James W. Kretchmer

Diffusion and tunneling currents in GaN/InGaN multiple quantum well light-emitting diodes

We have studied the electrical characteristics and optical properties of GaN/InGaN multiple quantum well (MQW) light-emitting diodes (LEDs) grown by metalorganic chemical vapor deposition. It appears that there is an essential link between material quality and the mechanism of current transport through the wide-bandgap p-n junction. Tunneling behavior dominates throughout all injection regimes in a device with a high density of defects in the space-charge region, which act as deep-level carrier traps. However, in a high-quality LED diode, temperature-dependent diffusion-recombination current has been identified with an ideality factor of 1.6 at moderate biases. Light output has been found to follow a power law, i.e., L /spl prop/ I/sup m/ in both devices. In the high-quality LED, nonradiative recombination centers are saturated at current densities as low as 1.4 /spl times/ 10/sup -2/ A/cm/sup 2/. This low saturation level indicates that the defects in GaN, especially the high density of edge dislocations, are generally optically inactive.

Silicon carbide UV photodiodes

SiC photodiodes were fabricated using 6 H single-crystal wafers. These devices have excellent UV responsivity characteristics and very low dark current even at elevated temperatures. The reproducibility is excellent and the characteristics agree with theoretical calculations for different device designs. The advantages of these diodes are that they will operate at high temperatures and are responsive between 200 and 400 nm and not responsive to longer wavelengths because of the wide 3-eV bandgap. The responsivity at 270 nm is between 70% and 85%. Dark-current levels have been measured as a function of temperature that are orders of magnitude below those previously reported. Thus, these diodes can be expected to have excellent performance characteristics for detection of low light level UV even at elevated temperatures. >

A comparative evaluation of new silicon carbide diodes and state-of-the-art silicon diodes for power electronic applications

Recent progress in silicon carbide (SiC) material has made it feasible to build power devices of reasonable current density. This paper presents recent results including a comparison with state-of-the-art silicon diodes. Switching losses for two silicon diodes (a fast diode, 600 V, 50 A, 60 ns Trr), an ultra-fast silicon diode (600 V, 50 A, 23 ns Trr) and a 4H-SiC diode (600 V, 50 A) are compared. The effect of diode reverse recovery on the turn-on losses of a fast WARP/sup TM/ IGBT are studied both at room temperature and at 150/spl deg/C. At room temperature, SiC diodes allow a reduction of IGBT turn-on losses by 25% compared to ultra-fast silicon diodes and by 70% compared to fast silicon diodes. At 150/spl deg/C junction temperature, SiC diodes allow a turn-on loss reduction of 35% and 85% compared to ultra-fast and fast silicon diodes respectively. The silicon and SiC diodes are used in a boost power converter with the WARP/sup TM/ IGBT to assess the overall effect of SiC diodes on the power converter characteristics. Efficiency measurements at light load (100 W) and full load (500 W) are reported. Although SiC diodes exhibit very low switching losses, their high conduction losses due to the high forward drop dominate the overall losses, hence reducing the overall efficiency. Since this is an ongoing development, it is expected that future prototypes will have improved forward characteristics.

Blue and near-ultraviolet light-emitting diodes on free-standing GaN substrates

Blue and near-ultraviolet (UV) InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with peak emission at 465 nm and 405 nm, respectively, were grown on GaN and sapphire substrates. The densities of surface and bulk defects in the homoepitaxially grown LEDs were substantially reduced, leading to a decrease in reverse currents by more than six orders of magnitude. At a typical operating current of 20 mA, the internal quantum efficiency of the UV LED on GaN was twice as high compared to the UV LED on sapphire, whereas the performance of the blue LEDs was found to be comparable. This suggests that the high-density dislocations are of greater influence on the light emission of the UV LEDs due to less In-related localization effects. At high injection currents, both the blue and UV LEDs on GaN exhibited much higher output power than the LEDs on sapphire as a result of improved heat dissipation and current spreading.

SiC MOS interface characteristics

It is well known that SiC can be thermally oxidized to form SiO/sub 2/ layers. And Si MOSFET ICs using thermally grown SiO/sub 2/ gate dielectrics are the predominant IC technology in the world today. However the SiC/SiO/sub 2/ interface has not been well characterized as was the case for Si MOS in the early 1960s. This paper presents data which for the first time characterizes the SiC/SiO/sub 2/ interface and explains one of the previously unexplained abnormalities observed in the characteristics of SiC MOSFETs. >

Nitrogen-implanted SiC diodes using high-temperature implantation

6H-SiC diodes fabricated using high-temperature nitrogen implantation up to 1000 degrees C are reported. Diodes were formed by RIE etching a 0.8- mu m-deep mesa across the N/sup +//P junction using NF/sub 3//O/sub 2/ with an aluminum transfer mask. The junction was passivated with a deposited SiO/sub 2/ layer 0.6 mu m thick. Contacts were made to N/sup +/ and P regions with thin nickel and aluminum layers, respectively, followed by a short anneal between 900 and 1000 degrees C. These diodes have reverse-bias leakage at 25 degrees C as low as 5*10/sup -11/ A/cm/sup 2/ at 10 V.<<ETX>>

Silicon carbide MOSFET technology

Abstract The research and development activities carried out to demonstrate the status of MOS planar technology for the manufacture of high temperature SiC ICs will be described. These activities resulted in the design, fabrication and demonstration of the worlds first SiC analog IC—a monolithic MOSFET operational amplifier. Research tasks required for the development of a planar SiC MOSFET IC technology included: characterization of the SiCSiO2 interface using thermally grown oxides; high temperature (350°C) reliability studies of thermally grown oxides; ion implantation studies of donor (N) and acceptor (B) dopants to form junction diodes; epitaxial layer characterization; device isolation methods; and finally integrated circuit design, fabrication and testing of the worlds first monolithic SiC operational amplifier IC. High temperature circuit drift instabilities at 350°C were characterized. These studies defined an SiC depletion model MOSFET IC technology and outlined tasks required to improve all types of SiC devices.

Phosphorus and boron implantation in 6H–SiC

Phosphorus and boron ion implantations were performed at various energies in the 50 keV–4 MeV range. Range statistics of P+ and B+ were established by analyzing the as-implanted secondary ion mass spectrometry depth profiles. Anneals were conducted in the temperature range of 1400–1700 °C using either a conventional resistive heating ceramic processing furnace or a microwave annealing station. The P implant was found to be stable at any annealing temperature investigated, but the B redistributed during the annealing process. The implant damage is effectively annealed as indicated by Rutherford backscattering measurements. For the 250 keV/1.2×1015 cm−2 P implant, annealed at 1600 °C for 15 min, the measured donor activation at room temperature is 34% with a sheet resistance of 4.8×102 Ω/□. The p-type conduction could not be measured for the B implants.

Microwave power SiC MESFETs and GaN HEMTs

Abstract We have fabricated SiC metal semiconductor field effect transistors (MESFETs) with more than 60 W of output power at 450 MHz from single 21.6 mm gate periphery devices (2.9 W/mm) and 27 W of output power at 3 GHz from single 14.4 mm SiC MESFET devices (1.9 W/mm). We have also demonstrated more than 6.7 W/mm CW power from 400 μm GaN/AlGaN high electron mobility transistors devices for X band (10 GHz) applications. These excellent device performances have been attributed to the improved substrate and epitaxial films quality, optimized device thermal management, and enhanced device fabrication technologies. The substrates and epitaxial films from different sources were compared and some showed significant less SiC substrate micropipes confirmed by X-ray topography and epitaxial defects characterized by optical defect mapping.

Boron‐implanted 6H‐SiC diodes

Ion implanted planar p‐n junctions are important for silicon carbide discrete devices and integrated circuits. Conversion to p‐type of n‐type 6H‐SiC was observed for the first time using boron implantation. Diodes were fabricated with boron implants at 25 and 1000 °C, followed by 1300 °C post‐implant annealing in a furnace. The best diodes measured at 21 °C exhibited an ideality factor of 1.77, reverse bias leakage of 10−10 A/cm2 at −10 V, and a record high (for a SiC‐implanted diode) breakdown voltage of −650 V.