Thermal Generation Currents Research Articles

5‐3: Experimental and Physics‐Based Analysis of Leakage Currents for LTPS TFTs in AMOLED Displays

This paper presents extremely‐low leakage technologies in low‐temperature polysilicon (LTPS) TFTs. Experimental and physics‐based analysis of leakage currents, emphasizing the effects of process technologies and device design, are described. Small‐geometry TFTs, controlled by optimal LTPS process, dramatically reduce off‐state leakage current (Ioff) by suppressing gate‐induced drain leakage (GIDL) and thermal generation currents, thus potentially offering lower frame rate operations by reducing IC clock power. Numerical device simulations, supplemented by physics‐based analysis, are performed to corroborate the remarkable low‐Ioff experimental results as well as more‐than‐twice enhanced on‐state current (Ion) in optimized LTPS devices.

LTPS TFTs with an Amorphous Silicon Buffer Layer and Source/Drain Extension

A low leakage poly-Si thin film transistor (TFT) is proposed featuring hydrogenated amorphous silicon (a-Si:H) buffer layer and source/drain extension (SDE) by using technology computer aided design (TCAD) simulation. This architecture reduces off-current effectively by suppressing two leakage current generation mechanisms with little on-current loss. The amorphous silicon buffer layer having large bandgap energy (Eg) suppresses both thermal generation and minimum leakage current, which leads to higher on/off current ratio. In addition, the formation of lightly doped region near the drain alleviates the field-enhanced generation in the off-state by reducing electric field. TCAD simulation results show that the proposed TFT shows more than three orders of magnitude lower off-current than low-temperature polycrystalline silicon (LTPS) TFTs, while maintaining on-current.

Temperature-Dependent Short-Circuit Capability of Silicon Carbide Power MOSFETs

This paper presents a comprehensive short-circuit ruggedness evaluation and numerical investigation of up-to-date commercial silicon carbide (SiC) MOSFETs. The short-circuit capability of three types of commercial 1200-V SiC MOSFETs is tested under various conditions, with case temperatures from 25 to 200 °C and dc bus voltages from 400 to 750 V. It is found that the commercial SiC MOSFETs can withstand short-circuit current for only several microseconds with a dc bus voltage of 750 V and case temperature of 200 °C. The experimental short-circuit behaviors are compared, and analyzed through numerical thermal dynamic simulation. Specifically, an electrothermal model is built to estimate the device internal temperature distribution, considering the temperature-dependent thermal properties of SiC material. Based on the temperature information, a leakage current model is derived to calculate the main leakage current components (i.e., thermal, diffusion, and avalanche generation currents). Numerical results show that the short-circuit failure mechanisms of SiC MOSFETs can be thermal generation current induced thermal runaway or high-temperature-related gate oxide damage.

Dark current mechanisms in amorphous selenium-based photoconductive detectors: an overview and re-examination

The transient and steady-state dark current behaviors in various amorphous selenium (a-Se) detectors are analyzed by developing mathematical models considering all possible mechanisms (e.g., carrier depletion, thermal generation, and carrier injection from electrodes) and charge carrier transport properties of a-Se. The theoretical models are validated by comparing them with recently published measured transient and steady-state dark currents in various a-Se detectors. The fittings of the models with the experimental results reveal various important material and device properties such as the optimum trap depth and concentrations in the blocking layers for faster stabilization and lower dark current, and the effective barrier height between the metal/a-Se contacts. The thermal generation current is significantly higher in avalanche detectors (at extremely high fields) than that in conventional detectors. The effective blocking layers for both holes and electrons are necessary for the minimum level of the dark current. The minimum dark current is determined by the thermal generation current. The thermal generation current can be lowered by reducing the mid gap defect states.

Spectroscopy of Technological Defects in Si Solar Cells by Analysis of Temperature Dependent Generation Currents

The efficiency of solar cells considerably depends on the technological defects introduced by the formation of junctions, passivation layers and electrodes. Identification of these defects present in the high conductivity base layer of modern solar cells by usage of the standard techniques, such as capacitance deep level spectroscopy, is restricted by extremely small size of samples with inherent enhanced leakage current on sample boundaries. Therefore, it is important to develop the alternative methods for the defect spectroscopy in the high conductivity junction structures, to directly control a relative low concentration of the technological defects. In this work, the spectroscopy of deep traps has been performed by combining the temperature scans of the thermal generation currents extracted from barrier capacitance charging transients and capacitance deep level transient spectroscopy techniques. The dominant carrier traps ascribed to the Cu and Ni impurities were revealed. DOI: http://dx.doi.org/10.5755/j01.ms.20.3.5194

Enhanced Dark Current Suppression of Amorphous Selenium Detector With Use of IGZO Hole Blocking Layer

We examined the potential application of indium gallium zinc oxide (IGZO) as a hole-blocking layer for an amorphous selenium (a-Se)-based detector to reduce the dark current and improve the sensitivity of the detector. By employing a thin layer of IGZO (375 nm), the dark current of an a-Se detector remains below 1 pA/mm 2 up to electric fields as high as 60 V/μm. The measured dark current at different electric fields is comparable to the thermal generation currents in a-Se, thus demonstrating the good hole-blocking properties of IGZO. The detector's photo response was characterized using a blue light-emitting diode at different electric fields. A factor of three improvement in external quantum efficiency was observed by increasing the electric field of the detector from 10 to 50 V/μm.

Dark current mechanisms in stabilized amorphous selenium based n-i detectors for x-ray imaging applications

The dark current behavior under operating bias is one of the important selection criteria for an x-ray photoconductor to be usable in a practical x-ray image detector. The authors have developed an analytical model for describing the transient and steady-state behavior of dark current in n-i-type amorphous selenium (a-Se) detectors by considering carrier injections from the metal contacts and thermally generated carriers. It has been found that the thermal generation current is almost two orders of magnitude smaller than the total steady-state dark current in n-i-type a-Se detectors. The main source of dark current is the injection of holes from the metal/n-layer interface which is described by the diffusion theory. The hole injection from the metal depends on the blocking layer (n-layer) thickness, the concentration of trap centers in the blocking layer, the characteristic carrier release time, and the effective barrier height. The fitting of the first principles model with the experimental results estimates the concentration of deep hole trap center in the n-layer, the trap depth from the valence band edge, and the effective barrier heights for the injecting carriers. The electron injection varies with the work function of the contact metal.

Modeling of transient and steady-state dark current in amorphous silicon p–i–n photodiodes

A theoretical model for describing the bias-dependent transient and steady-state behavior of dark current in hydrogenated amorphous silicon ( a-Si:H) p– i– n photodiode has been developed. An analytical expression for the bias-dependent steady-state thermal generation current is derived by solving the continuity equations for both electrons and holes. The model for describing transient dark current in a-Si:H p– i– n photodiode is developed by considering the depletion of electrons from the i-layer and carrier injection through p– i interface. For photodiodes that have very good junction properties, the high initial dark current decreases with time monotonously and reaches a plateau. However, in case of poor junctions, the injection current can be the dominating mechanism for transient leakage current at relatively high biases, the dark current decays initially and then rises to a steady-state value. The proposed physics-based dark current model is compared with published experimental results on several photodiodes. The comparison of the model with the experimental data allows an estimate of active dopant concentration in the p-layer and the defect density in the midgap of i-layer.

Experimental and numerical investigations on delayed short-circuit failure mode of single chip IGBT devices

This paper presents experimental and numerical investigations on the delayed failure mode of IGBT devices observed under short-circuit operations. For short-circuit energies lightly higher than a critical value, the default current may be successfully turned-off but a leakage current takes place leading to a thermal runaway and finally to a delayed failure. Numerical investigations have been carried out and a detailed analysis of the physical mechanisms occurring during the delayed failure mode is presented. We show that the classical short-circuit failure mode (energy limited) is a limit case of the delayed failure mode with zero delay time to failure. So, the analysis is extended to the energy limited case and give better understanding of the failure process of this last failure mode. An electro-thermal model including the device and the solder layer is presented as well as all leakage current components models like saturation, thermal and avalanche generation currents. A detailed description of the thermal runaway is given and main parameters influence on critical energy value are presented, especially, case temperature and solder layer thickness. Finally, numerical results allow to show that the critical energy which separates two failure modes corresponds to a threshold temperature of about 650 °C in the vicinity of the junction location which seems to be independent of test conditions.

Modeling of effects of material quality on fast characterization of state-of-the-art a-Si:H p +–i–n + diodes

High-quality amorphous silicon p +–i–n + diodes have very small dark leakage currents, allowing the detection of low intensity illuminations with a dynamic range of several orders of magnitude. Low currents imply that a change in the charging state of the device may take very long times, up to a few minutes. A correct current as a function of voltage measurement becomes time-consuming. We discuss and model the effects of the density of defects in intrinsic material on such measurements. The model takes into account Poole–Frenkel thermal generation currents and the rearranging of the equilibrium generation rates to explain the observed negative offset in the voltage at which zero current is measured in a fast measurement. The offset is related to the density of defects and the study, comparing model and experimental data, shows an approach to quickly evaluate the quality of the device with no need for a full time steady state measurements.

Dark current mechanisms and conditions of background radiation limitation of n-doped AlGaAs/GaAs quantum-well infrared detectors

The most important mechanism of dark current in n-doped quantum-well infrared photodetectors (QWIPs) is due to interaction of electrons with longitudinal optical phonons. Theoretical expressions are derived for the carrier lifetime, and for generation currents originating from both photoexcitation as well as from thermal excitation in a single quantum well. Detector gain is discussed briefly. Calculated values of thermal generation currents and the ratio of photocurrent to thermal current are found to accord well with experimental data. Finally the conditions of background radiation limitation of QWIPs are investigated and the theory gives a temperature of background radiation limitation TBLIP=81 K for a 9 μm cutoff detector with a two-dimensional grating and optical cavity, for 300 K background temperature, optics f number=1 with 100% optical transmission, provided that a photocurrent to dark current ratio of 1 criterion is used.

Thermal generation currents in hydrogenated amorphous silicon p-i-n structures

Dark conductivity in amorphous silicon p-i-n devices arising from thermal generation through bulk defect states is explored. The current decays slowly after a voltage is applied, due to depletion of charge from the undoped layer, and is voltage dependent due to a field-enhanced generation rate. Creation of metastable bulk defects by light soaking reversibly increases the current. The steady-state generation current is dervied from the measured relaxation time and depletion charge.

Investigation of the influence of the bias applied to phase electrodes on the thermal generation currents in charge-coupled devices

A study was made of the kinetics of thermal generation currents and of their dependence on the bias voltage applied to phase electrodes. A mechanism is proposed to explain the observed phenomena. The surface recombination velocity and the lifetime of the minority carriers are determined.

Thermal and optical generation current in reverse-biased gold-doped silicon p+n junctions without the depletion approximation

Optical and thermal generation currents in reverse-biased p-n junctions have been calculated for an exponential tail of free-charge carriers extending from the neutral regions into the transition region. Net generation of electron-hole pairs can then be neglected at the ends of the transition region because of recombination processes. The width W0 of these regions has been calculated for step junctions and asymmetrical linearly graded junctions. For a symmetrical step junction, W0 is independent of bias at constant illumination intensity. For a linearly graded junction, however, the widths of these regions become monotonically smaller with increasing reverse bias. In all cases, the regions with negligible net generation decrease logarithmically with intensity. The calculations are in good agreement with experimental results for gold-doped silicon diodes. Furthermore, it is shown that, for moderate average electric fields, neither the optical nor the thermal hole emission rate for the gold acceptor in silicon is field dependent.