Experimental results on a new type of light-emitting device, the light-emitting triode (LET), are presented. The LET is a three-terminal p-n junction device that accelerates carriers in the lateral direction, i.e. parallel to the p-n junction plane, by means of an electric field between two anodes. The lateral field provides additional energy to carriers thereby allowing them to overcome barriers and increasing the carrier injection efficiency into the active region. LETs were fabricated using a ultraviolet LED structure that has an AlGaN/GaN superlattice in the p-type confinement region for high-conductivity 2 dimensional hole gas. LET mesa structures were obtained by standard photolithographic patterning followed by chemically-assisted ion-beam etching using Cl2 and Ar to expose the n-type cladding layer. The n-type contact was fabricated by electron-beam evaporation of Ti/Al/Ni/Au. Ni/Au (50/50 Å) metallization was deposited for both anodes, Anode 1 and Anode 2, and subsequently annealed at 500 oC in an O2 ambient. It is shown that both the current between Anode 1 and the cathode, and the light-output power increase with increasing negative bias to the Anode 2. This is consistent with the expectation that a negative bias to the second anode allows carriers to acquire a high kinetic energy thereby enabling them to overcome the barrier for holes, resulting in high injection efficiency into the active region that lies beyond the barrier.
The junction temperature of red (AlGaInP), green (GaInN), blue (GaInN), and ultraviolet (GaInN) light-emitting diodes (LEDs) is measured using the temperature coefficients of the diode forward voltage and of the emission-peak energy. The junction temperature increases linearly with DC current as the current is increased from 10 mA to 100 mA. For comparison, the emission-peak-shift method is also used to measure the junction temperature. The emission-peak-shift method is in good agreement with the forward-voltage method. The carrier temperature is measured by the high-energy-slope method, which is found to be much higher than the lattice temperature at the junction. Analysis of the experimental methods reveals that the forward-voltage method is the most sensitive and its accuracy is estimated to be ± 3°C. The peak position of the spectra is influenced by alloy broadening, polarization, and quantum confined Stark effect thereby limiting the accuracy of the emission-peak-shift method to ±15°C. A detailed analysis of the temperature dependence of a tri-chromatic white LED source (consisting of three types of LEDs) is performed. The analysis reveals that the chromaticity point shifts towards the blue, the color-rendering index (CRI) decreases, the color temperature increases, and the luminous efficacy decreases as the junction temperature increases. A high CRI > 80 can be maintained, by adjusting the LED power so that the chromaticity point is conserved.
An electrically conductive omnidirectional reflector (ODR) is demonstrated as p-type ohmic contact for an AlGaInP light-emitting diode (LED). The ODR comprises the semiconductor, a metal layer and an intermediate low-refractive index dielectric layer. The SiO2 dielectric layer, located between a GaP and a silver layer, is perforated by an array of AuZn micro-contacts thus enabling electrical conductivity. It is shown that the ODR-LED has a significantly higher light-extraction efficiency as compared to LEDs employing distributed Bragg reflectors (DBRs). For devices emitting in the red wavelength range, external quantum efficiencies of 18 % and 11 % are obtained for ODR- and DBR-LEDs, respectively. The performance of the ODR-LED can be further increased by replacing the SiO2 dielectric with materials having a refractive index << 1.45. Performance characteristics of such powerful reflectors will be presented.
The performance characteristics of white light sources based on a multiple-LED approach, in particular dichromatic and trichromatic sources are analyzed in detail. Figures of merit such as the luminous efficacy, color temperature, and color rendering capabilities are provided for a wide range of primary emission wavelengths. Spectral power density functions of LEDs are assumed to be thermally and inhomogeneously broadened to a full width at half maximum of several kT, in agreement with experimental results. A gaussian line shape is assumed for each of the emission bands. It is shown that multi-LED white light sources have the potential for luminous efficacies greater than 400 lm/W (dichromatic source) and color rendering indices of greater than 90 (trichromatic source). Contour maps for the color rendering indices and luminous efficacies versus three wavelengths are given.
A high-reflectivity omni directional reflector (ODR) has been incorporated into a GaInN light-emitting diode (LED) structure. The ODR comprises a transparent, electrically conductive quarter-wave layer of indium tin oxide clad by silver and serves as an ohmic contact to p-type GaN. It is shown that ODR-LEDs have low optical losses and high extraction efficiency. Mesa-structure GaInN/GaN ODR-LEDs emitting in the blue wavelength range are demonstrated and compared to GaInN/GaN LEDs with semitransparent Ni/Au top contacts. The extraction efficiency of ODR-LEDs is higher as compared to conventional LEDs with Ni/Au contacts.
A novel AlGaInP light-emitting diode (LED) is presented that employs high-reflectivity omni-directional reflector (ODR) submounts. It is shown that the reflective-submount (RS) LED has a higher light-extraction efficiency than conventional LEDs. Red AlGaInP RS-LEDs bonded to Si-substrates are demonstrated using a silver-based ODR. The ODR is perforated by an array of small-area low-resistance ohmic contacts. The optical and electrical characteristics of the RS-LEDs are presented and compared to conventional AlGaInP absorbing substrates (AS) LEDs with distributed Bragg reflectors (DBR). It is shown that the light output from the RS-LED exceeds that of AS-LEDs by about a factor of two.