We have investigated magneto-transport properties of differently strained Si/ Si1−xGex resonant tunneling devices. The built-in strain was either put in the Si layers, by means of a thick., relaxed Si1−xGex buffer layer, or in the Si1−xGex layers, in which case all Si1−xGex layers were grown below the critical thickness, and a Si1−xGex spacer layer with graded Ge content was used. Magnetic fields parallel to the interface have been employed to probe the in-plane dispersion in the quantum well. This is used to study the effect of band-mixing in the two strain configurations. A field perpendicular to the interface resolves some Landau level splitting. Most strikingly, however, is the similarity in the spectra with the case when the magnetic field is applied parallel to the interfaces. This indicates broadening of the levels, possibly due to scattering, and the importance of 3-dimensional band structure effects.

Thin GaAs-oxide layers have been formed on GaAs (100) surfaces using deep UV light to enhance the oxidation reactions. Two different oxidation reactions were compared by using UV light of wavelengths below and above the O2 dissociation limit, viz, 248 nm and 193 nm respectively. For the light above the O2 dissociation limit which causes ozone formation, oxides formed by direct surface illumination were compared to oxides formed by illumination parallel to the GaAs surface. Metal contacts were deposited in situ to study the effect of the various thin oxides on the Schottky barrier height of contacts to GaAs.

High temperature superconductor (HTS) thin films have been applied in making a low loss RF receiver coil for improving magnetic resonance imaging image quality. However, the application of these coils is severely limited by their limited field of view (FOV). Stringent fabrication environment requirements and high cost are further limitations. In this paper, we propose a simpler method for designing and fabricating HTS coils. Using industrial silver alloy sheathed Bi(2−x)PbxSr2Ca2Cu3O10 (Bi-2223) HTS tapes, a five-inch single-turn HTS solenoid coil has been developed, and human wrist images have been acquired with this coil. The HTS tape coil has demonstrated an enhanced FOV over a six-inch YBCO thin film surface coil at 77 K with comparable signal-to-noise ratio.

An InGaP/GaAs heterojunction bipolar transistor (HBT) power amplifier is developed for WCDMA user equipment, specifically, band‐1‐power class‐2 application. The HBT power amplifier demonstrates maximum output power P out of 29.4 dBm and power‐added efficiency (PAE) of 48% at a frequency of 1.95 GHz. When operated according to the WCDMA standard, it achieves P out of 27 dBm and PAE of 32.4%. The adjacent channel leakage power ratio (ACLR) is −33 dBc. A compact microstrip resonant cell (CMRC) circuit is implemented on the HBT amplifier in order to further improve the PAE, ACLR, and IM3 performances. This results in improvements of 8 dB and 6% for the ACLR and PAE, respectively. © 2005 Wiley Periodicals, Inc. Microwave Opt Technol Lett 46: 84–88, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.20908

We have developed InGaP∕GaAsSb∕GaAs double-heterojunction bipolar transistors (DHBTs) with low turn-on voltage and high current gain by using a narrow energy bandgap GaAsSb layer as the base and an InGaP layer as the emitter. The current transport mechanism is examined by measuring both of the terminal currents in forward and reverse mode. The results show that the dominant current transport mechanism in the InGaP∕GaAsSb∕GaAs DHBTs is the transport of carriers across the base layer. This finding suggests that the bandgap offset produced by incorporating Sb composition into GaAs mainly appears on the valence band and the conduction-band offset in InGaP∕GaAsSb heterojunction is very small.

The thermal stability of current gain in InGaP∕GaAsSb∕GaAs double-heterojunction bipolar transistors (DHBTs) is investigated. The experimental results show that the current gain in the InGaP∕GaAsSb∕GaAs DHBTs is nearly independent of the substrate temperature at collector current densities >10A∕cm2, indicating that the InGaP∕GaAsSb∕GaAs DHBTs have excellent thermal stability. This finding suggests that the InGaP∕GaAsSb∕GaAs DHBTs have larger emitter-base junction valence-band discontinuity than traditional GaAs-based HBTs.

An InGaP/GaAs heterojunction bipolar transistor (HBT) is developed. By using this HBT, a power amplifier is designed for WCDMA user equipment, band-1 power class-2 application. The HBT power amplifier demonstrates a maximum output power (Pout) of 29.4 dBm and a PAE of 48% at the frequency of 1.95 GHz. When it operates in the WCDMA standard, it achieves a Pout of 27 dBm and a PAE of 32.4%. The adjacent channel leakage power ratio (ACLR) is -33 dBc. To improve the PAE, ACLR and IM3 performance further, a CMRC (compact microstrip resonance cell) circuit has been implemented on the HBT amplifier. The effect of CMRC on PAE and ACLR is investigated using a low power HBT amplifier. The results show that the ACLR can be improved by the CMRC.

A study of the InGaP/GaAsSb/GaAs double heterojunction bipolar transistor (DHBT) is presented. Novel device structure is designed. A fully strained pseudomorphic GaAsSb with 8.0% Sb composition is used as the base layer, while an InGaP layer as the emitter which both eliminates the misfit dislocations and increases the valence band discontinuity at the InGaP/GaAsSb interface. A current gain of 22.6 has been obtained from the InGaP/GaAsSb/GaAs DHBT. Typical turn-on voltage of the device is 0.973 V which is 0.116V lower than that of traditional InGaP/GaAs HBT. Moreover, the current transport mechanism of the InGaP/GaAsSb/GaAs DHBTs is investigated. These results show that GaAsSb is a promising base material for reducing the turn-on voltage of GaAs HBTs.

High efficiency, low offset voltage InGaP/GaAs power heterostructure-emitter bipolar transistors (HEBTs) have been demonstrated. The large signal performance of the HEBTs is characterized. Output power of 0.25 W with power added efficiency (PAE) of 63.5% at 1.9 GHz has been achieved from a 26-finger HEBT with total emitter area of 873.6 /spl mu/m/sup 2/. Output power of 1.0 W with PAE of 63% has been obtained from the composition of four above-mentioned power cells at the optimum conditions of impedance matching. The thermal performance of HEBT is presented and the results show better thermal management than conventional HBT. The experimental results demonstrate good power performance and capability of HEBTs.

A novel InGaP/GaAs0.94Sb0.06/GaAs double heterojunction bipolar transistor is presented. It features the use of fully strained pseudomorphic GaAs0.94Sb0.06 as the base layer and an InGaP layer as the emitter, which both eliminate misfit dislocations and current blocking, and increase the valence band discontinuity at the InGaP/GaAsSb interface. The device demonstrates a high current gain and a low turn-on voltage.

A novel InGaP/GaAs0.92Sb0.08/GaAs double heterojunction bipolar transistor (DHBT) with low turn-on voltage has been fabricated. The turn-on voltage of the DHBT is typically 150 mV lower than that of the conventional InGaP/GaAs HBT, indicating that GaAsSb is a suitable base material for reducing the turn-on voltage of GaAs HBTs. A current gain of 50 has been obtained for the InGaP/GaAs0.92Sb0.08 /GaAs DHBT. The results show that InGaP/GaAsSb/GaAs DHBTs have a great potential for reducing operating voltage and power dissipation.

Hole-initiated avalanche multiplication is investigated using an AlGaAs/InGaAs p-n-p heterojunction bipolar transistor (HBT). Both experimental measurements and theoretical calculation are used to determine the avalanche multiplication factor. A large departure is observed at low electric field when comparison is made between the measured data and theoretical results obtained from the standard ionization model. The comparison shows that the conventional impact ionization model, based on local electric field, substantially overestimates the hole avalanche multiplication factor Mp−1 in the AlGaAs/InGaAs p-n-p HBT, where a significant dead space effect occurs in the collector space-charge region. A simple correction model for the dead space is proposed, that allows the multiplication to be accurately predicted, even in a heavily doped structure. Based on this model, multiplication characteristics for different threshold energy of the hole are calculated. A threshold energy of 2.5 eV was determined to be suitable for describing the hole-initiated impact ionization process.

InGaP/GaAs heterojunction bipolar transistors (HBTs) with and without passivation ledge in the extrinsic base region were investigated. Gummel plot changes before and after reliability testing were compared. The experimental results demonstrated that the devices featuring the lower quality of the extrinsic base surface are more sensitive to a temperature–current stress. The HBTs with a passivation ledge have an activation energy of 1.41 eV and a mean time to failure (MTTF) of 106 h whereas the HBTs without passivation ledge have an activation energy of 1.24 eV and a MTTF of 105 h.

The first microwave large-signal power results measured from InGaP/GaAs heterostructure-emitter bipolar transistors (HEBT) are reported. A continuous wave output power of 0.25 W with power added efficiency of 63.5% at 1.9 GHz has been achieved from a 26-finger HEBT with total emitter area of 873.6 mum(2). The experimental results demonstrate excellent power performance and capability of HEBTs.

A self-aligned fabrication process for AlGaAs/GaAs heterojunction bipolar transistors (HBT's) is presented. The advantage of this process is that self-aligned structure and device passivation can be realized simultaneously using silicon nitride sidewall technique. The silicon nitride sidewall functions both as an isolation layer to prevent shorting between the base metal and the emitter mesa and as an etching mask to prevent AlGaAs passivation layer to be removed. A current gain cutoff frequency f(T) of 30GHz and a maximum oscillation frequency f(max) of 50 GHz have been obtained from the device with 3 mu mx15 mum emitter size.

The temperature dependence of the current gain is investigated for GaAs-based heterostructure-emitter bipolar transistors (HEBT's). With the separation of the p-n junction and the heterojunction, the mechanism of hole injection from the base to emitter in the HEBT is different from that of a conventional HBT. Theoretical results demonstrate that the thermionic emission current plays an important role for the hole current which results ina smaller negative or even positive temperature coefficient for the current gain. Experimental data show that the base current for HEBT's is indeed dominated by thermionic emission as predicted. This finding indicates that the HEBT structure is the suitable choice for high power and high speed applications.

The temperature effect on current gain is presented for GaInP/GaAs heterojunction and heterostructure-emitter bipolar transistors (HBT's and HEBT's), Experimental results showed that the current gain of the HEBT increases with the increase of temperature in the temperature range of 25-125 degrees C and decreases slightly at temperatures above 150 degrees C, The smaller the collector current, the larger is the positive differential temperature coefficient. At high current levels, the current gain dependence on temperature is significantly reduced. On the other hand, a large negative coefficient is observed in the HBT in all current range. This finding indicates that the HEBT is a better candidate than the HBT for power devices.

The temperature effect on current gains is presented for an AlGaAs/GaAs heterostructure-emitter bipolar transistor (HEBT). Experimental results show that the HEBT has much less temperature sensitivity in current gain than a heterojunction bipolar transistor. The current gains for the HEBT are almost constant with the substrate temperature at a high current regime. This indicates that the HEBT could be a good candidate for power applications.

The use of nitrogen as the carrier gas in metalorganic chemical vapor deposition (MOCVD) for the growth of carbon-doped GaInP/GaAs heterojunction bipolar transistors (HBTs) is reported. The material quality grown using a nitrogen carrier gas is the same as that of using a hydrogen carrier gas. High carbon doping and hole concentrations of 3×1020 and 2×1020 cm−3 in GaAs were obtained. The fabricated HBTs showed very good DC and RF performances indicating that nitrogen can be a promising carrier gas for MOCVD growth.

A C-doped GaInP/GaAs heterojunction bipolar transistor (HBT) with a selective buried sub-collector has been fabricated by two growth steps. The active HBT region was made on the selective buried sub-collector layer with minimum overlap of the extrinsic base and the sub-collector region resulting in substantial reduction of the base-collector capacitance. The experiment shows that the base-collector capacitance is reduced to about half of that of a conventional HBT while the base resistance remains unchanged resulting in a 40-50% increase in the maximum oscillation frequency. Both DC and RF characteristics are investigated and compared with a conventional HBT. A current gain of 40, cutoff frequency of 50 GHz and maximum oscillation frequency of 140 GHz were obtained for the GaInP/GaAs HBT. It is demonstrated that the selective buried sub-collector provides an effective means for enhancing RF performance of an HBT.