conference report nitrides
Russell Dupuis from Georgia Institute of Technology, who has been working with Fernando Ponce’s group at Arizona State University, one way to do this is to replace the conventional p-type InGaN layer with p-type InAlN.
This US partnership fabricated a series of LEDs with In0.18
Ga0.82 N electron-blocking layers of differing
thicknesses, and found that device performance peaked for a 15 nm-thick blocking layer. The team have simulated device performance with a modified version of the ABC model that includes a term for electron spill- over. In their work, they found a good fit to their experimental data using values for A, B and C of 1x107
s-1 , 2 x 10-10 cm3 s-1 and 4.5 x 10-29 cm6 s-1 .
One of the great strengths of the widely used ABC model is its simplicity. However, it has its weaknesses too, according to Mary Crawford from Sandia National Laboratories, Albuquerque, NM. She explained that the radiative recombination in real devices is not actually proportional to Bn2
Toyota gears up HEMTs for HEVs
Toyota is developing vertical HEMTs for incredibly efficient, high-power switching in hybrid electric vehicles (HEVs), and lateral equivalents for lower power applications in cars. The motivation behind these efforts, according to Tetsu Kachi from Toyota Central R&D Labs, Japan, is that the silicon electronics used today to convert DC power from the battery to an AC form to power the motor throws away 5-10 percent of the energy, which is wasted as heat. Managing this requires a water-cooling system, and the amount of energy lost could increase because the trend is towards higher and higher powers in HEVs. The first generation Prius used a 30 kW motor, but second and third generations have increased this figure to 50 kW and 70 kW. Even higher figures are being used by Lexus, which has a motor with a power of more than 150 kW in its model LS600h.
at high carrier densities, and claimed
that more realistic modelling is possible by using momentum-resolved carrier distributions, which allow direct implementation of bandstructure properties into the rate equations. This provides a more accurate description of carrier-carrier and carrier-phonon interactions, includes a treatment of carrier leakage and capture, and derives radiative recombination via bandstructure and carrier distributions. One tremendous benefit of this approach is that it can be used to consider contributions not present in the ABC model, such as recombination at defects with a microscopically determined, carrier-density-dependent A coefficient.
Droop: The case for carrier overflow Going against the grain, Akio Kaneta from Kyoto University, Japan, did not attribute any of the droop he observed in his light-emitting nitride samples to Auger recombination. Instead, he pointed the finger at carrier overflow for the droop in blue LEDs, and blamed carrier diffusion from radiative recombination domains to non- radiative ones for the decline in green LED efficiency at higher drive currents.
Kaneta and his colleagues came to these conclusions after scrutinizing the photoluminescence produced by blue and green single-quantum-well structures with a scanning near-field optical microscope, a form of scanning probe microscope that can realise incredibly high spatial resolution in the optical domain by illuminating a sample through a sub-wavelength aperture formed at the apex of an optical fibre.
The researchers illuminated both samples through the aperture at two excitation powers, differing by a factor of 100, and recorded the local emission intensity. The two samples featured domains of strong photoluminescence that were several hundred nanometers across,
18
www.compoundsemiconductor.net August / September 2011
Losses in the DC-to-AC conversion process results from imperfect switching. The ideal: No current would be drawn in the off-state; in the on-state there would be no resistance; and switching speeds would be instantaneous. Switching losses promise to fall by a factor of about six by replacing silicon switches with those made from GaN. But that’s not the only benefit – switch to GaN and the electronics no longer needs its own dedicated water-cooling system, explained Kachi. Instead, it can tap into the one used for the engine, which one day might just require air cooling.
Smaller power modules operating at a few kilowatts are also needed in HEVs for air-conditioning, and emergency and outdoor applications.
Toyota is developing two types of GaN HEMT for the HEV: vertical devices for high-powers, which have the merits of high current density, high breakdown voltage and a high on-resistance; and lateral transistors for lower powers, which combine high frequencies with a low resistance, and high breakdown voltage and low cost. The vertical devices feature a novel U-shaped trench that is formed by dry etching with an inductively coupled plasma, followed by wet etching for 60 minutes at 85 °C. The sidewall forms a metal-oxide-semiconductor channel that works well up to 300 °C. Normally-off operation is realised with this device, which has a breakdown of 180 V. The goal is to increase this to 600 V. Efforts on the lateral devices have created transistors that are normally off above 3 V, and have issues related to current collapse and reliability of the gate insulator. Engineers at Toyota will work to improve this device and its vertical cousin.
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