AlGaN/GaN HEMT TRAP CHARACTERISTIC FREQUENCY DEPENDENCE ON TEMPERATURE AND ITS IMPACT ON THE RF POWER AMPLIFIER LINEARIZABILITY

This paper presents a study of the linearizability of AlGaN/GaN HEMT based RF power amplifiers, RFPAs, and its relation with the active device trap activation energy. Based on the theory of thermally activated traps and on the experimental determination of the trap activation energy, we could show that despite different devices may exhibit traps with the same emission timeconstant at room temperature, their characteristic frequency may change significantly under nominal operation because of their temperature rise. And this was found to be key to explain the distinct linearizability performance of the tested devices because different stimulus dynamics excite the long-term memory effects imposed by traps with sensible different levels.

Recombination and diffusion processes in electronic grade 4H silicon carbide

Carrier dynamics in n-type 4H-SiC epilayers of varying thicknesses and low Z1/2 defect concentrations are investigated here in wide ranges of excess carrier density and temperature. Several techniques are employed to monitor carrier diffusion and recombination processes, including light induced transient grating (LITG), microwave photoconductance decay (MPCD) and free carrier absorption (FCA) using ps-laser pulses at 355 nm. The observed increase of the diffusion coefficient with the increasing excitation level is explained by the transition from the minority to the bipolar transport regime. Its subsequent decrease with even higher excitations is found to be governed by band-gap renormalization and degeneracy effects. The bulk lifetime, limited by hole traps at 0.19–0.24 eV above the valence band at lower excitations, was found to decrease from few microseconds to hundreds of nanoseconds during the transition regime from the minority to the bipolar transport. Our temperature-dependent measurements confirmed the trap activation energy and provided the approximate functional form of electron and hole lifetimes as τe = 340 × (T/300 K)3/2 ns and τh = 100 × (T/300 K)–1/2 ns, for the temperature T range 80–800 K. It was found to hold for 65 and 120 μm sample thicknesses, while the lifetimes were found to be twice shorter for the sample 35 μm thick.

Threshold-Voltage Instability in SiC MOSFETs Due to Near-Interfacial Oxide Traps

There are two basic mechanisms that affect the threshold-voltage (VT) stability: oxide-trap activation and oxide-trap charging. Once additional oxide traps are activated, then they are free to participate in the charge-trapping processes that can, especially for older vintage devices, result in large VT shifts and potential device failure. More recent commercially-available devices show much smaller effects, and minimal trap activation. Given the dramatic improvements, it is now imperative that improved test methods be employed to properly separate out bad devices from good devices.

How to catch more prey with less effective traps: explaining the evolution of temporarily inactive traps in carnivorous pitcher plants

Carnivorous Nepenthes pitcher plants capture arthropods with specialized slippery surfaces. The key trapping surface, the pitcher rim (peristome), is highly slippery when wetted by rain, nectar or condensation, but not when dry. As natural selection should favour adaptations that maximize prey intake, the evolution of temporarily inactive traps seems paradoxical. Here, we show that intermittent trap deactivation promotes ‘batch captures' of ants. Prey surveys revealed that N. rafflesiana pitchers sporadically capture large numbers of ants from the same species. Continuous experimental wetting of the peristome increased the number of non-recruiting prey, but decreased the number of captured ants and shifted their trapping mode from batch to individual capture events. Ant recruitment was also lower to continuously wetted pitchers. Our experimental data fit a simple model that predicts that intermittent, wetness-based trap activation should allow safe access for ‘scout’ ants under dry conditions, thereby promoting recruitment and ultimately higher prey numbers. The peristome trapping mechanism may therefore represent an adaptation for capturing ants. The relatively rare batch capture events may particularly benefit larger plants with many pitchers. This explains why young plants of many Nepenthes species additionally employ wetness-independent, waxy trapping surfaces.

On Photoelectrical Properties of 6H-SiC Bulk Crystals PVT-Grown on 6H- and 4H-SiC Substrates

Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN0from 1017cm-3to 1019cm-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energyEa= 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN01019cm3), both crystals revealed the bipolar diffusion coefficientDa4 cm2/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.

A Study of High Temperature DC and AC Gate Stressing on the Performance and Reliability of Power SiC MOSFETs

Although high-temperature measurements show a dramatic reduction in the bias-temperature stress-induced threshold-voltage instability of present state-of-the-art devices, a more thorough test methodology shows that several different conclusions may actually be drawn. The particular conclusion depends on the specific post-BTS measurement technique employed. Immediate room-temperature measurements suggest that significant oxide-trap activation may still be occurring. A significant, yet rapid, post-BTS recovery is observed as well. These results underline the importance of making both high-temperature and room-temperature measurements, as a function of stress and recovery time, to better ensure that the full effect of the BTS is observed. Initial AC BTS results suggest a similar level of device degradation as occurs from a DC BTS.