Virginia M Robbins

Very high‐efficiency semiconductor wafer‐bonded transparent‐substrate (AlxGa1−x)0.5In0.5P/GaP light‐emitting diodes

Data are presented demonstrating the operation of transparent‐substrate (TS) (AlxGa1−x)0.5In0.5P/GaP light‐emitting diodes (LEDs) whose efficiency exceeds that afforded by all other current LED technologies in the green to red (560–630 nm) spectral regime. A maximum luminous efficiency of 41.5 lm/W (93.2 lm/A) is realized at λ∼604 nm (20 mA, direct current). The TS (AlxGa1−x)0.5In0.5P/GaP LEDs are fabricated by selectively removing the absorbing n‐type GaAs substrate of a p‐n (AlxGa1−x)0.5In0.5P double heterostructure LED and wafer bonding a ‘‘transparent’’ n‐GaP substrate in its place. The resulting TS (AlxGa1−x)0.5In0.5P/GaP LED lamps exhibit a twofold improvement in light output compared to absorbing‐substrate (AS) (AlxGa1−x)0.5In0.5P/GaAs lamps.

HIGH PERFORMANCE ALGAINP VISIBLE LIGHT-EMITTING DIODES

The performance of surface‐emitting visible AlGaInP light‐emitting diodes (LEDs) is described. The devices have external quantum efficiencies greater than 2% and luminous efficiencies of 20 lm/A in the yellow (590 nm) spectral region. This performance is roughly ten times better than existing yellow LEDs and is comparable to the highest performance red AlGaAs LEDs currently available. The devices also perform favorably compared to existing devices in the orange and green spectral regions. Low‐pressure organometallic vapor phase epitaxy (OMVPE) is used to grow the epitaxial layers. The devices consist of a double heterostructure with an AlGaInP active region grown on a GaAs substrate.

Unusual properties of photoluminescence from partially ordered Ga0.5In0.5P

Ga0.5In0.5P grown lattice matched to GaAs by metalorganic chemical vapor deposition (MOCVD) exhibited domains of varying degrees of column III sublattice ordering. Continuous‐wave photoluminescence spectra were single peaked and relatively narrow, but the peak wavelengths from samples grown at low (630–670 °C) temperatures varied strongly with excitation density at low measurement temperatures, while peak wavelength did not vary for high (775 °C) temperature growth. The half width was 6.5 meV in the latter case, the narrowest reported from MOCVD‐grown Ga0.5In0.5P. Time‐resolved photoluminescence of partially ordered GaInP at liquid‐helium temperatures is reported for the first time. For samples grown at low temperatures, the spectral peak displayed a slow (τ≳1 μs) decay at low excitation density. The decay was more rapid (τ=1.8 ns) at higher excitations and at higher photon emission energies. Possible explanations discussed include spatial separation of carriers and trapping.

Drain resistance degradation under high fields in AlInAs/GaInAs MODFETs

Lattice-matched AlInAs/GaInAs modulation-doped FETs (MODFETs) demonstrate excellent high-frequency, small-signal performance. However, high-power, large-signal applications of these devices may be limited. Impact ionization and tunneling reduce the breakdown voltage, which limits the upper end of the output voltage swing, thus reducing the output power. Our results indicate that the lower end of the voltage swing (knee voltage) is also degraded by increased drain resistance when impact ionization occurs. In this work, we correlate R/sub d/ degradation in the AlInAs/GaInAs material system to the presence of impact ionization. The magnitude of R/sub d/ degradation depends on the applied drain bias and drain current. These factors affect the degree of impact ionization, and thus the extent of the degradation. Since R/sub d/ increases and R/sub s/ does not, only the high field side of the FET is affected. This increase in R/sub d/ is attributed to a wider carrier depletion region between the gate and drain after stress, which results in reduced device performance.

Low-noise bias reliability of AlInAs/GaInAs MODFETs with linearly graded low-temperature buffer layers grown on GaAs substrates

AlInAs/GaInAs MODEETs lattice-matched to InP have been shown to be reliable at low bias (V/sub ds/=0.75 to IV) for low-noise applications. Mean-times to failure (MTTF) from 10/sup 5/ to 10/sup 7/ hrs., based on various failure criteria, have been reported for lattice-matched FETs. To improve manufacturability of these FETs we have fabricated 0.1 /spl mu/m T-gate AlInAs/GaInAs MODFETs on mismatched GaAs substrates by the insertion of a compositionally linearly graded low-temperature buffer (LGLTB) layer. In this work, we demonstrate that such FETs show comparable reliability at low bias under high temperature operating life (HTOL) tests to FETs on InP. Although the LGLTB layer is highly defective, there is no indication that the low-bias reliability of these devices is compromised. Our AlInAs/GaInAs MODFETS, grown on GaAs, have an extrapolated MTTF, based on I/sub dss/ drift, exceeding 10/sup 6/ hours at a channel temperature of 125/spl deg/C.

A 0.1-μm MHEMT millimeter-wave IC technology designed for manufacturability

Abstract We describe and discuss our approach to improved manufacturability of millimeter-wave InP-type HEMT IC technology. The main ingredients are the use of (1) GaAs (rather than InP) as substrate, with a buffer that grades the lattice constant to that of the high-mobility Ga 0.47 In 0.53 As channel; (2) low-resistance non-alloyed ohmic contacts with good reliability and reproducibility; (3) an e-beam process that produces 0.1-μm T-gate fingers with one exposure and (4) two-step selective etching of the gate trough for good uniformity. The process is compatible with standard GaAs FET IC front and back end unit processes, including backside vias. The yield, uniformity and performance of devices and circuits made on this metamorphic (MHEMT) material are consistently as good as those we get using InP substrates. The reliability of our devices, whether made on GaAs or InP substrates, is limited by impact ionization in the channel, which has also been linked to R d -degradation and burnout. At low-noise drain bias ( V d =1 V), where the impact ionization is small, we have demonstrated reliability, i.e. extrapolated MTTF at 125°C channel temperature is >10 6 h. This is as good as FETs made on InP substrates.

High-performance AlGaInP light-emitting diodes

A new class of LEDs based on the AlGaInP material system first became commercially available in the early 1990s. These devices benefit from a direct bandgap from the red to the yellow-green portion of the spectrum. The high efficiencies possible in AlGaInP across this spectrum have enabled new applications for LEDs including automotive lighting, outdoor variable message signs, outdoor large screen video displays, and traffic signal lights. A review of high-brightness AlGaInP LED technology will be presented.