85) offer comparable performance to GaN-channel devices. Unlike previous assumptions of a constant two-dimensional electron gas (2DEG) density across the entire composition range x, we show that the 2DEG density is highly sensitive to both the Al content and thickness of the individual layers in a HEMT structure. We demonstrate that the superior performance of Al-rich (Al,Ga)N-channel HEMTs is driven by a competing effect between 2DEG density and electron mobility. This work challenges the assumptions of prior studies, which can result in a significant under or overestimation of the potential of high Al content HEMTs. The insights gained from our work provide a comprehensive understanding of the trade-offs between device and material parameters, thus helping to guide the design of future Al-rich (x = 0.5–1.0) AlxGa1−xN-channel HEMTs for high-power applications.

In this paper, we have characterized an AlGaN/GaN High Electron Mobility Transistor (HEMT) with a short gate length (Lg $\approx$ 0.15$\mu$m). We have studied the effect of short gate length on the small signal parameters, linearity parameters and gm-gd ratio in GaN HEMT devices. To understand how scaling results in the variation of the above-mentioned parameters a comparative study with higher gate length devices on similar heterostructure is also presented here. We have scaled down the gate length but the barrier thickness(t$_{bar}$) remained same which affects the aspect ratio (L$_{g}$/t$_{bar}$) of the device and its inseparable consequences are the prominent short channel effects (SCEs) barring the optimum output performance of the device. These interesting phenomena were studied in detail and explored over a temperature range of -40$^\circ$C to 80$^\circ$C. To the best of our knowledge this paper explores temperature dependence of SCEs of GaN HEMT for the first time. With an approach to reduce the impact of SCEs a simulation study in Silvaco TCAD was carried out and it is observed that a recessed gate structure on conventional heterostructure successfully reduces SCEs and improves RF performance of the device. This work gives an overall view of gate length scaling on conventional AlGaN/GaN HEMTs.

An algorithm is proposed to implement digital peeling to determine dominant time constants of an exponential transient process. The method is simpler to implement and reduces computational time to a large extent in comparison to other techniques widely used. Apart from a synthetic test function, the algorithm has been implemented on reported experimental transient decay curves of Cs2HfCl6 (CHC) single crystal scintillation to verify its efficacy. Finally, drain current detrapping transients of unpassivated AlGaN/GaN high electron mobility transistors (HEMTs) are analyzed to determine the trap energy levels and concentrations. The validation of this digital peeling technique is also carried out by comparing with conventional method of time constant extraction from HEMT current transients. The extracted exponentials from the transient data efficiently fits well with the experimental data and can be extensively used for transient analysis. The digital peeling technique has wide applicability and can be used to analyze all exponential processes which occur in all domains of science.

Trap characterization on GaN Schottky barrier diodes (SBDs) has been carried out using deep-level transient spectroscopy (DLTS). Selective probing by varying the ratio of the rate window values ( r ) incites different trap signatures at similar temperature regimes. Electron traps are found to be within the values: 0.05–1.2 eV from the conduction band edge whereas the hole traps 1.37–2.66 eV from the valence band edge on the SBDs. In the lower temperature regime, the deeper electron traps contribute to the capacitance transients with increasing r values, whereas at the higher temperatures >300 K, a slow variation of the trap levels (both electrons and holes) is observed when r is varied. These traps are found to be mainly contributed to dislocations, interfaces, and vacancies within the structure.

An InAlN/GaN HEMT device was studied using extensive temperature dependent DC IV measurements and CV measurements. Barrier traps in the InAlN layer were characterized using transient analysis. Forward gate current was modelled using analytical equations. RF performance of the device was also studied and device parameters were extracted following small signal equivalent circuit model. Extensive simulations in Silvaco TCAD were also carried out by varying stem height, gate length and incorporating back barrier to optimize the suitability of this device in Ku-band by reducing the detrimental Short Channel Effects (SCEs). In this paper a novel structure i.e., a short length T gate with recess, on thin GaN buffer to achieve high cut-off frequency (f$_T$) and high maximum oscillating frequency (f$_{max}$) apt for Ku-band applications is also proposed.

The recovery of high reverse gate leakage current in AlGaN/GaN HEMTs has been analyzed in this study. To demonstrate the recovery after a thermal stress for large time periods (8-12 h), two mechanisms: phonon assisted tunnelling (PAT) and trap assisted tunnelling (TAT) have been utilized. Charge-based analytical models from literature have been used to match experimental leakage currents in reverse bias conditions. The degradation in the devices has been activated through thermal stress and the effect of high electric field induced inverse piezo electric effect has been eliminated. PAT component shows dominance in higher temperature regime (T > 500 K) and the slow recovery at 300 K with characteristic tau values around 5-13 h has been attributed to the TAT component through a deep trap level within the AlGaN barrier.

In this work an AlGaN/GaN HEMT device with thin (200 nm) buffer structure and moderate C-doping has been studied. DC characterization, Capacitance-Voltage (CV) measurements and RF measurements were carried out to investigate the effect of bulk traps on the device. Less vulnerability of this buffer structure towards trap was revealed by the observation of nominal kink in DC-IV and less current degradation (13% current slump) in Pulsed IV (PIV) characteristics. This is also confirmed by negligible threshold voltage (V T ) shift and hysteresis from CV measurement. A good isolation is observed from the low values of buffer leakage. Minimum noise figure (NF min ) is also calculated for this structure. Further, a Silvaco TCAD based simulation study is also performed considering the acceptor traps in buffer. From this, quite small drain lag is noticed which indicates that the effect of traps in buffer is less.

The effect of the thermal storage test on GaN high electron mobility transistor (HEMT) is investigated in this study by observing off‐state drain leakage current and on‐state hysteresis and maximum transconductance. The recovery post thermal stress is also observed which points to native defects within the heterostructure as the leakage values after 72 hrs of recovery are measured to be one order higher than the pristine device. On‐state hysteresis recovery and off‐state drain leakage are dominated by similar defects as observed from the characteristic recovery times. Trap analysis through drain current spectroscopy reveals an additional trap level of EC – 0.83 eV from the thermal storage test. The electrical measurements are in close correlation with the Raman active mode leading to reduced lifetimes (≈0.44 ps) at higher temperatures, where anharmonic phonon decay is observed following the reported Balkanski model. This gives an overall picture of the phonon‐induced defect generation from the thermal stress and the recovery from the reduced scattering probability within the material observed after 72 hrs recovery; also observed within the HEMTs from electrical characterization.

This article presents the study of trapping effects in AlGaN/GaN high electron mobility transistors (HEMTs). Conventional methods like temperature-dependent capacitance-voltage (CV) and pulsed-IV (PIV) have been employed to quantify the traps present in the device layers. Apparent threshold voltage (V-TH) instability as well as lag in the drain current after pulsing are evident. The kink in the drain current has been taken up to observe the trapping signature for devices with two different gate lengths. A modified drain and gate pumping methodology which has been designated as pre-measurement run (PMR) has been presented to get a deeper insight into the kink effect in these devices. Constant maximum field (CMF) PMR shows substantial change in the drain current, whereas negligible change for variable maximum field (VMF) PMR is observed, which confirms the dominant impact of trapping within the heterostructure and field assisted detrapping through Poole-Frenkel emission. Multiple traps have also been identified within the epitaxial layers with the E2 trap (E-a (activation energy) = 0.69 eV) which can be held responsible for the kink observed in the DC characteristics as observed from pulsed drain lag characteristics.

In this paper we have studied AlGaN/GaN high electron mobility transistor devices with three different buffer layer structures and their effect on the RF performance of the devices. Detailed DC and pulsed IV measurements were carried out to investigate the effect of bulk traps on the performance of the devices. Activation energy of buffer traps is also investigated by low frequency S-parameter measurements of the devices at variable temperature. The effect of buffer structure and quality of buffer on RF gain has been observed for the devices. Using the S-parameters, the equivalent circuit model parameters of the devices were also extracted. All the characterizations revealed that the presence of low C-doped buffer shows less vulnerability towards traps and produces better RF performance. It was also observed that the device with only low C-doped thin buffer reproduces better RF characteristics.

A study of pulsed-IV and drain current transients for unpassivated and SiN passivated AlGaN/GaN high electron mobility transistors (HEMTs) have been carried out in this report to observe the impact of traps in these devices. Pulsed-IV (PIV) characteristics for gate turn-on and gate turn-off to semi-on state for different pulsewidths (PW) reveal the effect of passivation. The unpassivated HEMT shows degraded characteristics suffering from significant current collapse due to surface states in the gate-drain access regions. Both devices observe kink in the drain current with passivated HEMT exhibiting kink for $\text{PW}\ > \ 30\ \ \mu s$. Drain current transient study from emission characteristics reveal traps with activation energies from 0.43-0.68 eV in the devices. The trapping phenomenon in unpassivated HEMT is governed by surface traps through virtual gate formation. The passivated HEMT checks the surface trapping but is vulnerable due to energetically deep bulk traps.

In this paper, a suspended, compact, circularly polarized microstrip antenna with “SWASTIKA” shaped slot has been introduced which can be operated within the ISM band covering the range of 433 MHz-434.79 MHz. The antenna has been designed using two 1.59 mm FR4 substrate layers with an air gap of 1.59 mm. The antenna has been optimized using method of moment based commercially available electromagnetic simulator. A fair response of less than -10 dB return loss and less than 3 dB axial ratio has been observed for the proposed bandwidth. Based on the optimized configuration, an antenna is fabricated for ground penetrating radar (GPR) application.