Congyong Zhu

Degradation in InAlN/GaN-based heterostructure field effect transistors: Role of hot phonons

We report on high electric field stress measurements at room temperature on InAlN/AlN/GaN heterostructure field effect transistor structures. The degradation rate as a function of the average electron density in the GaN channel (as determined by gated Hall bar measurements for the particular gate biases used), has a minimum for electron densities around 1×1013 cm−2, and tends to follow the hot phonon lifetime dependence on electron density. The observations are consistent with the buildup of hot longitudinal optical phonons and their ultrafast decay at about the same electron density in the GaN channel. In part because they have negligible group velocity, the build up of these hot phonons causes local heating, unless they decay rapidly to longitudinal acoustic phonons, and this is likely to cause defect generation which is expected to be aggravated by existing defects. These findings call for modified approaches in modeling device degradation.

Effect of hot phonon lifetime on electron velocity in InAlN/AlN/GaN heterostructure field effect transistors on bulk GaN substrates

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructure field effect transistors (HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields present in the HFET channel, and thus, we interpret the minimum intrinsic transit time in terms of the hot phonon decay. At the gate bias associated with the minimum transit time, we determined the average electron velocity for a 1.1 μm gate length device to be 1.75±0.1×107 cm/sec.

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 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.

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.

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.