Carlo De Santi

Deep-Level Characterization in GaN HEMTs-Part I: Advantages and Limitations of Drain Current Transient Measurements

This paper critically investigates the advantages and limitations of the current-transient methods used for the study of the deep levels in GaN-based high-electron mobility transistors (HEMTs), by evaluating how the procedures adopted for measurement and data analysis can influence the results of the investigation. The article is divided in two parts within Part I. 1) We analyze how the choice of the measurement and analysis parameters (such as the voltage levels used to induce the trapping phenomena and monitor the current transients, the duration of the filling pulses, and the method used for the extrapolation of the time constants of the capture/emission processes) can influence the results of the drain current transient investigation and can provide information on the location of the trap levels responsible for current collapse. 2) We present a database of defects described in more than 60 papers on GaN technology, which can be used to extract information on the nature and origin of the trap levels responsible for current collapse in AlGaN/GaN HEMTs. Within Part II, we investigate how self-heating can modify the results of drain current transient measurements on the basis of combined experimental activity and device simulation.

Time-Dependent Failure of GaN-on-Si Power HEMTs With p-GaN Gate

This paper reports an experimental demonstration of the time-dependent failure of GaN-on-Si power high-electron-mobility transistors with p-GaN gate, submitted to a forward gate stress. By means of combined dc, optical analysis, and 2-D simulations, we demonstrate the following original results: 1) when submitted to a positive voltage stress (in the range of 7-9 V), the transistors show a time-dependent failure, which leads to a sudden increase in the gate current; 2) the time-to-failure (TTF) is exponentially dependent on the stress voltage and Weibull-distributed; 3) the TTF depends on the initial gate leakage current, i.e., on the initial defectiveness of the devices; 4) during/after stress, the devices show a localized luminescence signal (hot spots); the spectral investigation mainly reveals a peak corresponding to yellow luminescence and a broadband related to bremsstrahlung radiation; and 5) 2-D simulations were carried out to clarify the origin of the degradation process. The results support the hypothesis that the electric field in the AlGaN has a negligible impact on the device failure; on the contrary, the electric field in the SiN and in the p-GaN gate can play an important role in favoring the failure, which is possibly due to a defect generation/percolation process.

Reliability and failure analysis in power GaN-HEMTs: An overview

Power GaN transistors have recently demonstrated to be excellent devices for application in power electronics. The high breakdown field and the superior mobility of the 2-dimensional electron gas allow to fabricate transistors with low resistive and switching losses, that permit to increase the efficiency of switching mode power converters beyond 99 %. GaN-based transistors are currently supposed to be adopted in KW-range power converters; 650 V transistors are already available on the market, and 1200 V devices are currently under development. During operation, GaN power transistors can reach critical conditions, especially in the off-state (with a high VDS, in excess of 650 V), during hard-switching (where high current and voltage can be simultaneously present), and for high positive gate voltages (in the case of normally-off devices). This paper reports our most recent results on the gradual and catastrophic degradation of GaN-based power HEMTs. We present the results of three different case studies, on: (i) the time-dependent breakdown of power HEMTs submitted to high off-state stress; (ii) the degradation of HEMTs with p-GaN gate submitted to high gate stress; (iii) the hot electron effects in GaN-MISHEMTs submitted to high-temperature source current stress.

Experimental Demonstration of Time-Dependent Breakdown in GaN-Based Light Emitting Diodes

This letter reports the first experimental demonstration of time-dependent breakdown in GaN-based light-emitting diodes (LEDs); based on a number of constant voltage stress tests, carried out below the breakdown limit of the devices, we show that: 1) when submitted to reverse-bias stress, the LEDs show a gradual increase in current, well correlated with an increase in the breakdown luminescence signal; 2) for sufficiently long stress times, the LEDs can reach a catastrophic (sudden) breakdown, which leads to the failure of the devices; 3) the breakdown process is time-dependent and the time to failure (TTF) has an exponential dependence on stress voltage; and 4) TTF is Weibull distributed. The results presented within this letter demonstrate that the GaN-based heterostructures can show a TDDB-like behavior, and can be useful for the interpretation of the degradation data of LEDs and HEMTs.

Degradation of GaN-HEMTs with p-GaN Gate: Dependence on temperature and on geometry

This paper investigates the time-dependent degradation of normally-off transistors with p-GaN gate submitted to constant voltage stress. Based on combined dc characterization and temperature-dependent analysis, we study the dependence of time-to-failure on stress temperature and device geometry. The results of this analysis indicate that: (i) normally-off transistors with p-GaN gate have a good stability, reaching a 20 years lifetime with a 7.2 V gate bias; (ii) at higher stress voltages, a time-dependent failure is observed. Time-to-failure (TTF) depends exponentially on stress voltage, while failure is ascribed to a localized breakdown process that takes place in the p-GaN/AlGaN stack; (iii) TTF scales with device area only if the area is changed by increasing the gate width (and not if area is increased by modifying gate length). This result suggests that degradation occurs mostly in proximity of the gate edge, rather than at the center of the gate. (iv) finally, stress tests carried out at different temperature levels indicate that TTF is dependent on temperature, with activation energy of 0.48–0.50 eV.

Defect-Related Degradation of AlGaN-Based UV-B LEDs

This paper describes an extensive analysis of the degradation of (InAlGa)N-based UV-B light-emitting diodes (LEDs) submitted to constant current stress. This paper is based on combined electrical characterization, spectral analysis of the emission, deep-level transient spectroscopy (DLTS) and photocurrent (PC) spectroscopy. The results of this analysis demonstrate that: 1) UV-B LEDs show a gradual degradation when submitted to constant current stress; the decrease in optical power is stronger for low measuring current levels, indicating that degradation is related to the increase in Shockley-Read-Hall (SRH) recombination; 2) the current-voltage characteristics measured before/during stress show an increase in the current below the turn-on voltage, that is ascribed to the increase in trap-assisted tunneling (TAT) components; and 3) DLTS analysis and PC spectroscopy measurements were carried out to identify the properties of the defects responsible for the degradation of the optical and electrical characteristics. The results indicate that stress induces or activates defects centered around 2.5 eV below the conduction band edge. These defects, close to midgap, can explain both the increased SRH recombination and the increase in TAT components detected after stress. Moreover, the DLTS measurements allowed to identify the signature of Mg-related acceptor traps.

Nanoscale Investigation of Degradation and Wavelength Fluctuations in InGaN-Based Green Laser Diodes

We present a study of the optical inhomogeneities and degradation of InGaN-based green laser diodes based on high-resolution cathodoluminescence (CL) investigation of the output facets and inner cross-section of the devices. The results indicate that 1) degradation originates from a diffusion process, which causes an increase in the threshold current. 2) Nanoscale-level CL points out the circular symmetry of the degraded area, which is wider than the ridge and includes not only the quantum wells but also the waveguiding and cladding layers. 3) The yellow luminescence decreases within the degraded region, whereas its intensity increases outside of the degraded region. 4) Wavelength fluctuations are found in both quantum wells and waveguides, which are critically analyzed and ascribed to inhomogeneity in indium concentration. Their possible effect in the filamentation of the laser emission is discussed.

Towards high reliability GaN LEDs: Understanding the physical origin of gradual and catastrophic failure

This paper reviews the most relevant mechanisms responsible for the gradual and catastrophic failure of InGaN-based light emitting diodes, for application in general lighting. Based on recent results obtained on state-of-the-art LEDs and lamps, we discuss the following: (i) the lifetime of 800 lm retrofit lamps is still limited by the early degradation of the InGaN-LEDs, of the plastic encapsulant/reflectors, and of the driving circuitry. High temperatures and/or poor thermal management can significantly reduce the lifetime of the lamps in real-life applications. (ii) mid-power LEDs, that represent a low-cost widely adopted alternative to power devices, can suffer from both chip and package degradation, that result in a significant decrease in optical power and shift in the chromatic properties. (iii) Electrical overstress (EOS) can result in the sudden failure of power LEDs; failure can be due to the fusion of the bonding wires, to the degradation of the metal lines, or to the cracking of the semiconductor material. The results presented within this paper provide an update on the state-of-the-art of LED reliability.

Defect generation in deep-UV AlGaN-based LEDs investigated by electrical and spectroscopic characterisation

The paper reports the analysis of (In)AlGaN-based UV-B LEDs degradation under constant current stress, and investigates the impact of defects in changing the devices electro-optical performance. The study is based on combined electro-optical characterization, deep-level transient- (DLTS) and photocurrent spectroscopy. UV-B LEDs show a decrease of the optical power during stress, more pronounced at low measuring current levels, indicating that the degradation is related to an increase of Shockley-Read-Hall (SRH) recombination. DLTS measurements allowed the identification of three defects, in particular one ascribed to Mg-related acceptor traps presence. Photocurrent spectroscopy allows the localization of the defects close to the mid-gap.

Performance-Limiting Traps in GaN-Based HEMTs: From Native Defects to Common Impurities

This chapter describes the properties of the performance-limiting defects in GaN-based transistors. The first part of the chapter summarizes the properties of the most common defects in GaN, by describing a database of defects that has been prepared by our group based on a collection of more than 80 papers on the topic. The second part of the chapter describes the results of our most recent experiments on the impact of common native defects (vacancies, surface states, etc.) and impurities (such as Fe and C) on the dynamic performance of GaN HEMTs. Information on the correlation between epitaxial structure, process quality, and dynamic performance is given in the text.