Cemil Kayis

Low-Frequency Noise Measurements of AlGaN/GaN Metal–Oxide–Semiconductor Heterostructure Field-Effect Transistors With HfAlO Gate Dielectric

We report on the low-frequency phase-noise measurements of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors employing HfAlO as the gate dielectric. Some devices tested exhibited noise spectra deviating from the well-known 1/fγ spectrum. These devices showed broad peaks in the noise spectral density versus frequency plots, which shifted toward higher frequencies at elevated temperatures. The temperature dependence of the frequency position of this peak allowed us to determine the energy level of these excess traps as 0.22 ± 0.06 eV below the conduction band for the bias conditions employed.

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.

Degradation and phase noise of InAlN/AlN/GaN heterojunction field effect transistors: Implications for hot electron/phonon effects

In15.7%Al84.3%N/AlN/GaN heterojunction field effect transistors have been electrically stressed under four different bias conditions: on-state-low-field stress, reverse-gate-bias stress, off-state-high-field stress, and on-state-high-field stress, in an effort to elaborate on hot electron/phonon and thermal effects. DC current and phase noise have been measured before and after the stress. The possible locations of the failures as well as their influence on the electrical properties have been identified. The reverse-gate-bias stress causes trap generation around the gate area near the surface which has indirect influence on the channel. The off-state-high-field stress and the on-state-high-field stress induce deterioration of the channel, reduce drain current and increase phase noise. The channel degradation is ascribed to the hot-electron and hot-phonon effects.

Field-assisted emission in AlGaN/GaN heterostructure field-effect transistors using low-frequency noise technique

We utilized low-frequency noise measurements to probe electron capture and emission from the traps in AlGaN/GaN heterostructure field-effect transistors as a function of drain bias. The excess noise-spectra due to generation-recombination effect shifted higher in frequency with the elevated temperature from room temperature up to 446 K. These temperature dependent noise measurements were carried out for four different drain-bias values from 4 up to 16 V with 4 V increments. The shift of the excess-noise in frequency was also seen with increasing drain bias. The characteristic recharging times for the trapped electrons varied within the range of 26 μs − 32 ms for the highest and lowest values of the drain voltage and temperature used in the experiment, respectively. The activation energies of the traps corresponding to the four different voltage values were extracted using temperature dependence by Arrhenius analysis. The trap energy at zero drain-bias was obtained as 0.71 eV by the extrapolation technique...

Reduction of Flicker Noise in AlGaN/GaN-Based HFETs After High Electric-Field Stress

We report on the evolution of AlGaN/GaN-based heterojunction field-effect transistor (HFET) operation under high-electric-field stress. Specifically, a 10 ~ 15 dB decrease in the flicker noise is observed after stress in contrast with what has been nominally observed and reported in the literature in the realm of direct-current characteristics. Gate lag measurements revealed a trap state with an activation energy of 0.20 eV in the pristine devices, which manifests itself as a generation-recombination peak in the flicker noise spectrum. This trap state becomes undetectable in gate lag and noise measurements after high-field stress. Analysis shows that the phenomena observed are consistent with the change of surface charge profile during high-electric-field stress.

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.

Low-frequency noise measurements of electrical stress in InAlN/GaN and AlGaN/GaN heterostructure field-effect transistors

We report on the low-frequency noise (LFN) measurements on GaN based heterostructure field-effect transistors (HFETs) on sapphire with InAlN and AlGaN barriers to investigate the effects of electrical stress. The HFETs with InAlN barrier undergone a DC stress at bias conditions of VDS=20V and VG= -4.5 for up to 4 hours in aggregate. These devices exhibited an LFN in the form of 1/fγ and a significant increase in the noise spectrum up to 15 dB for 2 hours and then the noise saturated for further stress durations. We also monitored the LFN for the HFETs with AlGaN barriers. The devices were stressed by applying 20V DC drain bias for up to 64 hours at various gate voltages. Stressing at a gate bias (VG) of -2V showed negligible degradation. On the other hand, stressing at VG=0V surprisingly reduced the noise power by about 4 to 15 dB in the frequency range of 1 Hz-100 kHz. Additionally, the InAlN-barrier HFETs exhibited 20-25 dB lower noise power than the ones with the AlGaN layer for the tested devices within the entire frequency range. The results suggest that the trap generation increases due to electrical stress in devices with InAlN barrier, whereas the noise power decreases as a function of stress in AlGaN/GaN HFETs due to an increase in the activation energy of the excess traps.

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.