Alfonso Patti

Nondestructive testing of power MOSFET's failures during reverse recovery of drain-source diode

The failure of power MOSFET during reverse recovery of its intrinsic drain-source diode is experimentally studied by means of a nondestructive tester. The analysis is based on the observation of the waveforms during the failure which shows the evidence of the activation of parasitic bipolar transistor. This paper advances the hypothesis that the failure mechanism is strictly related to how this transistor is being activated during reverse recovery. In particular, for higher values of dI/sub D//dt, the bipolar parasitic transistor on small area of the chip is activated at the early beginning voltage rise. Device failure is then caused by the second breakdown of this transistor which takes place during its turn-off.

Slow and fast traps in metal-oxide-semiconductor capacitors fabricated on recessed AlGaN/GaN heterostructures

In this letter, slow and fast trap states in metal-oxide-semiconductor (MOS) capacitors fabricated on recessed AlGaN/GaN heterostructures were studied by frequency dependent conductance measurements. In particular, the comparison of devices before and after annealing in forming gas allowed to ascribe the fast states (with characteristic response time in the range of 5–50 μs) to SiO2/GaN “interface traps,” and the slow states (50–100 μs) to “border traps” located few nanometers inside the SiO2 layer. These results can be important to predict and optimize the threshold voltage stability of hybrid MOS-based transistors on GaN.

Effects of Annealing Treatments on the Properties of Al/Ti/p-GaN Interfaces for Normally OFF p-GaN HEMTs

This paper reports on the behavior of Al/Ti/ p-GaN interfaces as gate contacts for p-GaN/AlGaN/GaN normally off high electron mobility transistor (HEMTs), highlighting the impact of the thermal budget on the metal gate on the device characteristics. In fact, while the devices subjected to an annealing at 800°C show a considerable high leakage current, those with nonannealed Al/Ti gate contacts exhibit a normally off behavior, with a pinch-off voltage Vpo = +1.1 V and an on/off current ratio of 3 × 108. Temperature-dependent electrical measurements on back-to-back Schottky diodes allowed to determine a Schottky barrier height ΦB of 2.08 and 1.60 eV, for the nonannealed and 800°C annealed gate contacts, respectively. Hence, the increase in the leakage current observed upon annealing at 800°C was attributed to the lowering of the Schottky barrier height ΦB of the metal gate. The interfacial structural characterization explained the barrier lowering induced by the annealing. This scenario was discussed through the simulated band diagram of the heterostructures, considering the experimental values of ΦB. These results provide useful information for the device makers to optimize the fabrication flow of normally off HEMTs with p-GaN gate.

Experimental and Numerical Analysis of Hole Emission Process From Carbon-Related Traps in GaN Buffer Layers

The role of carbon-related traps in GaN-based ungated high-electron mobility transistor structures has been investigated both experimentally and by means of numerical simulations. A clear quantitative correlation between the experimental data and numerical simulations has been obtained. The observed current decrease in the tested structure during backgating measurements has been explained simply by means of a thermally activated hole-emission process with EA = 0.9 eV, corresponding to the distance of the acceptor-like hole-trap level from the GaN valence band. Moreover, it has been demonstrated by means of electrical measurements and numerical simulations that only a low percentage of the nominal carbon doping levels induces the observed current reduction when negative substrate bias is applied to the tested structure.

GaN HEMT devices: Experimental results on normally-on, normally-off and cascode configuration

Power electronics systems play key function in power management and motion control: power consumption and volume reduction are strongly required in new applications oriented to protect environment. Moreover, a switching frequency increase is demanded for microwave and power switching applications, thus reducing passive component and converter volume: however, increasing the switching frequency directly increases switching losses. Also, switching applications are very demanding, because semiconductor switches are required to withstand high voltage in reverse condition and handle large current in forward operation mode. New material and new devices are studied to satisfy such requirements, like SiC and GaN devices. In this work, several 600-V class GaN-on-Si HEMT prototypes are presented. These devices have been designed and developed for power switching converter applications. In order to offer a complete scenario of GaN-on-Si HEMT technology, normally-on, normally-off, and cascode-connected devices have been characterized. Achieved experimental results of static and pulsed measurements are then shown.

Correlation between dynamic Rdsou transients and Carbon related buffer traps in AlGaN/GaN HEMTs

The on resistance increment observed when the device is operated at high drain-source voltages is one the topics that limits the performance of the AlGaN/GaN HEMT devices. In this paper, the physical mechanisms responsible of the RDSon degradation are investigated. The dynamic RDSon transient method is used in order to get insight to characterize the traps states. By calculating the Arrhenius plot associated with the RDSon transients an activation energy of 0.86eV was extracted, that can be correlated to the traps due to the incorporation of Carbon inside the buffer. This hypothesis was further supported by the analyses performed on a simpler structure (TLM). By applying a negative substrate bias the effect of only the buffer traps was studied. A fairly close value of the activation energy (0.9eV) to the one extracted when analyzing the RDSon transient was obtained.

Advanced RBSOA analysis for advanced power BJTs

Abstract Performances of some modern power BJTs in inductive turn-off are experimentally evaluated, by means of an unclamped non-destructive method. The different instabilities exhibited are classified and their influence on device performances is discussed both in clamped and unclamped applications An “Instability Map” is proposed both as a synthetic picture which eases comparison of reverse-bias performances of devices having different ratings, and as an investigation tool for linking device behaviour to its physical features. It results that RBSOA performances are not just related to lateral dimensions of the emitter, but also to metallization lay-out of the chip, which evidently influences current distribution among cells. Finally, stray elements of testing circuit which affect results of RBSOA measurements are investigated, and some suggestions are proposed in order to let measurement results become independent of testing circuit.

Metal/P-GaN Contacts on AlGaN/GaN Heterostructures for Normally-Off HEMTs

In this paper, the electrical properties of different metal/p-GaN contacts (Ti/Al, TiN/Ti/Al and Ni/Au) have been investigated to get a deeper understanding of the behavior of p-GaN/AlGaN/GaN heterostructures for normally-off HEMTs. In particular, the study of the temperature dependent current-voltage characteristics allowed to identify the dominant carrier transport mechanism at the metal/p-GaN interface (Thermionic Field Emission). From the fit of the experimental current-voltage data it was possible to determine the Schottky barrier height values for the three systems, 2.08 eV (Ti/Al), 1.57 eV (TiN/Ti/Al) and 1.89 eV (Ni/Au). Hence, choosing the highest barrier height contact (Ti/Al) as gate electrode on a p-GaN/AlGaN/GaN heterostructure, optimized based on simulations, allowed to obtain devices with a normally-off behavior and a positive Vth of +1.3 V.

SiC- and GaN-based power devices: Technologies, products and applications

Compound semiconductors (and mainly at the moment SiC and GaN) power devices have practically shown a quantum leap in the performances of power devices and the possibility to enlarge the use of power electronics at very high voltages, high temperature and high power. However, the status of SiC and GaN devices today is much less mature than that of Si power devices in terms of manufacturability, material quality, process control, cost and reliability. In this paper activities on SiC and GaN power devices at STMicroelectronics will be presented and the perspectives of a large adoption of compound semiconductors power devices highlighted.

Trapping States in SiO2/GaN MOS Capacitors Fabricated on Recessed AlGaN/GaN Heterostructures

In this work, the different nature of trapping states in metal-oxide-semiconductor (MOS) capacitors fabricated on recessed AlGaN/GaN heterostructures has been investigated. In particular, the origin of both fast and slow states has been elucidated by frequency dependent conductance measurements. Using post deposition annealing in forming gas, which is known to induce hydrogen incorporation at the interface, allowed to ascribe the fast traps (with characteristic response time in the range of 5–50 μs) to SiO2/GaN interface states and the slow traps (50–100 μs) to “border traps” located few nanometers inside the SiO2 layer. The results can be useful to predict the behavior of GaN switching devices, like hybrid MOSHEMTs.