With the DARPA Wide Bandgap Semiconductor Technology RF Thrust Contract, TriQuint Semiconductor and its partners,
BAE Systems, Lockheed Martin, IQE-RF, II-VI, Nitronex, M.I.T., and R.P.I. are achieving great progress towards the
overall goal of making Gallium Nitride a revolutionary RF technology ready to be inserted in defense and commercial
applications. Performance and reliability are two critical components of success (along with cost and manufacturability). In
this paper we will discuss these two aspects. Our emphasis is now operation at 40 V bias voltage (we had been working at
28 V). 1250 µm devices have power densities in the 6 to 9 W/mm with associated efficiencies in the low- to mid 60 % and
associated gain in the 12 to 12.5 dB at 10 GHz. We are using a dual field-plate structure to optimize these performances.
Very good performances have also been achieved at 18 GHz with 400 µm devices. Excellent progress has been made in
reliability. Our preliminary DC and RF reliability tests at 40 V indicate a MTTF of 1E6hrs with1.3 eV activation energy at
150 0C channel temperature. Jesus Del Alamo at MIT has greatly refined our initial findings leading to a strain related
theory of degradation that is driven by electric fields. Degradation can occur on the drain edge of the gate due to excessive
strain given by inverse piezoelectric effect.
The factors critical to the fabrication of high-lumen InGaN flip chip LEDs are discussed. It is shown that as the die size and the current density increase, the n-GaN sheet conductivity becomes extremely critical to uniform current spreading and the corresponding uniformity of light emission. It is observed that a thick p-metal is important in reducing hot spot formation. The p-contact is also critical to increasing light extraction efficiency and wall-plug efficiency. The merits and reliability of Al- and Ag-based p-contacts are compared. Reliability issues related to the n-contact are also discussed. Finally, performance data for InGaN blue lamps, for drive currents up to 900 mA is shown.
The requirements for maximizing the external quantum efficiency of UV nitride LEDs are discussed. It is shown that as the chip wavelength progressively decreases, nitride epi growth on a sapphire substrate becomes advantageous in terms of light extraction. The epilayer requirements for UV LEDs dictate the growth of n-AlGaN, with increasing Al contents, and the growth of UV-transparent p-GaN. It is shown that MOCVD growth in a Emcore D-180 or Ganzilla reactor is ideal for meeting the stringent epilayer requirements. Increasing light extraction efficiency and wall-plug efficiency also requires optimization of the reflecting P-contact. The relative merits of Al- and Ag-based reflecting contacts are discussed. Performance data for UV LEDs on sapphire, for drive currents up to 700 mA is shown. Finally, a practical high power UV-based white lamp is demonstrated.
In modern GaN-based light-emitting diodes (LEDs) structures, total internal reflection (TIR) limits light extraction, and consequently, overall efficiency of the light source. Proper chip and package material combinations as well as surface property modifications offer the opportunity to reduce the luminous flux lost due to TIR and absorption. Different sepa-ration techniques are taking influence on substrate surface properties and thus on light extraction improvement. Imple-menting all these factors in a flexible ray tracing model and applying effective mathematical optimization, helps to refine a chip design in a fast and accurate way to achieve a significant increase of the light extraction. Based on experimental data and ray trace modeling, the effects of chip size scaling, surface roughness and encapsulation on light extraction val-ues will be demonstrated.
The use of laser technology for the separation of gallium nitride-based light-emitting diodes (LEDs) on sapphire substrates overcomes many of the problems associated with standard separation techniques. Scribe-and-break and sawing have many drawbacks and limitations due to the hardness of the sapphire substrate and to its wurtzite crystal lattice. Laser ablation has an inherent advantage over other laser separation techniques that heat the crystal to the point of causing damage. That difference can be used to enhance throughput and yield without sacrificing valuable wafer area. This work illustrates the process advantages of using the laser die separation technology. These advantages include the flexibility of chip shaping and surface modification, minimization of street and kerf width, and chip aspect ratio. A discussion of the wider process window and ease of use of the laser separation system will be demonstrated. In addition, the electrical and optical characteristics of laser separated die will be compared with die separated by competing technologies.
Increasing optical power and electrical-to-optical conversion efficiency enable visible light-emitting diodes to advance into new applications and wider markets. InGaAlP/GaAs and InGaN/sapphire material systems cover the whole visible spectrum of saturated colors used for display, signage, and automotive use. A combination of blue InGaN LEDs with phosphor delivers a 'white' spectrum adequate for most lighting needs. Demand for high optical power requires larger chips suitable for high-current operation. Current crowding effects and their negative consequences for chip performance and reliability limit the performance of high-power chips based on both material systems. Despite the differences between InGaAlP/GaAs and InGaN/sapphire chip structures, a number of common design concepts leading to higher external efficiency and total luminous output have been proposed, including large chips operating at high drive currents. This paper highlights fundamental current spreading and reliability issues related to the chip size and operating current density, outlines a framework for quantitative analysis, proposes and compares a number of novel high-power chip designs.
As more advances are made in the performance of GaN-based devices, a trend toward the use of large scale MOCVD reactors for epitaxial growth of GaN-based device structures is clear. In this paper we describe the use of Emcore's SpectraBlueTM reactor for large-scale manufacturing of Blue and Green LEDs. The high throughput growth of GaN based LEDs is demonstrated without compromising LED uniformity or overall performance. In-situ control of key parameters critical to the production of high quality LEDs, such as buffer layer growth is now feasible using in-situ reflectance spectroscopy. Film properties as well as LED device performance are discussed.
In the extensive research dedicated recently to metal- organic chemical vapor deposition (MOCVD)-grown high- efficiency GaN LED device design, a significant effort has been made to increase the conductivity of p-GaN layers, while n-GaN layers received relatively little attention. We demonstrated, both experimentally and theoretically, that the resistivity of n-GaN layers has a profound effect on blue InGaN LED performance. Optimization of n-GaN epitaxial layers allows the achievement of device series resistances below 15 Ohms and forward voltages as low as 2.9 Volts at 20 mA. We have also shown that contactless measurements of sheet resistivity of the entire LED epitaxial structure closely correlate with the ohmic resistance of the GaN layer measured in the fabricated devices. This provides an excellent non-destructive characterization tool for n-GaN optimization. Insufficient n-GaN conductivity is shown to trigger a distinct degradation mechanism by initiating current crowding in a localized device area. InGaN LED lamps with optimized n-GaN layers had a high external quantum efficiency and a good long-term reliability.
The theory, structure, and current manufacturing technologies for InGaAlP high brightness light emitting diodes (HB-LED) emitting in the range of 650 to 585 nm are described in this paper. A state-of-the-art HB-LED MOCVD reactor designed for high volume manufacturing (42 - 2' or 16 - 3' wafers) is demonstrated. Data for thickness and compositional uniformity and reproducibility are presented showing the material quality and reactor stability that can currently be achieved. In addition, device data for InGaAlP HB-LEDs is reported, including brightness, forward voltage, and emission wavelength with excellent intra and inter wafer uniformity and run-to-run reproducibility.
980 nm GaInAs/GaAs/GaInP separate-confinement heterostructure single quantum well lasers are fabricated by LP-MOCVD. The lasers exhibit threshold current density of 170 A/cm2, output light power 2W in continuous wave, slope efficiencies of 0.91 W/A without mirror coating. The characteristic temperature T0 is 330 degree(s)K.
GaN homojunction and InGaN/GaN single quantum well (SQW) light-emitting diodes (LEDs) were fabricated and characterized. The blue LED has a typical operating voltage of 3.6 V at 20 mS. Temperature dependence of the emission characteristics of the GaN-based LEDs was studied from 25 degrees C to 130 degrees C. The emission intensity of the InGaN/GaN SQW LED decays exponentially with the increase of temperature. The temperature coefficient Lc is 2.5 X 10-2/degrees C. The emission wavelength of the InGaN/GaN SQW LED was found to be relatively independent of the LED operation temperature while the UV emission of the GaN homojunction LED has a red-shift with the increase of temperature. The temperature coefficient (alpha) of the bandgap energy of Si-doped n-type GaN derived from the EL measurement is 8.5 X 10-4/K. The low temperature coefficient of emission wavelength of the InGaN/GaN SQW LED indicates that the recombination processes involves localized states. The localized states are attributed to excitons localized at the potential minima in the quantum well due to In content fluctuation.
The operating characteristics of Al-free InGaAsP/GaAs separate confinement heterostructure single quantum well high power laser grown by low-pressure metalorganic chemical vapor deposition are reported. The internal differential quantum efficiency (eta) i is closed 98 percent. The external differential quantum efficiency (eta) d of 75 percent and characteristics temperature To of 146 degrees C are achieved, CW total output power both facets of 2.6 W single quantum well laser with 100 micrometers width, 1.1 mm cavity length is obtained. Threshold current density Jth, reciprocal differential quantum efficiency l/(eta) d, emission wavelength (lambda) and characteristics temperature To as function of laser cavity length L respectively have been measured and researched. Dependence of Jth (T), (lambda) (T), and (eta) d (T) respectively on temperature T have been given and explained.
In this paper, we studied the effects of the active region structure (one, two and three quantum wells with same total thickness) for high-power InGaAsP-GaAs separate confinement heterostructure lasers emitting at 0.8 micrometers wavelength. Experimental results for the lasers grown by low pressure metalorganic chemical vapor deposition show excellent agreement with the theoretical model. Total output power of 47 W from an uncoated 1 cm-wide laser bar was achieved in quasi-continuous wave operation.