Active Region Temperature Research Articles

Time-resolved nanosecond sub-micron resolution thermal analysis of high-power AlGaN/GaN HFETs

A novel time-resolved micro-Raman thermography technique has been developed to measure the active region temperature in AlGaN/GaN electronic devices, achieving a temporal and spatial resolution of 200 ns and 0.5-0.7 μm, respectively. AlGaN/GaN heterostructure field effect transistors (HFETs) grown on SiC and sapphire substrates are compared. Rapid temperature changes within 200 ns of current changes are followed by a slower change in device temperature. For a device on a SiC substrate an approximate temperature plateau is reached within 1-2 μs after switching the device on or off, illustrating its fast thermal response. This is in contrast to the slower time constant of devices on sapphire substrates where (is-separated electrical pulses do not allow the devices to cool to room temperature between the electrical pulses.

Active region temperatures of quantum cascade lasers during pulsed excitation

Temperature changes in quantum cascade laser active regions can be accurately estimated, during pulsed excitation, by monitoring the spectral shift of laser’s longitudinal modes. Relatively small differences in the amplitude and widths of excitation current pulses yielded large, and therefore easily measurable, longitudinal mode shifts. Thus, straightforward experiments can be used estimate active region temperatures and physical parameters.

Thermal effects in InGaAs/AlAsSb quantum-cascade lasers

A quantum-cascade laser (QCL) thermal model is presented. On the basis of a finite-difference approach, the model is used in conjunction with a self-consistent carrier transport model to calculate the temperature distribution in a near-infrared InGaAs/AlAsSb QCL. The presented model is used to investigate the effects of driving conditions and device geometries on the active-region temperature, which has a major influence on the device performance. A buried heterostructure combined with epilayer-down mounting is found to offer the best performance compared with alternative structures and has thermal time constants up to eight times smaller. The presented model provides a valuable tool for understanding the thermal dynamics inside a QCL and will help to improve operating temperatures.

Investigation of Thermal Effects in Quantum-Cascade Lasers

The development of a thermal model for quantum cascade lasers (QCLs) is presented. The model is used in conjunction with a self-consistent scattering rate calculation of the electron dynamics of an InGaAs-AlAsSb QCL to calculate the temperature distribution throughout the device which can be a limiting factor for high temperature operation. The model is used to investigate the effects of various driving conditions and device geometries, such as epilayer down bonding and buried heterostructures, on the active region temperature. It is found that buried heterostructures have a factor of eight decrease in thermal time constants compared to standard ridge waveguide structures in pulsed mode and allow a /spl sim/78% increase in heat sink temperature compared to epilayer down mounted devices in continuous-wave mode. The model presented provides a valuable tool for understanding the thermal dynamics inside a quantum cascade laser and will help to improve their operating temperatures.

Thermal relaxation time and heat distribution in pulsed InGaAs quantum dot lasers

Using a charge coupled device-based thermoreflectance technique, we achieve a high-resolution (∼700nm) cross-sectional temperature profile of a semiconductor laser. This two-dimensional profile allows us to identify separate heat sources due to contact heating and nonradiative recombination in the active region. By adapting the technique to pulsed operation and varying the laser’s duty cycle, we measure the thermal relaxation time constant. We also quantitatively determine the heat transfer from device-internal heat sources and demonstrate both the large effect of lateral heat spreading and the distinction between a laser’s top surface temperature and its active region temperature.

Thermal resistance and optimal fill factor of a high power diode laser bar

Using analytical methods, we obtain compact theoretical expressions for the electrical and thermal resistances of a laser bar as a function of its fill factor. The theoretically determined values of the thermal resistance are in excellent agreement with the measured ones. We use the theoretical values of the electrical and thermal resistances to find the fill factor that gives the maximal output power for a given active region temperature and the fill factor that gives the minimal active region temperature for a given output power. The optimal fill factor is dependent on the bar operating regime; higher power operation favours the use of higher fill factors to minimize the active region temperature rise.

High-power semiconductor red laser arrays for use in photodynamic therapy

Semiconductor laser ridge arrays emitting 250 mW at a wavelength of 635 nm are designed for photodynamic therapy applications. Although ridge laser arrays are less efficient than broad area lasers, they are more reliable and can produce higher power from the same lateral width due to thermal considerations. An analytic expression for the active layer temperature of the laser array as a function of the ridge spacing, number of ridges, and width is derived and has excellent agreement with a finite-element analysis. This analytic expression allows optimization of the laser and the submount geometry to minimize the active region temperature with the constraint of a small submount, heat sink, and package.

Designing strategy to enhance mode selectivity of higher-output oxide-confined vertical-cavity surface-emitting lasers

Performance of modern oxide-confined (OC) vertical-cavity surface-emitting diode lasers (VCSEL s) is more sensitive to the construction details than in the case of other VCSEL s. In particular, a stable single-fundamental-mode operation is difficult to be achieved in these VCSEL s especially in higher-output large-size continuous-wave (cw) operating devices at higher temperatures. In the present paper, an operation of OC VCSEL s has been investigated with the aid of the comprehensive fully self-consistent model using the (GaIn)(NAs)/GaAs quantum-well VCSEL with two oxide apertures as a typical example. A new approach is proposed to enhance cw RT single-fundamental-mode low-threshold operation in higher-output OC VCSEL s. One of their oxide apertures should be shifted to the node position of the resonator standing wave where it is working as the electrical aperture only. Then diameters of both apertures may be changed independently giving an additional degree of freedom for VCSEL designing, which enables their optimisation. While the larger-diameter optical aperture placed in the anti-node position creates an efficient radial waveguiding effect, the smaller-diameter electrical aperture enhances a more uniform current injection into the VCSEL active region. Due to combining influence of both the apertures, the single-fundamental-mode operation is predicted in a large device with the 10-μm-diameter active region even for 80 K active-region temperature increase over RT of the ambient. An impact of intentional detuning at room-temperature (RT) of VCSEL active-region gain spectrum towards shorter wavelengths with respect to the resonator mode improves mode selectivity is also analysed.

Transient thermal analysis of InGaAsP-InP high-power diode laser arrays with different fill factors

The temperature kinetics of InGaAsP∕InP high-power laser diode arrays with different fill factors was studied experimentally and theoretically. It was shown that except during a short initial period, the laser array heating is determined by the heat flux propagation through the heat spreader. To characterize this heating, we developed a two-dimensional analytical model. Experimentally, the temperature change in the active region was obtained by measuring the laser spectrum’s temporal evolution during a single current pulse. Three distinctive periods in the transient heating process were clearly identified—an initial temperature rise, a square-root-of-time dependence of the active-region temperature increase, and an exponential approach of the active-region temperature to its steady-state value. We demonstrated that in the initial period of time, the heat propagates within the laser bar structure, and the laser bar design (fill factor) strongly affects the active-region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor.

Optical pumping of IV-VI semiconductor multiple quantum well materials using a GaSb-based laser with emission at λ=2.5μm

Room-temperature photoluminescence (PL) measurements of IV-VI semiconductor multiple quantum well (MQW) structures using diode laser optical pumping at two different excitation wavelengths, 2.5 and 0.91μm, are described. Active region temperatures during continuous-wave optical pumping were determined from blueshifts in PL emission energies. Temperatures were between 22.7 and 29.5°C lower for 2.5-μm pumping as compared to 0.91-μm pumping at the same power level of 100mW. Heating effects are also shown to be smaller for MQW samples with more narrow PbSe wells.

Current spread and overheating of high power laser bars

The heating of a semiconductor laser bar imbedded between two heat spreaders has been studied theoretically and experimentally. The model included the p-cladding layer, active region, n-cladding layer, and n-substrate. Heat sources from the active region and both cladding layers were considered. An analytical relation was obtained between the temperature distribution in the laser and the bar geometry. The analytical approach revealed features that are usually missing in purely numerical modeling. It was shown that the current density across the active region is nonuniform, and that the magnitude of the nonuniformity grows when the resistance of the p-cladding layer decreases. This nonuniform current distribution can lead to higher temperatures at the edges of the laser stripe than in the middle. It was found that mutual heating of the individual lasers in the bar is controlled mainly by the overlap of the temperature fields in the heat spreaders. The theoretical results obtained without any fitting parameters show good agreement with the experimentally measured dependence of the active region temperature on pumping current.

A theoretical study of differing active region doping profiles for W-band (75–110 GHz) InP Gunn diodes

InP Gunn diodes are widely used as high power microwave sources in the W-band frequency range (75–110 GHz), but interesting questions remain as to the optimal active region doping profile for such devices. In this paper we carry out a detailed theoretical study, using Monte Carlo simulations, of three types of doping profiles: (i) uniform doping, (ii) graded doping (i.e. increasing linearly from cathode to anode) and (iii) notch doping (a lightly doped region adjacent to the cathode). By studying the effects of varying all the relevant parameters, such as doping levels, active region length, bias levels and temperature, under both DC and RF conditions, we argue that the notch doping approach is superior, offering high performance combined with a greater tolerance to doping fluctuations.

Using microchannels to cool microprocessors: a transmission-line-matrix study

As microprocessors components density and clock frequency increase, so do heat dissipation. The heat results from Joule effect due to leakage currents in the components area or active region. This region is only few microns thick and can quickly reach destructing temperatures if heat is not quickly removed. On this critical issue depends the system reliability. The active region is separated from the ventilated heat sink by a silicon substrate and a metal integrated heat spreader, both hundreds of microns thick. This interface region is the microprocessor's heat transfer plate where heat exchange is achieved by conduction. Because of the localized heat source, the thermal spreading resistance of the interface region can be high. A novel way of spreading heat in that region is the use of microchannel arrays where an appropriate thermal compound or a phase change liquid can be trapped to increase heat transfer by conduction or to create micro-heat-pipes. Traditional cooling methods, with conventional and well optimised heat sinks, can then be used with less burden.In this paper, the Transmission-Line-Matrix (TLM) technique is used to simulate the effect of microchannels on the temperature distribution in the active region. To minimize the interface heat resistance various microchannel and patterns are examined. In this part of the work, the microchannels are filled with the heat spreader material copper or aluminium. The results show an improved thermal transient behaviour and a reduced active region temperature in steady state.

Self-heating effects in red (665 nm) VCSELs

The threshold current (Ith) of GaInP/AlGaInP based vertical cavity surface emitting lasers (VCSELs) is very sensitive to temperature. The extent to which self-heating in a compact VCSEL structure couples with temperature sensitive loss mechanisms is investigated. In particular, the authors consider the strong increase in threshold current (Ith) due to carrier leakage into the X-minima and gain-cavity detuning effects. They find, using two experimental methods and from a thermal model, that the current-induced increase of the active region temperature (ΔT) increases linearly with Ith at a rate of ∼ 14.5 K mA−1 over a limited substrate temperature range. They estimate that the additional losses due to self-heating rise from 5% of the total Ith at room temperature (20°C) to 30% at 47°C. To uncouple the effects of leakage and gain-cavity alignment, they have calculated the gain spectra to find the radiative current as a function of temperature. They thus verify that leakage coupled to self-heating is the major cause of the increase in Ith in these devices at room temperature and above. Gain-cavity detuning has minimal effect on Ith due to the broad short wavelength tail of the quantum well gain spectrum.

The Extreme‐Ultraviolet Structure and Properties of a Newly Emerged Active Region

The structure and properties of a newly emerged solar active region (NOAA Active Region 7985) are discussed using the Coronal Diagnostic Spectrometer (CDS) and the Extreme-Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory. CDS obtained high-resolution EUV spectra in the 308-381 ? and 513-633 ? wavelength ranges, while EIT recorded full-disk EUV images in the He II (304 ?), Fe IX/X (171 ?), Fe XII (195 ?), and Fe XV (284 ?) bandpasses. Electron density measurements from Si IX, Si X, Fe XII, Fe XIII, and Fe XIV line ratios indicate that the region consists of a central high-density core with peak densities of the order of 1.2 ? 1010 cm-3, which decrease monotonically to ~5.0 ? 108 cm-3 at the active region boundary. The derived electron densities also vary systematically with temperature. Electron pressures as a function of both active region position and temperature were estimated using the derived electron densities and ion formation temperatures, and the constant pressure assumption was found to be an unrealistic simplification. Indeed, the active region is found to have a high-pressure core (1.3 ? 1016 cm-3 K) that falls to 6.0 ? 1014 cm-3 K just outside the region. CDS line ratios from different ionization stages of iron, specifically Fe XVI (335.4 ?) and Fe XIV (334.4 ?), were used to diagnose plasma temperatures within the active region. Using this method, peak temperatures of 2.1 ? 106 K were identified. This is in good agreement with electron temperatures derived using EIT filter ratios and the two-temperature model of Zhang et al. The high-temperature emission is confined to the active region core, while emission from cooler (1-1.6) ? 106 K lines originates in a system of loops visible in EIT 171 and 195 ? images. Finally, the three-dimensional geometry of the active region is investigated using potential field extrapolations from a Kitt Peak magnetogram. The combination of EUV and magnetic field extrapolations extends the core-halo picture of active region structure to one in which the core is composed of a number of compact coronal loops that confine the hot, dense, high-pressure core plasma while the halo emission emerges from a system of cooler and more extended loops.

On Stellar Coronae and Solar Active Regions

Based on Yohkoh Soft X-Ray Telescope (SXT) observations of the Sun near peak activity level obtained on 1992 January 6, we search for coronal structures that have emission measure distributions EM(T ) that match the observed stellar coronal emission measure distributions derived for the intermediate-activity stars v Eri (K2 V) and m Boo A (G8 V) from Extreme Ultraviolet Explorer spectro- scopic observations. We —nd that the temperatures of the peaks of the observed stellar distributions EM(T ), as well as their slopes in the temperature range are very similar to those 6.0 ( log T ( 6.5, obtained for the brightest of the solar active regions in the 1992 January 6 SXT images. The observed slopes correspond approximately to EM P T b with b D 4, which is much steeper than predicted by static, uniformly heated loop models. Plasma densities in the coronae of v Eri and m Boo A are also observed to be essentially the same as the plasma densities typical of solar active regions. These data provide the best observational support yet obtained for the hypothesis that solar-like stars up to the activity levels of v Eri (K2 V) and m Boo A are dominated by active regions similar to, though possibly considerably larger than, those observed on the Sun. The surface —lling factor of bright active regions needed to explain the observed stellar emission measures is approximately unity. We speculate on the scenario in which small-scale ii nano—ares ˇˇ dominate the heating of active regions up to activity levels similar to those of v Eri (K2 V) and m Boo A. At higher activity levels still, the interactions of the active regions themselves may lead to increasing —aring on larger scales that is responsible for heating plasma to the observed coronal temperatures of on very active stars. Observations of X-ray and T Z 107 K EUV light curves using more sensitive instruments than are currently available, together with determi- nations of plasma densities over the full range of coronal temperatures (106¨107 K and higher), will be important to con—rm —are heating hypotheses and to elicit further details concerning coronal structures at solar-like active region temperatures and the temperatures that characterize the most (T ( 5 ) 106 K) active stars (T Z 107 K). Subject headings: stars: coronaestars: individual (v Eridani, m Bootis) ¨ Sun: corona ¨ Sun: X-rays, gamma raysX-rays: stars

Large-scale evolution of the active region NOAA 7978, 7981, 7986 observed by GOES, SOHO, and Yohkoh

We took part in a joint project aimed to study the large-scale evolution of an active region from its emergence throughout its decay for several solar rotations. Our interest focuses on the understanding of how energy is generated, released, deposited, and transformed in active regions. To this end, we determined physical parameters like intensity, temperature, and emission measure of the whole active region as a function of time for the entire period selected. We present the preliminary results of the analysis of GOES (Geosynchronous Operational Environmental Satellite), SOHO (Solar and Heliospheric Observatory) and Yohkoh data of the active region named NOAA 7978, 7981, and 7986 observed between July and October 1996.

Polarization selection in VCSELs due to current carrier heating

The problem of polarization switching of light from a vertical cavity surface emitting laser (VCSEL) is discussed. It is shown that heating of holes in the p-type distributed Bragg reflector and of electrons and holes in the quantum wells of the active region lead to switching of the polarization of the VCSEL. The model developed here makes it possible to explain the results of experiments on switching of the polarization of VCSELs at fixed active region temperatures.

Micromachined widely tunable vertical cavity laser diodes

Wavelength tunable vertical-cavity surface-emitting lasers (VCSELs) are potentially useful for future optical communications. Traditionally, the emission wavelength of a vertical cavity laser was tuned by modulating the active region temperature. However, thermal tuning is slow, and the realized tuning range is quite limited. Micromachined tunable VCSELs (Mi-T-VCSELs) combine the traditional vertical cavity laser structure with a monolithically micromachined deformable membrane, enabling continuous wavelength tuning without mode hopping. In addition to a large wavelength tuning range, this technique does not suffer from the shortcomings of the thermal tuning technique. This paper presents the background theory, processing sequence, and experimental results for Mi-T-VCSELs.

Radiation spectra in GaN-based LEDs under thermal-injection processes

The effects of series current magnitude ( I) upon the intrinsic UV luminescence band ( λ∼370 nm) and the impurity of the blue band ( λ∼430 nm) of a GaN LED has been investigated. The excess device temperature (Δ T AR) of the GaN LED active region has been determined over a wide range of working currents. It has a linear behaviour for currents over 15 mA. The weaker dependence Δ T AR ( i) at i<15 mA is due to the fact that at forward biases V< E g/ e all the power terminated to the device is released in the space charge region ( w≤0.3 μm).