A physical, yet simple, small-signal equivalent circuit for the heterojunction bipolar transistor (HBT) is proposed. This circuit was established by analysing in detail the physical operation of the HBT. The model verification was carried out by comparison of the measured and simulated S- and Z-parameters for both passive (reverse-biased) and active bias conditions. A feature of this model is that it uses a direct extraction method to determine the parasitic elements, in particular, the parasitic capacitances. The excellent agreement between the measured and simulated parameters was verified all over the frequency range from 0.25 to 75 GHz
By a combination of conventional, HREM and CBED TEM experiments we have studied wurtzite GaN layers grown by Metal-Organic Chemical Vapour Deposition (MOCVD) on (0001)Al2O3. We experimentally determine the structure of the macroscopic hexagonal pyramids that are visible at the surface of the layers when no optimised buffer is introduced. These pyramids look like hexagonal volcanoes with one hexagonal microscopic chimney (up to 75nm wide) at their core. The crystal inside the chimney is a pure GaN crystal with a polarity opposed to the one of the neighbouring material : the GaN layers grown on (0001)Al2O3 are everywhere Ga-terminated except in the chimneys where they are N-terminated. Some of the N-terminated chimneys grow faster and form macroscopic hexagonal pyramids. Chimneys bounded by Inversion Domains Boundaries (IDBs) originate from steps at the surface of the substrate and may be suppressed by an adapted buffer layer.
The temperature and excitation power dependence of a bound exciton photoluminescence line S with a localization energy Q=11.5 meV has been studied in undoped and moderately Mg-doped wurtzite GaN of high resistivity. The data provide strong evidence that line S is due to recombination of excitons bound to ionized shallow donors. The consistency of this assignment with theoretical predictions is demonstrated
Metastable GaAs1−ySby with 0.22<y<0.70 has been grown pseudomorphically strained on (001) InP substrates using metalorganic chemical vapor deposition. The Sb concentration and layer thicknesses, ranging from 24 to 136 nm, were determined by high resolution x-ray diffraction (HRXRD) measurements. Low-temperature photoluminescence (PL) spectroscopy revealed spatially indirect band-to-band emission of electrons localized in the InP and holes in the GaAs1−ySby. At increased excitation power densities samples with layer thicknesses above 65 nm showed, also, spatially direct PL across the band gap of the strained GaAs1−ySby. From the PL data the band gap energy and the band offsets of GaAs1–ySby relative to InP were derived and compared with the predictions of the Model Solid Theory.
Oxygen doped GaN has been grown by metalorganic chemical vapor deposition using N2O as oxygen dopant source. The layers were deposited on 2" sapphire substrates from trimethylgallium and especially dried ammonia using nitrogen (N2) as carrier gas. Prior to the growth of the films, an AlN nucleation layer with a thickness of about 300 AA was grown using trimethylaluminum. The films were deposited at 1085 degrees C at a growth rate of 1.0 mu m/h and showed a specular, mirrorlike surface. Not intentionally doped layers have high resistivity (>20 kW/square). The gas phase concentration of the N2O was varied between 25 and 400 ppm with respect to the total gas volume. The doped layers were n-type with carrier concentrations in the range of 4*1016 cm-3 to 4*1018 cm-3 as measured by Hall effect. The observed carrier concentration increased with increasing N2O concentration. Low temperature photoluminescence experiments performed on the doped layers revealed besides free A and B exciton emissi on an exciton bound to a shallow donor. With increasing N2O concentration in the gas phase, the intensity of the donor bound exciton increased relative to that of the free excitons. These observations indicate that oxygen behaves as a shallow donor in GaN. This interpretation is supported by covalent radius and electronegativity arguments
We have realized a (GaAs1−xSbx-InyGa 1−yAs)/GaAs bilayer-quantum well (BQW), which consists of two adjacent pseudomorphic layers of GaAs1−xSbx and InyGa1−yAs sandwiched between GaAs barriers. Photoluminescence was observed at longer wavelengths than those found for corresponding InyGa1−yAs/GaAs and GaAs1−xSbx/GaAs single quantum wells (SQW), which indicates a type-II band alignment in the BQW. The longest 300 K emission wavelength achieved so far was 1.332 μm. For an accurate determination of the band offset between GaAs1−xSbx and GaAs, required for a theoretical modeling of the interband transition energies of these BQWs, a large set of GaAs1−xSbx /GaAs SQWs was prepared from which a type-II band alignment was deduced, with the valence band discontinuity ratio Qv found to depend on the Sb concentration x (Qv=1.76+1.34 x). With this parameter it was possible to calculate the expected interband transition energies in a BQW structure without any adjustable parameters. The calculations are in agreement with experimental data within a range of ±4%.
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