Diode Turn-on Voltage Research Articles

Covalently Bound Hole-Injecting Nanostructures. Systematics of Molecular Architecture, Thickness, Saturation, and Electron-Blocking Characteristics on Organic Light-Emitting Diode Luminance, Turn-on Voltage, and Quantum Efficiency

Hole transporting materials are widely used in multilayer organic and polymer light-emitting diodes (OLEDs, PLEDs, respectively) and are indispensable if device electroluminescent response and durability are to be truly optimized. This contribution analyzes the relative effects of tin-doped indium oxide (ITO) anode-hole transporting layer (HTL) contact versus the intrinsic HTL materials properties on OLED performance. Two siloxane-based HTL materials, N,N'-bis(p-trichlorosilylpropyl)-naphthalen-1-yl)-N,N'-diphenyl-biphenyl-4,4'-diamine (NPB-Si(2)) and 4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPD-Si(2)), are designed and synthesized. They have the same hole transporting triarylamine cores as conventional HTL materials such as 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) and N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl)-4,4-diamine (TPD), respectively. However, they covalently bind to the ITO anode, forming anode-HTL contacts that are intrinsically different from those of the anode to TPD and NPB. Applied to archetypical tris(8-hydroxyquinolato)aluminum(III) (Alq)-based OLEDs as (1) the sole HTLs or (2) anode-NPB HTL interlayers, NPB-Si(2) and TPD-Si(2) enhance device electroluminescent response significantly versus comparable devices based on NPB alone. In the first case, OLEDs with 36 000 cd/m(2) luminance, 1.6% forward external quantum efficiency (eta(ext)), and 5 V turn-on voltages are achieved, affording a 250% increase in luminance and approximately 50% reduction in turn-on voltage, as compared to NPB-based devices. In the second case, even more dramatic enhancement is observed (64 000 cd/m(2) luminance; 2.3% eta(ext); turn-on voltages as low as 3.5 V). The importance of the anode-HTL material contact is further explored by replacing NPB with saturated hydrocarbon siloxane monolayers that covalently bind to the anode, without sacrificing device performance (30 000 cd/m(2) luminance; 2.0% eta(ext); 4.0 V turn-on voltage). These results suggest new strategies for developing OLED hole transporting structures.

Electrical Characterization of Blue Light Emitting Diodes as a Function of Temperature

AbstractBlue light emitting diodes (LEDs) based on an AlGaN/GaN/AlGaN double heterojunction structure were electrically characterized as a function of temperature. Current-voltage (I-V), capacitance-voltage (C-V) and reverse recovery storage time measurements were conducted at temperatures in the range between -8° and 75° C. Capacitance-voltage measurements as a function frequency (20 Hz – 1 MHz) and electroluminescence study at room temperature were also performed. It was observed that the diode turn-on voltage decreased with increasing temperature, however, reverse leakage currents monitored at -1, -5 and -10 V showed only a slight increase with increasing temperature. The concentration of deep states and their position in the bandgap, as extracted from logarithmic plots of the forward characteristics, were not influenced by the measurement temperature. Recombination lifetimes, as obtained from experimentally determined reverse recovery storage times, remained constant over the range of temperature considered. A higher value of diode capacitance was observed at low measurement frequencies (20 Hz – 1 kHz), gradually dropping to a lower value over a frequency range between (1 kHz – 100 kHz) and remained constant from 100 kHz to 1 MHz. A loss peak centered about 10 kHz was observed in the corresponding plot of gm/ω as a function of frequency, f. The position of the peak in the gm/ω - frequency (f) plot and dC/dω(for f in the range 1 kHz – 100 kHz ), yielded a concentration of deep-states of approximately 2.2 × 1015/cm3, located at 0.39 eV above the valence bandedge.

Electro-optical properties of fluorene derivative with acetylenic functionalities

An aromatic compound with acetylenic functionalities, dipropargylfluorene (DPF), was synthesized and characterized. DPF itself was thermally crosslinkable and showed wide processing window. DPF showed characteristic UV-visible absorption band at 337 and 354 nm and blue PL spectrum at 432 nm corresponding to the photon energy of 2.87 eV. DPF-based EL device showed a typical rectifying diode characteristics, blue EL and turn-on voltage of 12 V. The thermal reaction properties of DPF were studied by differential scanning calorimeter. DPF exhibited to melt and react at 117 and at 247 °C, respectively.

Self-heating effects at CW operation of MQW long-wavelength lasers

Based on a thermal lumped element model and on experimental results concerning the active region heating as a function of several injected current values, a very simple model involving self-heating effects has been implemented. The temperature rise in the active region of the laser due to current injection of magnitude I was calculated according to Δ T= aI+ bI 2− cL where a, b and c are coefficients which depend on the thermal resistance of the device and on the laser diode series resistance and turn-on voltage. L is the laser output power. Combining these results with those of the threshold current as a function of temperature, obtained for MQW long-wavelength lasers, we were able to analyze separately the effect of the diode thermal resistance, series resistance, turn-on voltage and characteristic temperature on the L– I curves. The calculated results seem to agree well with the experimental ones and may be used in the design of the laser structure in order to reduce self-heating effects.

DC and high-frequency performance of AlGaN/GaN Heterojunction Bipolar Transistors

The DC and high frequency performance of AlGaN/GaN Heterojunction Bipolar Transistors (HBTs) is analyzed using an enhanced drift-diffusion model and GaN/AlGaN material parameters, which were previously verified by modeling experimental device characteristics. The emitter–base diode turn-on voltage is as high as 2.7 V while the collector and base ideality factors are 1.16 and 1.46 respectively. A DC current gain of β=15 is found at a collector current density of 2.5 kA/cm 2 and the gain is maintained at this value up to 4.1 kA/cm 2. The devices show a small offset voltage of 0.5 V. A forward breakdown voltage BV CEO of 70 V is found for designs with collector doping of 5×10 16 cm −3. The current gain varied from 22 to 6 when the base doping was increased from 5×10 17 to 2×10 18 cm −3. At the same time, the maximum oscillation frequency f MAX increased from 3 to 6 GHz. A severe degradation of the current gain, f T, and f MAX was observed for HBT designs with a base thickness wider than 1000 Å. By optimizing the transistor design and bias, an f T of 44 GHz and an f MAX of 24 GHz are predicted. Results of a large-signal analysis are also presented to evaluate the power capability and efficiency of AlGaN/GaN HBTs.

Optimisation of DC and RF performance of GaAs HEMT-basedSchottky diodes

The authors show that in a GaAs HEMT-based monolithic millimetre-wave integrated circuit process flow, prior to gate metallisation the introduction of a hydrofluoric acid cleaning process after gate recess etching results in a significant reduction in the Schottky diode turn-on voltage, series resistance and bias dependent RF junction resistance.

Electrical deactivation of interstitial Zn in heteroepitaxial InP by hydrogen and its effect on electronic properties

Hydrogen passivation of InP layers grown on lattice-mismatched substrates can achieve thermally stable deactivation of dislocation-related deep levels, making this a promising process for improving the performance of heteroepitaxial InP space solar cells. However, in addition to dislocation-related defects, interstitial Zn (Zni) defects that are characteristic of Zn-doped InP and which form deep donor states within the InP band gap, are important considerations for optimizing the electronic quality of these layers. Here, we show that hydrogen forms complexes with and deactivates Zni donor states within Zn-doped InP grown by metalorganic chemical vapor deposition. A combination of photoluminescence (PL), electrochemical capacitance–voltage dopant profiling, secondary ion mass spectroscopy and current–voltage (I–V) measurements are applied to a set of samples receiving systematic hydrogenation and annealing treatments. We find that the deactivation of Zni deep donors, as detected by monitoring the evolution of the donor–acceptor transition using PL measurements, causes an increase of ∼50% in the net acceptor concentration of heavily Zn-doped heteroepitaxial InP by elimination of the acceptor compensation effect due to active Zni donors. Analysis of I–V characteristics indicates that Zni passivation sharply reduces depletion region recombination and shunt currents within heteroepitaxial diodes, causing an increase in the diode turn-on voltage from 680 to 960 mV. Subsequent annealing above 500 °C reactivates the Zni defects, resulting in a systematic increase in doping compensation as well as a decrease in VTO toward the original, as-grown value. A study of the reactivation kinetics for the H–Zni complex reveals a greater thermal stability than that of H–Zn acceptor complexes but less than that of H-dislocation complexes in InP, with an estimated dissociation energy for the H–Zni complex of 2.3 eV. While these effects are observed for both homoepitaxial and heteroepitaxial Zn-doped layers, the effect is far more pronounced for the heteroepitaxial layers due to the relatively high Zni concentration in the latter.

Complementary non-destructive optical methods for characterizing Al xGa 1− xAs HEMSFET heterostructures

Spectroscopic ellipsometry and monochromatic ellipsometry were used complementarily to non-destructively characterize HMESFET heterostructures consisting of GaAs/Al x Ga 1− x As/n-type GaAs epitaxial layers on semi-insulating GaAs substrates. The Al x Ga 1− x As barrier layer was used as the experimental variable: four specimens with target Al concentration of x = 0.5 and variable target thicknesses of 10, 20, 30 and 40 nm were prepared by the AP-MOCVD technique. Data reduction with the Bruggeman effective-medium approximation (EMA) yielded mean Al concentrations of x = 0.45−0.56 and mean thicknesses of 12, 22, 28 and 40 nm, respectively. A native oxide of 2–3 nm was also found on the surface, and, partly as a result, the GaAs cap layers were usually 5–6 nm thinner than the target value of 20 nm. In terms of lateral uniformities, a nine-point measurement on a specimen indicates that epitaxial growth was fairly uniform except at the top and bottom edges in the direction perpendicular to the flow of the organometallic gases. At these two locations, the Al concentration was higher and the Al x Ga 1− x As layer was thinner than other locations. Monochromatic ellipsometry provided strong qualitative agreements with the spectroscopic ellipsometry in regards to the lateral epitaxial uniformities, including the edge effect. C V and I V tests were performed on the Schottky diodes fabricated on the heterostructure specimens: the barrier height ( ø Bn) and the diode turn-on voltage ( V on) both increased as a function of the measured Al x Ga 1− x As barrier layer thickness, giving credence to the spectroscopic ellipsometry results.

Heteroepitaxial S 1- x- yGe xC y films on (100) Si substrates for future low-power applications

Thin heteroepitaxial films of S 1- x- y Ge x C y have been investigated for potential use in low-power electronic applications. Films were grown on (100) Si substrates using atmospheric pressure chemical vapor deposition at 625 and 700 °C. The crystallinity, composition and microstructure of the SiGeC films were characterized using Rutherford backscattering spectrometry and secondary ion mass spectrometry. The crystallinity of the films was very sensitive to the flow rate of C 2H 4 that served as the C source. S 1- x- y Ge x C y films with up to 2.0 at.% C and 20 at.% Ge were epitaxial with good crystallinity. Current-voltage measurements were obtained from the electrical characterization of Si 1-x-yGe xC y Si heterojunction diodes. Stable layers and low diode turn-on voltage make the Si 1-x-yGe xC y Si structure an appropriate candidate for future low-power research.

Manufacturable vacuum field emission diodes

Realization of an easily manufacturable field emission device will be a key to the widespread acceptance of vacuum microelectronics technology. Many current devices under investigation work well in the laboratory but face serious manufacturing obstacles. With this in mind, a manufacturable cold field emission diode has been developed. The structure described here takes advantage of the edge emission cathodes similar to those developed by Gray et al., with the major difference being that the anode and cathode are not coplanar [H. F. Gray, Proceedings of the Third Annual IEEE Vacuum Microelectronics Conference, Monterey, CA, July 1990 (unpublished)]. The ability to easily define, nonphotolithographically, the anode-to-cathode spacing during the fabrication process allows control of the diode turn-on voltage over a wide range. Additionally, a symmetrical device geometry provides for increased field emission area. Extensive device testing has shown current–voltage characteristics that follow the Fowler–Nordheim model for cold field emission over several orders of magnitude of current. In this work, various metals have been examined for use as the cathode and their differences noted. Finally, it should be reported that the structure developed here will lend itself well to light emitting diode and triode devices.

An InAlAs/InGaAs heterojunction bipolar transistor clocked latch on InP

A clocked latch functioning as a simple comparator with InAlAs/InGaAs heterojunction bipolar transistors (HBTs) on an InP substrate is discussed. Typical comparator offset voltages of 2 mV, and as low as 0.2 mV, along with differential pair measurements indicate good V/sub be/ matching of the HBTs in the circuit. A thermal hysteresis of approximately 1 mV was measured. These results are comparable to those for the best GaAs HBT-based comparators, despite the unoptimized device fabrication procedure. Additionally, these comparators are able to operate at low power levels and with a 2-V input common-mode voltage range. Despite remaining materials problems, these results indicate that InP-based HBTs have great potential for high-speed, low-power digital and linear small- and medium-scale ICs due to the ease of good ohmic contact formation, the low base-emitter diode turn-on voltage, and the intrinsically high speed and scalability of InP-based HBTs. >