Heat transfer and residual stress modeling of a diamond film heat sink for high power laser diodes

A three-dimensional finite element model of heat transfer and residual stress within high power laser diodes and their heat sinks is developed. These components are typically used in telecommunication applications. The model addresses both p-side down and p-side up laser diodes mounted on a variety of commercially available gold plated diamond heat sinks. In addition, the model is optimized with respect to the dimensions of the diamond film, and the laser diode cavity lengths. Finally, the design and performance of diamond film heat sinks for high performance GaAs and InP laser diodes are discussed. The results demonstrate the superior performance achieved through thermal engineering of the dominant thermal transport path from the laser diode heat source through diamond films to the heat sink.     

High-efficiency V-band GaAs IMPATT diodes

Double-drift GaAs IMPATT diodes were designed for V-band frequency operations and fabricated using molecular-beam epitaxy. The diodes were fabricated in two configurations: (a) circular mesa diodes with silver-plated (integrated) heat sinks; (b) pill-type diodes bonded to diamond heat sinks. Both configurations utilised a miniature quartz-ring package. Output power greater than 1 W CW was achieved at V-band frequencies from diodes on diamond heat sinks. The best conversion efficiency was 13.3% at 55.5 GHz with 1 W output power.     

GaAs TUNNETT diodes on diamond heat sinks for 100 GHz and above

Single-drift GaAs TUNNETT diodes were mounted on diamond heat sinks for improved thermal resistance and evaluated around 100 GHz in a radial line full height waveguide cavity. The diodes were fabricated from MBE-grown material originally designed for diodes that operate in CW mode around 100 GHz on integral heat sinks. An RF output power of more than 70 mW with a corresponding DC to RF conversion efficiency of 4.9% was obtained at 105.4 GHz. This is the first successful demonstration of GaAs TUNNETT diodes mounted on diamond heat sinks. To the authors' knowledge, these DC to RF conversion efficiencies and RF power levels are the highest reported to date from TUNNETT diodes and exceed those of any single discrete device made of group III-V materials (GaAs, InP, etc.) at this frequency. Free-running TUNNETT diode oscillators exhibit clean spectra with an excellent phase noise of less than -94 dBc/Hz, measured at a frequency off-carrier of 500 kHz and an RF output power of 40 mW     

High-power operation mode of pulsed IMPATT diodes

Pulsed silicon double-drift IMPATT diodes that yield 42 W at 96 GHz are discussed. This is about twice the value reported previously. Owing to the considerable input powers (≈500 W), these diodes are mounted on diamond heat sinks. Because of the strong carrier injection, the field distribution in the diode is similar to that in a p-i-n diode. An attempt is made to explain the results using T. Misawa's (1966) p-i-n type theory. The large-signal; avalanche resonant frequency is close to the operation frequency. Conventional Read-type theory fails to explain these results because of the current densities employed in the experiments     

0.7 W single-drift GaAs IMPATT diodes for millimetre-wave frequencies

Single-drift flat-profile GaAs IMPATT diodes with diamond heat sinks have been fabricated in the 50 GHz band. The design of the diodes is based on a lower value of effective saturated drift velocity of electrons at high electric fields. Output power as high as 0.7 W at 53 GHz and an efficiency of 12.3% at 51 GHz have been obtained.     

GaAs Schottky—Read diodes for x-band operation

High-efficiency performance of GaAs Schottky-Read IMPATT diodes has been observed at X-band frequencies. The highest efficiency measured was 26.1 percent with 2.5-W continuous-wave (CW) output power at 8.8 GHz for a single-mesa diode while multiple-mesa diodes have delivered more than 7 W at X band. The diodes were fabricated from multiple-layer epitaxial material with gold-plated heat sinks. Details of materials preparation and diode fabrication are presented. Theoretical calculations of diode breakdown voltage and efficiencies have been made as a function of the structural properties of the diodes. Good agreement has been obtained between the experimental microwave oscillator performance and the theoretical calculations.     

Thermal properties of diamond/copper composite material

Thermal considerations are becoming increasingly important for the reliabilities of the electronics parts as the electronics technologies make continuous progress such as the higher output power of laser diodes or the higher level of integration of ICs. For this reason the desire for improving thermal properties of materials for electronics component parts is getting stronger and the material performance has become a critical design consideration for packages. To meet the demands for a high performance material for heat spreader materials and packages, a new composite material composed of diamond and copper was successfully manufactured under high pressure and high temperature. The effects of diamond particle sizes and the volume fractions of diamond on both thermal conductivity and the coefficient of thermal expansion (CTE) were investigated. The thermal conductivity of the composite material was dependent on both the particle size and the volume fraction of diamond, while the CTE was dependent only on the volume fraction of diamond. At the higher diamond volume fraction, the experimentally obtained thermal conductivities of the composite materials were above the theoretically expected values and the experimentally obtained CTE were between the two theoretical Kerner lines. This may be due to the fact that at the higher diamond volume fraction the diamond particles are closely packed to form bondings between each particle. The composite of diamond and copper have a potential for a heat spreading substrate with high performance and high reliability because not only its thermal conductivity is high but its coefficient of thermal expansion can be tailored according to a semiconductor material of electronics devices.     

Double-drift-region ion-implanted millimeter-wave IMPATT diodes

A detailed experimental comparison between double-drift-region (DDR) and single-drift-region (SDR) millimeter-wave avalanche diodes is presented. For 50-GHz CW operation, DDR diodes have given a maximum of 1-W output power compared to 0.53 W for the SDR diodes, while maximum efficiencies of 14.2 percent for the DDR and 10.3 percent for the SDR diodes have been obtained. These results are in agreement with the theory of Scharfetter et al. [1] for DDR IMPATT diodes. Both the DDR and SDR diode measurements were made on room temperature, metal heat sinks. The DDR diodes were shown to operate at significantly lower junction temperatures for the same value of output power, indicating a potential reliability advantage. Ion implantation was used to make the p drift region of the p⁺p-n-n⁺50-GHz DDR devices. Otherwise the fabrication (which includes diffusion and epitaxial technologies) and the microwave measurement methods were identical for both types of diodes. Capacitance measurements were compared with calculations to determine the desired doping concentrations for frequencies from 43 to 110 GHz. Experimental results for the higher frequency millimeter-wave region have been obtained on DDR structures with both p and n drift regions implanted. At 92 GHz an output power of 0.18 W and an efficiency of 7.4 percent have been obtained.     

Improved performance of silicon avalanche oscillators mounted on diamond heat sinks

Silicon avalanche diodes have been mounted on diamond to achieve a continuous power density more than twice that achieved on copper. A pair of parallel connected wafers oa a single sample of diamond produced 4.7 watts of CW power at 13.3 GHz using a room temperature heat sink.     

Fabrication and noise performance of high-power GaAs IMPATTS

Schottky-barrier GaAs IMPATT diodes have been fabrirated in a double epitaxial layer structure on low-etch-pit density substrates. The resulting low defect density in the active region permits high power outputs and low noise measure. Mounted on copper studs and a 20°C heat sink, such diodes have given a maximum CW power output of 2.94 W at 6.1 GHz with 13.8 percent efficiency. The small-signal amplifier noise measure was 25dB. Operated as injection-locked oscillators, the noise measure was 32 dB at an output of 1 W. These results show that in a suitable structure, GaAs can surpass the efficiency and noise performance of other materials, and demonstrate the capability of high power output in this frequency band.     

Thermal resistance and temperature distribution in double-heterostructure lasers: Calculations and experimental results

The thermal resistance and temperature distribution in double-heterostructure lasers have been calculated taking into account the characteristics of the different layers, the internal quantum efficiency, and allotment of the dissipated power, in order to optimize their structure. The influence of the different layers in the heterostructure and of the electrical contact is analyzed. Thermal resistance of CW, shallow proton-implanted lasers has been determined experimentally using the technique that relies upon a null measurement of the wavelength of a single Fabry-Perot mode. Statistical results on some hundreds of lasers with different stripe widths (6-125 mum), mounted on different heat sinks (copper, silicon, beryllium oxide) are given and compared to theoretical values. The model we propose gives good agreement with experimental results. The 6 μm width stripe laser is of special interest because this laser is transverse monomode up to an optical power of 6 mW. A value of 22° C/W has been achieved in a reproducible manner for6 times 300 mum lasers mounted on copper heat sinks. The effectiveness of the bonding technique is demonstrated. Si and BeO heat sinks are suitable for many applications because of their chemical (V grove etching in Si) and thermal properties (better linear expansion coefficient match to GaAs). We show that the increase of thermal resistance so introduced is still compatible with long CW operation.     

Multichip IMPATT Power Combining, a Summary with New Analytical and Experimental Results

X band IMPATT diode chips have been efficiently combined in parallel, in series on diamond heat sinks, and in series-parallel on diamond heat sinks. This paper summarizes experimental and analytical work performed over a four-year period and places some of the results in perspective with respect to practical applications. Several device types have shown to be compatible with series geometries. Analysis has shown that some device types, when connected in series, form a combination which is neither open-circuit nor short-circuit stable, an intrinsically unstable condition. It has been shown further that capacitors of practical size can be placed in parallel with each chip of such an assembly to prevent the occurrence of the unstable condition. Thus unique problems reported earlier with some types of IMPATT's are understood and can be prevented. These experimental and analytical results appear to eliminate any hypothetical barrier to the routine series power combining of at least several types of IMPATT device chips.     

Power-impedance-frequency limitations of IMPATT oscillators calculated from a scaling approximation

The obtainable CW power of silicon IMPATT oscillators, as a function of frequency, is calculated by scaling from reference results. The analysis differs from previous treatments in that the microwave circuit impedance limitation, as observed experimentally, is utilized simultaneously with thermal impedance limitations to uniquely determine device diameter, operating currents, and output power. Results are presented for single and multiple (parallel) units on copper and diamond mounting studs, and for both single (p⁺-n-n⁺) and double-drift-region (p⁺-p-n-n⁺) structures. Obtainable power falls off essentially as 1/f until an ultimate (nonthermal) space-charge-limited current density is reached. Beyond this point the obtainable power varies as f^-2.14. The calculated results on single-drift-region structures are in agreement with experimental observations over the range of frequencies from 13 to 55 GHz, and the analysis predicts an obtainable power of 300 mW at 110 GHz for a double-drift-region structure with 10 percent conversion efficiency.     

High temperature device simulation and thermal characteristics of GaAs MESFETs on CVD diamond substrates

The temperature stability and high temperature characteristics of GaAs FETs on CVD diamond heat sinks were investigated by modeling the high-temperature electrical characteristics for GaAs MESFETs and by experimentally measuring the elevated-temperature performance. The bias region for the zero temperature coefficient (ZTC) was determined. The thermal characteristics were determined by infrared microscopy and the results were correlated with a finite element analysis calculation of the GaAs FET thermal distribution. The utilization of CVD diamond as a heat spreading substrate is shown to reduce the peak channel temperature by 30 percent when the FET is biased at the 1 dB compression point.     

High-power high-efficiency c.w. Gunn oscillators in X band

Gunn oscillators on diamond heat sinks have produced up to 780 mW c.w. with 5.1% efficiency at 9.9 GHz. The devices were operated in full-height waveguide cavities. Similar devices on copper have produced up to 700 mW with 4.1% efficiency and typical powers of 600 mW with efficiencies of 3.9%. The effect of varying the 2nd-harmonic loading on devices mounted in fully-reduced-height waveguide has also been investigated, but no enhancement in output power at the fundamental frequency was observed.     

Degradation of the power- and frequency-temperature performance of IMPATT diodes at lower frequencies

Investigations of the effect of ambient temperature on the RF power and frequency ofX-band p+-n-n+ Si IMPATT diodes at frequencies and temperatures below their optimum conditions show considerable degradation of performance. A simple model is presented to explain these effects in terms of a lower limit to the instantaneous terminal voltage of the diode. Values of diode negative conductance are derived from the measurements and good agreement is obtained with independent measurements. The effects are relevant to both amplitude and frequency stability in wide band applications of IMPATT diodes.     

Experimental demonstration of a silicon carbide IMPATT oscillator

Silicon carbide (SiC) is an excellent material for high-power and high-frequency applications because of its high critical field, high electron saturation velocity, and high thermal conductivity. In this letter, we report the first experimental demonstration of microwave oscillation in 4H-SiC impact-ionization-avalanche-transit-time (IMPATT) diodes. The prototype devices are single-drift diodes with a high-low doping profile. DC characteristics exhibit hard, sustainable avalanche breakdown, as required for IMPATT operation. Microwave testing is performed in a reduced-height waveguide cavity. Oscillations are observed at 7.75 GHz at a power level of 1 mW