Characterizing threshold voltage shifts and recovery in Schottky gate and Ohmic gate GaN HEMTs

Threshold voltage shift in normally-OFF GaN High Electron Mobility Transistors (HEMTs) is an important reliability concern in GaN devices. Differences in device architecture between Schottky gate and Ohmic gate normally-OFF GaN HEMTs means that there are important differences in the physical mechanism behind threshold voltage shift due to gate stress. In this paper, a non-intrusive technique for the characterization of threshold voltage shift is applied to both technologies. The technique relies on using a sensing current to measure the third quadrant voltage before and after gate-voltage stress. The results show that in Schottky Gate GaN HEMTs, a positive threshold voltage shift occurs at low gate stress voltages due to electron trapping in the GaN/AlGaN interface while at higher gate stress voltages, the threshold voltage shift becomes negative due to hole trapping and accumulation. The stress time has a fundamental role on the measured threshold voltage shift at medium gate voltage levels and pulsed gate stresses are able to capture this phenomenon. For the Ohmic Gate GaN HEMTs, only a negative threshold voltage shift is observed for all stress currents with no apparent shift as the junction temperature is increased

Non-intrusive methodologies for characterization of bias temperature instability in SiC power MOSFETs

The gate oxide reliability of SiC power MOSFETs remains a challenge, despite the improvements of the new generation power devices. The threshold voltage drift caused by Bias Temperature Instability (BTI) has been subject of different studies and methods have been proposed to evaluate the real magnitude of the threshold voltage shift. These methodologies usually focus on the characterization of the threshold voltage shift, rather than its implications to the operation or how the threshold voltage shift can be detected during the application. This paper presents two non-intrusive methodologies which can assess and determine the impact of BTI-induced. The proposed methodologies are able to capture the peak shift and subsequent recovery after stress removal

Bias temperature instability and condition monitoring in SiC power MOSFETs

Threshold voltage shift due to bias temperature instability (BTI) is a major concern in SiC power MOSFETs. The SiC/SiO2 gate dielectric interface is typically characterized by a higher density of interface traps compared to the conventional Si/SiO2 interface. The threshold voltage shift that arises from BTI has significant implications on the reliability of SiC power MOSFETs, hence, techniques for detecting the change in electrical parameters due to gate oxide degradation are desirable. Using accelerated high temperature gate bias stress tests on SiC MOSFETs, it has been shown that the output and transfer characteristics are affected by BTI. This paper presents the impact BTI induced threshold voltage shift on the forward voltage of the SiC MOSFET body diode during third quadrant operation. Using the forward voltage of the body diode during reverse conduction of low currents, threshold voltage shift can be detected, hence, the impact of BTI can be evaluated. The implications of the body diode forward voltage shift on junction temperature measurements are also studied in the context of TSEPs. The findings in this paper are important for engineers seeking to implement condition and health monitoring techniques on SiC power devices

Instability mechanisms in amorphous oxide semiconductors leading to a threshold voltage shift in thin film transistors

Amorphous indium gallium zinc oxide (a-IGZO) has been successfully employed commercially as the channel layer in thin film transistors (TFTs) for active-matrix flat panel displays. However, these TFTs are known to suffer from a threshold voltage shift upon application of a gate bias. The threshold voltage shift is reversible through annealing. A similar phenomenon is observed in other TFTs with an amorphous oxide semiconductor channel. The migration of oxygen vacancies is proposed as being the microscopic mechanism causing this effect as it can lead to a change in the equilibrium distribution of defect states in the band gap of the semiconductor. This would manifest itself as a reversible threshold voltage shift in the TFT transfer characteristics, as observed experimentally.The support of this work by the Engineering and Physical Sciences Research Council (EPSRC) through project EP/M013650/1 is acknowledged

Crosstalk in SiC power MOSFETs for evaluation of threshold voltage shift caused by bias temperature instability

Threshold voltage drift from Bias Temperature Instability is known to be a reliability concern for SiC MOSFETs. Negative bias temperature instability (NBTI) results from positive charge trapping at the gate dielectric interface and is more problematic in SiC due to the higher interface trap density. Turning SiC MOSFETs OFF with negative voltages to avoid Miller coupling induced cross-talk can cause VTH shifts in periods with long standby duration and high temperatures. This paper proposes a novel test method for BTI characterization that relies on measuring the shoot-through current and charge during switching transients. The method exploits the Miller coupling between 2 devices in the same phase and uses the shoot-through current from parasitic turn-ON to monitor VTH. Standard techniques require the use of static measurements (typically from a parameter analyzer or a curve tracer) to determine the threshold voltage shift. These conventional methods can underestimate the VTH shift since the recovery from charge de-trapping can mask the true extent of the problem. The proposed methodology uses the actual converter environment to investigate the VTH shift and should therefore be of more interest to applications engineers as opposed to device physicists. Furthermore, it avoids the problem of VTH recovery and is therefore more accurate in VTH shift characterization

Operational stability of solution based zinc tin oxide/SiO2 thin film transistors under gate bias stress

In this study, we report solution-processed amorphous zinc tin oxide transistors exhibiting high operational stability under positive gate bias stress, translated by a recoverable threshold voltage shift of about 20% of total applied stress voltage. Under vacuum condition, the threshold voltage shift saturates showing that the gate-bias stress is limited by trap exhaustion or balance between trap filling and emptying mechanism. In ambient atmosphere, the threshold voltage shift no longer saturates, stability is degraded and the recovering process is impeded. We suggest that the trapping time during the stress and detrapping time in recovering are affected by oxygen adsorption/desorption processes. The time constants extracted from stretched exponential fitting curves are approximate to 10(6) s and 10(5) s in vacuum and air, respectively. (C) 2015 Author(s).FEDER through COMPETE 2020 Programme; European Communities 7th Framework Programme (i-FLEXIS project) [ICT-2013-10-611070]; National Funds through FCT-Portuguese Foundation for Science and Technology [UID/CTM/50025/2013, EXCL/CTM-NAN/0201/2012]info:eu-repo/semantics/publishedVersio

Numerical approach for retention characteristics of double floating-gate memories

We report on a numerical investigation in which memory characteristics of double floating-gate (DFG) structure were compared to those of the conventional single floating-gate structure, including an interference effect between two cells. We found that the advantage of the DFG structure is its longer retention time and the disadvantage is its smaller threshold voltage shift. We also provide an analytical form of charging energy including the interference effect.Comment: 4 pages, 4 figure

Application of pMOS Dosimeters in Radiotherapy

The results of a study on pMOS dosimeters manufactured by Tyndall National Institute, Cork, Ireland and their sensitivity on radiation doses used in radiotherapy are presented. Firstly, we deal with analysis of defect precursors created by ionizing radiation, responsible for increase in fixed and switching traps, which are further responsible for threshold voltage shift as a dosimetric parameter. Secondly, influence of some parameters, such as gate bias during irradiation, gate oxide thickness and photons energies, on threshold voltage shift is presented. Fading of irradiated pMOS dosimeters and possible application of commercial MOSFETs in ionizing radiation dosimetry are also presented

Bias-induced threshold voltages shifts in thin-film organic transistors

An investigation into the stability of metal-insulator-semiconductor (MIS) transistors based on alpha-sexithiophene is reported. In particular, the kinetics of the threshold voltage shift upon application of a gate bias has been determined. The kinetics follow stretched-hyperbola-type behavior, in agreement with the formalism developed to explain metastability in amorphous-silicon thin-film transistors. Using this model, quantification of device stability is possible. Temperature-dependent measurements show that there are two processes involved in the threshold voltage shift, one occurring at Tapproximate to220 K and the other at Tapproximate to300 K. The latter process is found to be sample dependent. This suggests a relation between device stability and processing parameters. (C) 2004 American Institute of Physics

Gamma-ray irradiation and post-irradiation at room and elevated temperature response of pMOS dosimeters with thick gate oxides

Gamma-ray irradiation and post-irradiation response at room and elevated temperature have been studied for radiation sensitive pMOS transistors with gate oxide thickness of 100 and 400 nm, respectively. Their response was followed based on the changes in the threshold voltage shift which was estimated on the basis of transfer characteristics in saturation. The presence of radiation-induced fixed oxide traps and switching traps - which lead to a change in the threshold voltage - was estimated from the sub-threshold I-V curves, using the midgap technique. It was shown that fixed oxide traps have a dominant influence on the change in the threshold voltage shift during gamma-ray irradiation and annealing