Temperature Tj Research Articles

Ionization equilibrium at the transition from valence-band to acceptor-band migration of holes in boron-doped diamond

A quasi-classical model of ionization equilibrium in the p-type diamond between hydrogen-like acceptors (boron atoms which substitute carbon atoms in the crystal lattice) and holes in the valence band (v-band) is proposed. The model is applicable on the insulator side of the insulator–metal concentration phase transition (Mott transition) in p-Dia:B crystals. The densities of the spatial distributions of impurity atoms (acceptors and donors) and of holes in the crystal are considered to be Poissonian, and the fluctuations of their electrostatic potential energy are considered to be Gaussian. The model accounts for the decrease in thermal ionization energy of boron atoms with increasing concentration, as well as for electrostatic fluctuations due to the Coulomb interaction limited to two nearest point charges (impurity ions and holes). The mobility edge of holes in the v-band is assumed to be equal to the sum of the threshold energy for diffusion percolation and the exchange energy of the holes. On the basis of the virial theorem, the temperature Tj is determined, in the vicinity of which the dc band-like conductivity of holes in the v-band is approximately equal to the hopping conductivity of holes via the boron atoms. For compensation ratio (hydrogen-like donor to acceptor concentration ratio) K ≈ 0.15 and temperature Tj, the concentration of “free” holes in the v-band and their jumping (turbulent) drift mobility are calculated. Dependence of the differential energy of thermal ionization of boron atoms (at the temperature 3Tj/2) as a function of their concentration N is calculated. The estimates of the extrapolated into the temperature region close to Tj hopping drift mobility of holes hopping from the boron atoms in the charge states (0) to the boron atoms in the charge states (−1) are given. Calculations based on the model show good agreement with electrical conductivity and Hall effect measurements for p-type diamond with boron atom concentrations in the range from 3 × 1017 to 3 × 1020 cm−3, i.e., up to the Mott transition. The model uses no fitting parameters.

Role of electrostatic fluctuations in doped semiconductors upon the transition from band to hopping conduction (by the example of p-Ge:Ga)

The electrostatic model of ionization equilibrium between hydrogen-like acceptors and v-band holes in crystalline covalent p-type semiconductors is developed. The range of applicability of the model is the entire insulator side of the insulator–metal (Mott) phase transition. The density of the spatial distribution of acceptor- and donor-impurity atoms and holes over a crystal was assumed to be Poissonian and the fluctuations of their electrostatic potential energy, to be Gaussian. The model takes into account the effect of a decrease in the energy of affinity of an ionized acceptor to a v-band hole due to Debye–Huckel ion screening by both free v-band holes and localized holes hopping over charge states (0) and (–1) of acceptors in the acceptor band. All donors are in charge state (+1) and are not directly involved in the screening, but ensure the total electroneutrality of a sample. In the quasiclassical approximation, analytical expressions for the root-mean-square fluctuation of the v-band hole energy Wp and effective acceptor bandwidth Wa are obtained. In calculating Wa, only fluctuations caused by the Coulomb interaction between two nearest point charges (impurity ions and holes) are taken into account. It is shown that Wp is lower than Wa, since electrostatic fluctuations do not manifest themselves on scales smaller than the average de Broglie wavelength of a free hole. The delocalization threshold for v-band holes is determined as the sum of the diffusive-percolation threshold and exchange energy of holes. The concentration of free v-band holes is calculated at the temperature Tj of the transition from dc band conductivity to conductivity implemented via hopping over acceptor states, which is determined from the virial theorem. The dependence of the differential energy of the thermal ionization of acceptors at the temperature 3Tj/2 on their concentration N and degree of compensation K (the ratio between the donor and acceptor concentrations) is determined. Good quantitative agreement between the results of the calculation and data on the series of neutron transmutation doped p-Ge samples is obtained up to the Mott transition without using any fitting parameters.

Estimation of the average junction temperature of two phosphors-converted white LED array based on (B + Y + R)/B ratio

The method of non-contact measurement of the junction temperature (Tj) for phosphor-converted white LEDs based on W/B ratio, the ratio of the total radiant energy (W) to the radiant energy of blue emission (B), is verified firstly. It is shown that for two phosphors (Y3Al5O12:Ce and CaAlSiN3:Eu)-converted white LEDs, an significant uncertainty is introduced into the linearity between Tj and W/B ratio. Then a new approach is proposed which uses (B+Y+R)/B ratio, the ratio of the sum of radiant energies of blue emission (B), yellow emission (Y) and red emission (R) to the radiant energy of blue emission, to establish the correlation with Tj. Result shows that the proposed approach is of a satisfactory linearity between Tj and (B+Y+R)/B ratio, with R-square equal to 0.9906 and RMSE equal to 2.27°C. It is also demonstrated that the proposed method is applicable to actual LED lighting system composed of large number of LEDs.

Microarc oxidation coated magnesium alloy radiator for light emitting diode: Microstructure, thermal radiative and dissipating property

Abstract Light emitting diode (LED) as a new generation light source whose lifetime and luminescence efficiency drop rapidly with the P-N junction temperature (Tj) increase. In this paper, a high emissivity ceramic coating fabricated by microarc oxidation (MAO) on magnesium alloy radiator enhanced the heat dissipation of LED. And the effects of MAO coatings with different phase composition on infrared emissivity and Tj were studied. The results show that Mg-Si-O and Ca-Mg-Si-O coatings exhibit a high emissivity up to 0.8 at 623 K within 8–20 μm wavelength range. The formation of CaSiO3 phase in Ca-Mg-Si-O coating contributes to the short wavelength shift of the high emissivity region due to the enhanced lattice vibration absorption. The increasing coating surface roughness is beneficial to promote the emissivity. The enhanced thermal radiation with coated magnesium radiator significantly promoted the heat dissipating of LED. Compared with the uncoated magnesium radiator, the Mg-Si-O and Ca-Mg-Si-O coated samples with oxidation time of 10 min enable the temperature of LED to drop approximately at 6.2 °C and 7.3 °C, respectively.

Metallic TIM Testing and Selection for Harsh Environment Applications for GaN RF Semiconductors

Summary An important determinant of device reliability is operating temperature control. Maintaining a semiconductor device at or below the maximum rated junction temperature (Tj) is accomplished through careful thermal management design and selection of well-performing thermal interface materials (TIM) that minimize efficiency losses in packaging and between the semiconductor device package and a heat sink or liquid cold plate. Thermal management design is increasingly important in harsh operating environments, especially where higher operating temperatures are specified. Minimizing thermal resistances through each semiconductor package material stack and at the external case or package surface of the device are important aspects of maintaining operating temperatures within specified maximum values. In addition, certain semiconductor devices require an electrical path from the semiconductor case to an external component. Maximizing electrical performance of gallium nitride (GaN) RF semiconductors is critical to system performance, as a primary example. The on-going transition within RF and microwave systems from silicon to GaN devices has increased the need for thermal interface materials which offer both improved thermal performance and electrical conductivity. Additionally, GaN semiconductor die are typically smaller in footprint and, even with equivalent power dissipation values, therefore may operate with higher heat flux values that require greater attention to proper thermal solution design. To address such needs, recently-developed forms of metallic TIM preforms are available for integrated circuits, power semiconductors, and RF devices. Understanding how these materials may be tested and selected for specific application requirements is the subject of this discussion.

A hybrid prediction method on luminous flux maintenance of high-power LED lamps

A hybrid method, applied thermal modeling and temperature measurement, is proposed to estimate the junction temperature (Tj) of high-power light-emitting diodes (LEDs) at system level, and further project long term lumen maintenance of LED lamps. First, 3D finite element modeling on commercial LED lamps is performed based on lamp structure and material information to find a steady and reliable relationship between the junction point and a referenced point on the heat sink. Then, accurate thermal measurement is conducted on the heat sink to calibrate and verify the finite element model. The predicted Tj of LEDs from modeling, in conjunction with LM-80 luminous maintenance data using TM-21, is applied to project the luminous flux depreciation at the system level. The proposed approach is validated by aging tests at room ambience. Results show the thermal resistance modeling can be simplified into a one-dimensional model when a LED lamp operates in a steady situation. Thus, the Tj of LED lamps operating at any specified ambient temperature can be achieved quickly. The estimation method for predicting luminous maintenance of LED lamps is efficient and fast. The proposed method is expected to be useful for the fast qualification of LED lamps.

Modeling the spectral shape of InGaAlP-based red light-emitting diodes

We have developed a spectral model for describing the shape of the emission spectrum of InGaAlP-based red light-emitting diodes (LEDs) with quantum-well structure. The model is based on Maxwell-Boltzmann distribution with junction temperature Tj and an experimental two-dimensional joint density of states (DOS). We model the DOS with a sum of two exponentially broadened step functions describing the two lowest sub-bands in semiconductor quantum well. The relative locations ΔE1 = 0 meV and ΔE2 = 112.7 meV above the band gap energy Eg = 1.983 eV and the ratio 2.13 of the step heights were fixed using an experimental DOS extracted from a LED spectrum measured at known Tj and driving current I. The model can then be fitted to other spectra of other LED samples at varied Tj and I by varying the fitting parameters Eg, Tj, and the broadening of the sub-band edges. The model was tested for three LED samples over I = 200–370 mA and Tj = 303–398 K. Junction temperatures obtained by modeling were compared with calibrated Tj obtained by the forward voltage method. The mean absolute difference was about 2.9 K (0.8%) over the whole region studied and the maximum difference was 8.5 K. The thermal coefficient measured for Eg was −0.509 meV K−1. For the first and second sub-band edges, the thermal broadening coefficients were 18 μeV K−1 and 37 μeV K−1, respectively.

Temperature‐dependent optical, spectral, and thermal characteristics of InGaN/GaN near‐ultraviolet light‐emitting diodes

The temperature‐dependent device characteristics of InGaN/GaN near‐ultraviolet light‐emitting diodes, operating at λ ∼380 nm, with a chip size of 0.5 × 1 mm2 were reported. Their optical and spectral properties were measured and analyzed at different injection current levels and heatsink temperatures. The device performance showed the optical output power of 92.8 mW, forward voltage of 4.30 V, and emission peak wavelength of 380 nm at 350 mA and 298 K. The junction temperature (Tj) was experimentally estimated via the forward voltage method, leading to a thermal resistance of ∼10.03 K W−1. For comparison with the simulated Tj, the three‐dimensional steady‐state heat transfer simulation based on the finite element method was also carried out.

The efficiency of UV LEDs based on GaN/AlGaN heterostructures

The UV LED GaN/AlGaN heterostructures obtained by HVPE approach were investigated. It was shown that the peak wavelength of UV LEDs was in the range of 360-380 nm with FWHM of 10-13 nm. At operating current of 20 mA, the active region temperature Tj was 43°C, the output optical power and efficiency - 1.14 mW and 1.46%, respectively. It was shown that the use of HVPE method allowed to achieve a high degree of structural perfection of epitaxial structures.

Thermal characterization system for transient thermal impedance measurement and power cycling of IGBT modules

This paper presents a custom-designed thermal characterization system for insulated-gate bipolar transistor (IGBT) modules. The main purpose of this system is to provide experimental capability for transient thermal impedance (Zth) measurement and power cycling reliability test on IGBT modules. LabVIEW programming was applied for circuit control and data acquisition. Gate-emitter voltage (Vge) of IGBT was used as the temperature-sensitive parameter for junction temperature (Tj) measurement. The linear relationship between Vge and Tj for IGBT can be measured by the system. Variation coefficient of lower than 0.5% was achieved for Zth measurement. IGBT modules with the same sample geometry but different die-attach processes were used for system verification: IGBT modules with nanosilver-sintered die-attach have on the average a 12% lower Zth than SAC305 soldered modules. The power cycling capability of the system was demonstrated by cycling a nanosilver-sintered IGBT module with Tj cycled between 45°C and 175°C. The cycling lifetime of the IGBT module was recorded as 48,000cycles with the failure criteria of 20% increase on Zth. Such lifetime is 50% longer than the predicted value from lifetime prediction model.

Reduced junction temperature and enhanced performance of high power light-emitting diodes using reduced graphene oxide pattern

Thermal management has become a crucial area for further development of high-power light-emitting didoes (LEDs) due to the high operating current densities that are required and result in additional joule heating. This increased joule heating negatively affects the performance of the LEDs since it greatly decreases both the optical performance and the lifetime. To circumvent this problem, a reduced graphene oxide (rGO) layer can be inserted to act as a heat spreader. In this study, current–voltage and light-output-current measurements are systematically performed at different temperatures from 30 to 190 °C to investigate the effect that the embedded rGO pattern has on the device performance. At a high temperature and high operating current, the junction temperature (Tj) is 23% lower and the external quantum efficiency (EQE) is 24% higher for the rGO embedded LEDs relative to those of conventional LEDs. In addition, the thermal activation energy of the rGO embedded LEDs exhibits a 30% enhancement as a function of the temperature at a bias of −5 V. This indicates that the rGO pattern plays an essential role in decreasing the junction temperature and results in a favorable performance in terms of the temperature of the high power GaN-based LED junction.

Thermal Performance of High Power LED on Boron Doped Aluminium Nitride Thin Film Coated Copper Substrates

Aims: The paper is aim to study the thermal performance of Boron doped Aluminium Nitride (B-AlN) thin film coated over Copper (Cu) substrate to improve surface configuration of the interface between two materials with different synthesis parameters. Study Design: Synthesis of Boron doped AlN thin film by sputtering and post processed for various temperatures. The processed samples were characterized to study the behavior of B doping as well as the annealing temperature in changing the properties of B doped AlN thin film. The structural and surface properties were studied and reported. Place and Duration of Study: Nano Optoelectronic Research Laboratory, School of Physics, University Sains Malaysia, Penang 11800, Malaysia, between December 2013 and July 2014. Methodology: B-AlN thin films were prepared with five different gas ratios over Cu substrates by DC-RF coupled sputtering method and suggested for thin film based thermal interface material Original Research Article Ong et al.; JSRR, 5(2): 109-119, 2015; Article no.JSRR.2015.078 110 (TIM). 3W green LED package was tested with B-AlN thin films coated Cu substrates through thermal transient and surface analysis. The results are compared with the performance of bare Cu substrates. Results: The thin film prepared with gas ratio of Ar 7: N2 13 coated at 200°C showed the lowest thermal resistance Rth (53.27 K/W), board to ambient thermal resistance RthB-A (36.03 K/W) and the lowest junction temperature Tj(120.98°C) at higher driving current (700 mA). Surface analysis results show that the thin film mentioned above exhibits low in surface roughness (8 nm) and range of valley depth (30-80 nm), which contribute in thermal performance of LED. Conclusion: Overall, B-AlN films coated with gas ratio of Ar 7: N2 13 are more favorable in reducing both total thermal resistance and junction temperature (Tj) of LED.

Straightforward electrical measurement of forward-voltage to investigate thermal effects in InGaN/GaN high-brightness light-emitting diodes

The junction temperature of InGaN/GaN MQW high-brightness light-emitting diodes is measured using an electrical method based on the dependence of diode forward voltage, Vf, on the junction temperature Tj. Electroluminescence (EL) data collected during this measurement are presented and used to compare the efficacy of this method for measuring Tj to the electrical technique. In devices based on polar III-nitride materials, the temperature dependence of the EL peak energy is obscured by carrier screening and bandfilling; the contributions of these effects are explored and presented. Additionally, the experimental method presented offers an application for extrapolating a thermal relaxation time during the junction temperature measurement. The results from this thermal time constant study are presented and discussed.

Fast power cycling protocols implemented in an automated test bench dedicated to IGBT module ageing

This paper presents fast test protocols for ageing IGBT modules in power cycling conditions, and a monitoring device that tracks the on-state voltage VCE and junction temperature TJ of IGBTs during ageing test operations. This device is implemented in an ageing test bench described in previous papers, but which has since been modified to perform fast power cycling tests.The fast test protocols described here use the thermal variations imposed on IGBT modules by a test bench operating under Pulse Width Modulation conditions. This test bench reaches the maximal values of power cycling frequencies attainable with a given module packaging in order to optimize test duration.The measurement device monitors VCE throughout the ageing test that is needed to detect possible degradations of wire bonds and/or emitter metallization. This requires identifying small VCE variations (a few dozen mV). In addition, the thermal swing amplitude of power cycling must be adjusted to achieve a given ageing protocol. This requires measuring junction temperature evolution on a power cycle, which is carried out by means of VCE measurement at a low current level (100mA).Experimental results demonstrate the flexibility of this test bench with respect to various power cycling conditions, as well as the feasibility of the proposed on-line monitoring methods.

A study on IGBT junction temperature (Tj) online estimation using gate-emitter voltage (Vge) at turn-off

A novel method is presented for online estimation of the junction temperature (Tj) of semiconductor chips in IGBT modules, based on evaluating the gate-emitter voltage (Vge) during the IGBT switch off process. It is shown that the Miller plateau width (in the Vge waveform) depend linearly on the junction temperature of the IGBT chips. Hence, a method can be proposed for estimating the junction temperature even during converter operation – without the need of additional thermal sensors or complex Rth network models. A measurement circuit was implemented at gate level to measure the involved time duration and its functionality was demonstrated for different types of IGBT modules. A model has been proposed to extract Tj from Vge measurements. Finally, an IGBT module with semiconductor chips at two different temperatures has been measured using Vge method and this method was found to provide the average junction temperature of all the semiconductor chips.

Determination of LED’s Efficacy for Lighting Application

The efficacy and its degradation of light emitting diode (LED) are related to its PN junction’s temperature (Tj). Currently efficacy in certain Tjand thermo-resistant are defined for the thermal characteristics. In the paper samples from different manufacturers were tested for their efficacies and dependency relationship on Tj, results were investigated and show that current definitions on thermal characteristics are quite inconvenient or unreasonable for general lighting application. Then a new definition of LED’s efficacy was suggested for general lighting application.

Impact of Bias and Device Structure on Gate Junction Temperature in AlGaN/GaN-on-Si HEMTs

The thermal impact of device bias-state and structures (such as source connected field plates, gate-pitch, back-vias, and number of gate fingers) in AlGaN/GaN-on-Si high electron mobility transistors (HEMTs) are measured using gate metal resistance thermometry (GMRT). The technique characterizes the thermal response of device gate metallization to determine the gate-epilayer junction temperature (Tj), which is directly influenced by the channel heat source due to its close proximity. It is found that low gate leakage levels in GaN HEMTs make them favorable candidates for GMRT. Bias-dependent self-heating, independent of power dissipation, is observed in the devices. Therefore, Tj of different device configurations are compared at constant bias state, as well as constant power density (3.75 W/mm) to improve accuracy. Tj reduction is observed at high drain bias due to the migration of the channel heat source toward the gate field plate edge. This provides independent experimental validation for a reported electrothermal model [7]. A 3-D thermal finite element method model is presented, which simulates measured Tj rise to within ~6% across a range of device configurations and operating conditions. This is ultimately made possible upon implementation of a thermal boundary resistance layer and extraction of its temperature response using GMRT data.

Thermal analysis of dual piezoelectric fans for cooling multi-LED packages

This paper reports on the dissipation of heat generated by a high power LED array using piezoelectric fans. Both numerical and experimental studies were carried out to evaluate the heat dissipation efficiency of high power LED package operating under multiple vibrating fans. Two vibrating fans were vertically oriented to the LED package and arranged according to configuration A (for edge to edge arrangement), and configuration B (for face to face arrangement). The junction temperature (Tj), thermal resistance (R) and average heat transfer coefficient h¯ were estimated. The results show that the single fan enhanced heat transfer performance approximately 1.8 times for the LED package. On the contrary, the dual fans enhanced heat transfer performance approximately by 2.3 times for configuration A and 2.4 for configuration B. A significant decrease in the thermal resistance was observed for all the configurations when fan separation distance δ was reduced. The best performance relative to natural convection was found to be at (δ=0.1) which decreased the thermal resistance using single fan by about 38%, whereas the dual fan accounted for 49.5% in case of configuration A, and 50.6% for configuration B.

40-nm Embedded Split-Gate MONOS (SG-MONOS) Flash Macros for Automotive With 160-MHz Random Access for Code and Endurance Over 10 M Cycles for Data at the Junction Temperature of 170 $^{\circ}$C

First-ever 40-nm embedded split-gate MONOS (SG-MONOS) flash macros for automotive micro-controller unit (MCU) have been successfully developed. A SG-MONOS cell realizes high performance with low power consumption and intrinsically high data reliability thanks to the combination of split-gate and charge-trapping structure. In addition, newly developed circuit techniques greatly enhance the advantages of SG-MONOS cells and enable fast and reliable operations even at the junction temperature (Tj) of 170°C with small peripheral circuit area; 1) a sense amplifier with digital offset cancellation (SA-DOC) provides fast read operation over 160 MHz; 2) adaptable program current control scheme (APCCS) and intelligent erase scheme (IES) significantly decrease the program and erase time, which also results in improving the memory cells' reliability; and, 3) 3-D stacked unit capacitors achieve area-efficient charge pump. Two types of embedded flash (eFlash) macros with these technologies, code macro of 2 MB and data macro of 64 KB, were fabricated in a 40-nm eFlash process. The code macro demonstrates the capability of 160-MHz random read operation at Tj= 170°C, reaching 5.1 GB/s read throughput by simultaneous 256 bits read-out from two code macros. The data macro achieves the program/erase endurance over 10 million cycles at Tj= 170°C without any software-assisted techniques.

Drying of Rubber Sheet Using Impingement of Multiple Hot Air Jets

In the process for producing ribbed smoked rubber sheet, the rubber sheet drying is the most time and energy consumption process. This research study the possibility in develop new drying system using array of hot air jets impinging directly on rubber sheet in order to reduce time for drying rubber in smoked room and increase productivity of rubber sheets. In the study, the array of jets from jet plate with drilled jet holes in staggered arrangement impinged on the both side of rubber sheet. The effects of jet velocity (Vj=10, 16, 23 m/s), jet temperature (Tj=50, 60, 70°C) and the distance from jet outlet to rubber surface (L=4D, 6D, 8D which D is diameter of jet hole) on drying rate were investigated by measuring weight of rubber sheet change with time. The heat transfer rate on impinged surface was also measured by attaching a heat flux sensor on impinged wall. The results showed that the convective heat transfer coefficient increased when the jet velocity was increased and when the distance from jet was decreased particularly in jet directly impinged region. It was found that the enhancement in heat transfer rate from jets cannot increase the rubber drying rate for all cases because the drying rate depend on rubber property. The process of rubber sheet drying can be divided in 2 periods; in the first drying period, the drying rate decreases with decreased moisture content. The drying rate depended on the initial moisture content and the condition of external effect such as jet velocity, jet temperature and distance from jet outlet to rubber surface. In second drying period, the moisture content is below 20% dry basis. In this period, the drying rate is almost constant near zero. It was also found that the drying for case of L=6D was higher than case of L=4D and 8D. The optimum condition for rubber sheet drying without defects on rubber property after drying was L=6D and Tj=70°C.