Application of amorphous silicon field effect transistors in integrated circuits

The work described in this paper is concerned with the possible applications in integrated circuits of thin-film field effect transistors (FETs) made from glow discharge amorphous (a-) silicon and silicon nitride. The construction and performance of inverter circuits, employing integrated a-Si load resistors, are described in some detail. The extension of this basic circuit to NAND and NOR gates, to a bistable multivibrator and to a shift register is reported. Based on the excellent photoconductive properties of a-Si an integrated addressable photosensing element has been constructed, which could have potential applications in imaging arrays.     

Amorphous silicon diodes and TFTs for active matrix flat panel display applications

The use of a-Si devices (diodes and transistors) in active matrix flat-panel displays is reviewed. The fabrication of the elements and their characteristics with steady and pulsed applied potentials are discussed. Two experimental arrays of 32×32 BBDs and 256×320 TFTs are shown.Part of this work is supported by CEE under Esprit program No. 833     

The use of amorphous and microcrystalline silicon for silicon heterojunction bipolar transistors

The modification of the emitter structure of silicon bipolar transistors results in more freedom in the choice between sometimes conflicting device parameters. The approach followed in this work is the use of an amorphous silicon (a-Si:H) or microcrystalline silicon (c-Si) emitter, creating a real heterojunction with the crystalline silicon base. Due to the larger bandgap of these emitter materials, the back injection of minority carriers is strongly suppressed in comparison with conventional bipolar transistors. Furthermore, the small temperature coefficient of the current gain allows the use of these heterojunction bipolar transistors (HBT) over a wide temperature range. Most likely, the biggest advantage of such HBTs is that a better high-frequency behaviour could be obtained. However, some problems still need to be solved such as the recombination at the emitter-base interface and the high resistivity of the emitter material.On leave from University of Florida, Gainesville, Florida, USA     

Application of amorphous silicon field effect transistors in addressable liquid crystal display panels

It is shown that thin-film field effect transistors (FETs) made from amorphous (a-) silicon deposited by the glow-discharge technique have considerable potential as switching elements in addressable liquid crystal display panels. The fabrication of the elements and their characteristics with steady and pulsed applied potentials are discussed in some detail. Two important points are stressed: (i) a-Si device arrays can be produced by well-established photolithographic techniques, and (ii) satisfactory operation at applied voltages below 15VV is possible. Small experimental 7×5 transistor panels have been investigated and it is shown that with the present design up to 250-way multiplexing could be achieved. The reproducibility of FET characteristics is good and in tests so far no change has been observed after more than 10⁹ switching operations.     

Carrier frequency-dependent intrinsic parameters in an ion-implanted silicon photo-MESFET

Carrier frequency-dependent intrinsic parameters of an ion-implanted silicon photo-MESFET have been analysed theoretically. The internal gate source capacitanceC _gs is found to increase with increasing carrier frequency under the normally OFF condition and the change is small under the normally ON condition. Also, the internal drain-source resistanceR _ds increases with frequency at a fixed flux density and wavelength of operation. The ion-implanted photo-MESFET could become useful as optically controlled switching device in digital circuits.     

Design of multiple layered a-Si:H(F)/a-SiGex:H(F) films for enhancement in photoresponse in the near-infrared spectrum

Photo-electric properties of a-Si:H(F)/a-SiGe_x:H(F) multilayer films were investigated by measurements of optical absorption, and photoconductivity in both steady and transient modes with the repetition length and the difference in the optical gap between a-Si:H(F) and a-SiGe_x:H(F) as the variables. Measurements of primary photocurrent clarified that photosensitivity for the multilayer films extended to longer wavelengths of around 725 nm, while high resistivity was maintained despite of lowering the band gap.The drift mobility of electrons was measured by the time-of-flight technique, showing 10^–2-10^–3 cm²/Vs, while the drift mobility-lifetime products of electron was maintained to be 10^–7 cm²/V. On the other hand, the drift mobility of holes was 10^–3 cm²/Vs, which was the similar magnitude to that of a-Si:H(F).     

Microcrystalline silicon and micromorph tandem solar cells

“Micromorph” tandem solar cells consisting of a microcrystalline silicon bottom cell and an amorphous silicon top cell are considered as one of the most promising new thin-film silicon solar-cell concepts. Their promise lies in the hope of simultaneously achieving high conversion efficiencies at relatively low manufacturing costs. The concept was introduced by IMT Neuchatel, based on the VHF-GD (very high frequency glow discharge) deposition method. The key element of the micromorph cell is the hydrogenated microcrystalline silicon bottom cell that opens new perspectives for low-temperature thin-film crystalline silicon technology. According to our present physical understanding microcrystalline silicon can be considered to be much more complex and very different from an ideal isotropic semiconductor. So far, stabilized efficiencies of about 12% (10.7% independently confirmed) could be obtained with micromorph solar cells. The scope of this paper is to emphasize two aspects: the first one is the complexity and the variety of microcrystalline silicon. The second aspect is to point out that the deposition parameter space is very large and mainly unexploited. Nevertheless, the results obtained are very encouraging and confirm that the micromorph concept has the potential to come close to the required performance criteria concerning price and efficiency. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 1 July 1999     

Electrical behaviour of thermally diffused silicon solar cells submitted to rapid annealing

Conventional thermally diffused silicon solar cells have been submitted to subsequent rapid thermal annealing (1–10 s) in the temperature range 600–1000 °C, in order to investigate the effects of such treatments on the electrical behaviour. It appears that a decrease of the minority carrier diffusion length in the base region of the device is produced due to the generation of microscopic defects (as measured by DLTS) induced by the rapid high temperature cycling.     

Low cost wavelength filter of SiGe photodetector with a-Si:H capped layer

The strained Si_0.8Ge_0.2 film has been prepared onto Si substrate by using an ultrahigh-vacuum chemical vapor deposition system. A low cost wavelength filter of photodetector has been demonstrated for the first time. This filter was simply carried out by just inserting a 60 nm thick a-Si:H capped layer onto Si_0.8Ge_0.2 thin film. The room-temperature photoluminescence shows that the sample with Si_0.8Ge_0.2 layer has a tendency to shift wavelength into longer regime than that of Si substrate. The full width at half maximum (FWHM) was 185 nm for Si_0.8Ge_0.2 photodetector without a-Si:H capped. By inserting a 60 nm thick a-Si:H capped layer, the FWHM was narrowed into 97 nm. This demonstrates that the a-Si:H capped layer has an ability acted as wavelength filter in our study.     

High performance a-Si solar cells and new fabrication methods for a-Si solar cells

The super chamber, a separated UHV reaction-chamber system has been developed. A conversion efficiency of 11.7% was obtained for an a-Si solar cell using a high-quality i-layer deposited by the super chamber, and a p-layer fabricated by a photo-CVD method.As a new material, amorphous superlattice-structure films were fabricated by the photo-CVD method for the first time. Superlattice structure p-layer a-Si solar cells were fabricated, and a conversion efficiency of 10.5% was obtained.For the fabrication of integrated type a-Si solar cell modules, a laser pattering method was investigated. A thermal analysis of the multilayer structure was done. It was confirmed that selective scribing for a-Si, TCO and metal film is possible by controlling the laser power density. Recently developed a-Si solar power generation systems and a-Si solar cell roofing tiles are also described.     

Effect of signal-modulated optical radiation on the characteristics of a Modfet

The effect of signal-modulated optical radiation on the characteristics of a GaAlAs/GaAs MODFET has been studied analytically. It is found that the offset voltage increases with modulation frequency and the effect of frequency is negligible above 5 MHz. The drain-source current decreases with increase in signal frequency at a constant radiation flux density, doping concentration and drain-source voltage. Studies on sheet concentration and transconductance also show that the signal frequency has a significant effect upto a certain modulation frequency (5 MHz) above which the effect of frequency is insignificant.     

On the thermodynamic limit of photovoltaic energy conversion

An infinite stack ofp—n junctions with smoothly varying bandgap from ∞ to 0 is considered. AnI —V characteristic is derived, which is more correct than the classical exponential characteristic. It is shown that open-circuit operation is a reversible process and leads to the Carnot efficiency, if one defines the efficiency in the way that is usual in the theory of thermodynamic engines. If instead one uses the definition of efficiency usual in photovoltaics, open-circuit mode gives rise to zero efficiency. Then operation at maximum efficiency equals operation at maximum power and is not reversible.     

Reduction of the threshold temperature sensitivity of 1.55 μm InGaAsP lasers by subnanosecond optical excitation

In GaAsP lasers operating at 1.5 to 1.6 m were pumped optically with a pulsed 1.06 m source. The temperature dependence of the pump energy at laser threshold has been measured for temperatures from 170 to 330 K. Pump pulse widths of 300 ns and 150–300 ps were employed, long and short compared to the carrier life-time in the laser material. Over the high-temperature range of 260 to 330 K short pulse excitation gives a considerable reduction of the threshold temperature sensitivity with a characteristic temperatureT ₀ =85 K compared to T ₀ ^h =45 K for long-pulse excitation. This is in qualitative agreement with previous results on electrically excited lasers although the temperature sensitivity of the optically excited lasers is larger. At temperatures between 170 to 260 K no reduction of the temperature sensitivity was observed.     

Fiber optical communications devices

Both low attenuation silica optical fibers with peak transmission in the wavelength regions of 0.85 μm, and 1.05 μm, and improved lasers at both wavelengths are now available. In this review paper, the principal components for emission, modulation and detection are described. The characteristics of both semiconductor lasers, made of GaAs and related compounds, emitting at 0.85 μm or 1.05 μm and high neodymium-content lasers are discussed. For modulation, current modulation of GaAs lasers and external electro-optic modulation are considered. Concerning detection, the realisation of Si photodetectors suitable at 0.85 μm and the new photodetectors at 1.05 μm from Ga_1−x In _x As are reviewed.     

Influence of the broken symmetry of defect state distribution at the a-Si:H/c-Si interface on the performance of hetero-junction solar cells

Taking into account the fact that the distribution of defect states at the interface does not have strictly symmetrical shape, we present a simulation study of a-Si:H(n)/c-Si(p) and a-Si:H(p)/c-Si(n) structures with regard to the defect states at the interface, band offsets and doping concentration of the emitter. The presented results suggest for a-Si:H(n)/c-Si(p) solar cells a strong influence of the introduced broken symmetry between acceptor and donor defect states on the open-circuit voltage, whereas the a-Si:H(p)/c-Si(n) structure benefits from inherent favorable band alignment and remains unaffected.     

NaCl/Ca/Al as an efficient cathode in organic light-emitting devices

An efficient cathode NaCl/Ca/Al used to improve the performance of organic light-emitting devices (OLEDs) was reported. Standard N,N′-bis(1-naphthyl)-N,N′-diphenyl-1,1′ biphenyl 4,4′-dimaine (NPB)/tris-(8-hydroxyquinoline) aluminum (Alq₃) devices with NaCl/Ca/Al cathode showed dramatically enhanced electroluminescent (EL) efficiency. A power efficiency of 4.6 lm/W was obtained for OLEDs with 2 nm of NaCl and 10 nm of Ca, which is much higher than 2.0 lm/W, 3.1 lm/W, 2.1 lm/W and 3.6 lm/W in devices using, respectively, the LiF (1 nm)/Al, LiF (1 nm)/Ca (10 nm)/Al, Ca (10 nm)/Al and NaCl (2 nm)/Al cathodes. The investigation of the electron injection in electron-only devices indicates that the utilization of the NaCl/Ca/Al cathode substantially enhances the electron injection current, which in case of OLEDs leads to the improvement of the brightness and efficiency.     

Design and Fabrication of 1.06 μm Resonant-Cavity Enhanced Reflective Modulator with GaInAs/GaAs Quantum Wells

A resonant-cavity enhanced reflective optical modulator is designed and fabricated, with three groups of three highly strained InGaAs/GaAs quantum wells in the cavity, for low voltage and high contrast ratio operation. The quantum wells are positioned in antinodes of the optical standing wave. The modulator is grown in a single growth step in an molecular beam epitaxy system, using GaAs/AlAs distributed Bragg reflectors as both the top and bottom mirrors. Results show that the reflection device has a modulation extinction of 3 dB at -4.5 V bias.     

Investigation of an efficient YbF3/Al cathode for tris-(8-hydroxyquinoline)aluminum-based small molecular organic light-emitting diodes

Efficient tris-(8-hydroxyquinoline)aluminum (Alq₃)-based organic light-emitting diodes (OLEDs) using YbF₃ as the electron injection layer have been investigated. With an YbF₃ (3.0 nm)/Al cathode, the device with Alq₃ as the emitting layer achieved a better performance than the control device with a LiF (0.5 nm)/Al cathode. The release of the low-work-function metal Yb is responsible for the performance enhancement. From the analysis by atomic force spectroscopy and X-ray photoemission spectroscopy, it is observed that the Alq₃-cathode interface could be well covered by YbF₃ at an optimum thickness of 3.0 nm, which helps to prevent the contact between Alq₃ and Al, and to reduce the destruction of Alq₃ by Al.     

Polarization management for silicon photonic integrated circuits

Polarization management is very important for photonic integrated circuits (PICs) and their applications. Due to geometrical anisotropy and fabrication inaccuracies, the characteristics of the guided transverse‐electrical (TE) and transverse‐magnetic (TM) modes are generally different. Polarization‐dependent dispersion and polarization‐dependent loss are such manifestations in PICs. These issues become more severe in high index contrast structures such as nanophotonic waveguides made of silicon‐on‐insulator (SOI), which has been regarded as a good platform for optical interconnects because of the compatibility with CMOS processing. Recently, polarization division multiplexing (PDM) with coherent detection using silicon photonics has also attracted much attention. This trend further highlights the importance of polarization management in silicon PICs. The authors review their work on polarization management for silicon PICs using the polarization independence and polarization diversity methods. Polarization issues and solutions in PICs made of SOI nanowires and ridge waveguides are discussed.