Romualdo A. Ferreyra

Degradation in InAlN/AlN/GaN heterostructure field-effect transistors as monitored by low-frequency noise measurements: Hot phonon effects

Low-frequency noise technique was applied to analyze performance of nearly lattice-matched InAlN/AlN/GaN heterostructure field-effect transistors and their degradation caused by electrical stress. Nearly identical devices from the same wafer have undergone a 7 h DC electrical stress at a fixed DC drain bias of VDS = 20 V and different gate biases. We noted up to 32 dB/Hz higher low-frequency noise for stressed devices over the entire frequency range of 1 Hz-100 kHz. The measurements showed the minimum degradation at a gate-controlled two-dimensional electron gas density of 9.4 × 1012 cm−2. This result is in good agreement with the reported stress effect on drain-current degradation and current-gain-cutoff-frequency measurements and consistent with the ultrafast decay of hot-phonons due to the phonon–plasmon coupling.

Threshold field for soft damage and electron drift velocity in InGaN two-dimensional channels

Experimental investigation of electron transport along a two-dimensional channel confined in an InGaN alloy of AlInN/AlN/InGaN/GaN structure was performed at room temperature under near-equilibrium thermal-bath temperature. A soft damage was observed at a threshold electric field applied in the channel plane. The threshold current for soft damage and the supplied electric power were lower in the channels with a higher electron density. The results are interpreted in terms of plasmon-assisted heat dissipation. In agreement with ultra-fast decay of hot phonons in the vicinity of the resonance with plasmons, the electron drift velocity acquires a highest value of ~2 × 107 cm s−1 at 180 kV cm−1 in channels with 1 × 1013 cm−2 and decreases as the electron density increases. No negative differential resistance is observed. The effective hot-phonon lifetime is estimated as ~ 2 ps at 1.6 × 1013 cm−2 at low electric fields and is found to decrease as the field increases.

Microwave performance of AlGaN/AlN/GaN -based single and coupled channels HFETs

In this work we compare electronic transport performance in HFETs based on single channel (SC) GaN/Al0.30GaN/AlN/GaN (2nm/20nm/1nm/3.5μm) and coupled channel (CC) GaN/Al0.285GaN/AlN/GaN/AlN/GaN (2nm/20nm/1nm/4nm/1nm/3.5μm) structures. The two structures have similar current gain cut-off frequencies (11.6 GHz for SC and 14 GHz for CC for ~ 1μm gate length) however, the maximum drain current, IDmax, is nearly doubled in the CC HFET (0.64 A/mm compared to 0.36 A/mm in SC). HFETs exhibit maximum transconductance (Gmmax) at a bias point close to where maximum f T occurs: VGS =-2.25 V and VDS =12 V and VGS = -2 V and VDS= 15 V for SC and CC HFETs, respectively. Since threshold voltage (Vth) is ~ -3.75 V for both SC and CC structures, devices are able to work at high frequencies with a high gm delivering higher ID. This is in contrast with device performance reported by others where f T is attained at VGS closer to Vth and therefore with lower ID/IDmax ratios and low Gm. Results are consistent in that CC HFET delivers higher IDmax because of the higher electron mobility (μ) and higher carrier density (n) in the channel. As the saturation drain current, IDsat, is attained at electric fields (~40KV/cm) lower than the critical electric field, Ecr , (~ 150KV/cm for GaN ) the higher f T in CC HFETs can be attributed, mainly, to a higher μ, which is in agreement with the Hall measurements. A higher μ in CC HFET is attributed to a shorter hot phonon lifetime.

Degradation in AlGaN/GaN heterojunction field effect transistors upon electrical stress: Effects of field and temperature

AlGaN/GaN heterojunction field effect transistors (HFETs) with 2 μm gate length were subjected to on-state-high-field (high drain bias and drain current) and reverse-gate-bias (no drain current and reverse gate bias) stress at room and elevated temperatures for up to 10 h. The resulting degradation of the HFETs was studied by direct current and uniquely phase noise before and after stress. A series of drain and gate voltages was applied during the on-state-high-field and reverse-gate-bias stress conditions, respectively, to examine the effect of electric field on degradation of the HFET devices passivated with SiNx. The degradation behaviors under these two types of stress conditions were analyzed and compared. In order to isolate the effect of self-heating/temperature on device degradation, stress experiments were conducted at base plate temperatures up to 150 °C. It was found that the electric field induced by reverse-gate-bias mainly generated trap(s), most likely in the AlGaN barrier, which initially were manifested as generation-recombination (G-R) peak(s) in the phase noise spectra near 103 Hz. Meanwhile electric field induced by on-state-high-field stress mainly generated hot-electron and hot-phonon effects, which result in a nearly frequency independent increase of noise spectra. The external base plate temperatures promote trap generation as evidenced by increased G-R peak intensities.

Hot-electron real-space transfer and longitudinal transport in dual AlGaN/AlN/{AlGaN/GaN} channels

Real-space transfer of hot electrons is studied in dual-channel GaN-based heterostructure operated at or near plasmon–optical phonon resonance in order to attain a high electron drift velocity at high current densities. For this study, pulsed electric field is applied in the channel plane of a nominally undoped Al0.3Ga0.7N/AlN/{Al0.15Ga0.85N/GaN} structure with a composite channel of Al0.15Ga0.85N/GaN, where the electrons with a sheet density of 1.4 × 1013 cm−2, estimated from the Hall effect measurements, are confined. The equilibrium electrons are situated predominantly in the Al0.15Ga0.85N layer as confirmed by capacitance–voltage experiment and Schrodinger–Poisson modelling. The main peak of the electron density per unit volume decreases as more electrons occupy the GaN layer at high electric fields. The associated decrease in the plasma frequency induces the plasmon-assisted decay of non-equilibrium optical phonons (hot phonons) confirmed by the decrease in the measured hot-phonon lifetime from 0.95 ps at low electric fields down below 200 fs at fields of 4 kV cm−1 as the plasmon–optical phonon resonance is approached. The onset of real-space transfer is resolved from microwave noise measurements: this source of noise dominates for 8 kV cm−1. In this range of fields, the longitudinal current exceeds the values measured for a mono channel reference Al0.3Ga0.7N/AlN/GaN structure. The results are explained in terms of the ultrafast decay of hot phonons and reduced alloy scattering caused by the real-space transfer in the composite channel.

Investigation of microwave and noise properties of InAlN/GaN HFETs after electrical stress: role of surface effects

In an effort to investigate the particulars of their stability, In18.5%Al81.5%N/GaN HFETs were subjected to on-state electrical stress for intervals totaling up to 20 hours. The current gain cutoff frequency fT showed a constant increase after each incremental stress, which was consistent with the decreased gate lag and the decreased phase noise. Extraction of small-signal circuit parameters demonstrated that the increase of fT is due to a decrease in the gate-source capacitance (Cgs) and gate-drain capacitance (Cgd) as well as the increased microwave transconductance (gm). All these behaviors are consistent with the diminishing of the gate extension (“virtual gate”) around the gate area.

Measurements of off-state electrical stress in InAlN/AlN/GaN heterostructure field-effect transistors with varying In compositions

We report on the electrical stress results in GaN-based heterostructure field-effect transistors (HFETs) with InAlN barriers. We monitored the DC characteristics and low-frequency phase noise behavior for the devices at pre- and poststress conditions for five different wafers with In compositions varying from 12% to 20% in the barriers of the structures. The devices were stressed under off-state conditions with a gate bias of -10V (pinch-off condition) and zero drain bias for 10hr. From the acquired data we observed that at higher In composition, HFETs became less sensitive to the stress. At lower In composition we noted up to 30 dBc/Hz higher low frequency noise for stressed devices over the entire frequency range of 1 Hz-100 kHz. The change in drain current and change in noise power due to electrical stress decrease as the In composition in the barriers of the HFETs increases. The most relevant stress effect is revealed by a drain current reduction which is consistent with higher noise level measured. It was found that the HFET degradation is minimum for nearly lattice matched condition InAlN barriers, i.e.; 17% In composition, at which the sheet electron density (channel current) is comparable with that in lower In composition (12% In). This latter result is promising for power applications in which reliability of devices functioning at higher drain current is crucial. The results may also be beneficial to decouple the effect of off-state stress from the hot electron and self heating effects.

Degradation mechanism of InAlN/GaN based HFETs under high electric field stress

Degradation of InAlN/GaN based HFETs under stress for four bias conditions, namely, on-state high field stress (hot phonon, hot electron and self heating effect), off-state high field stress (hot electron effect), onstate low field stress (self heating effect), and reverse gate bias stress (inverse piezoelectric effect) has been examined. The degradation is characterized by monitoring electrical properties, such as, drain current reduction, gate lag, and low frequency noise. On-state high field stress has shown more than 50% reduction in the drain current and approximately 25-30 dBc/Hz increase in low frequency noise after 25 hours of stress, while other stress conditions led to much lesser degradation. It is demonstrated that the major degradation mechanism in InAlN/GaN HFETs is the hot-phonon and hot-electron effect in the realm of short term effects.

Degradation analysis of InAlN/AlN/GaN heterostructure field-effect transistors using low-frequency noise and current-transient methods: hot-phonon effects

Low-frequency noise and current-transient measurements were applied to analyze the degradation of nearly latticematched InAlN/AlN/GaN heterostructure field-effect transistors caused by electrical stress. Almost identical devices on the same wafer were stresses 7 hr. at a fixed DC drain bias of VDS=20 V and different gate biases. We noted up to 32 dB/Hz higher low-frequency noise for stressed devices over the entire frequency range of 1 Hz- 100 kHz. The measurements showed the minimum degradation at a gate-controlled two-dimensional electron gas density of 9.4x1012 cm-2. This result is in good agreement with the reported stress effect on drain-current degradation and current-gain-cutoff-frequency measurements, and consistent with the ultrafast decay of hot-phonons due to the phonon-plasmon coupling. Moreover, the current transient (gate-lag) measurements were also performed on pristine and highly degraded devices up to 5 ms pulse durations. Drain current is almost totally lost in degraded HFETs as opposed to a very small drop for pristine devices and no recovery observed for both indicating that generation of deep traps in GaN buffer.