New self-aligned T-gate InGaP/GaAs field-effect transistors grownby LP-MOCVD

This paper reports on self-aligned T-gate InGaP/GaAs FETs using n ⁺/N⁺/δ(P⁺)/n structures. N⁺ -InGaP/δ(P⁺)-InGaP/n-GaAs forms a planar-doped barrier. The inherent ohmic gate of camel-gate FETs together with a highly selective etch between an InGaP and a GaAs layers offers a self-aligned T-shape gate with a reduced effective length. A fabricated device with a reduced gate dimension of 1.5×100 (0.6×100) μm² obtained from 2×100 (1×100) μm² gate metal exhibits an extrinsic transconductance, unity-current gain frequency, and unity-power gain frequency of 78 (80) mS/mm, 9 (19.5), and 28 (30) GHz, respectively     

An analytic and accurate model for the threshold voltage of short channel MOSFETs in VLSI

Based on the two-dimensional Poisson equation, the surface potential distribution along the surface channel of a MOSFET has been analytically derived by assuming negligible source and drain junction depths and its minimum potential is then used to determine the threshold voltage. The existence of a minimum surface potential point along the channel of a MOSFET under an applied drain bias is consistent with the numerical results of the two-dimensional analysis. The effects of finite source and drain junction depths have been elegantly included by modifying the depletion capacitance under the gate and the resulted threshold voltage model has been compared to the results of the two-dimensional numerical analysis. It has been shown that excellent agreement between these results has been obtained for wide ranges of substrate doping, gate oxide thickness, channel length (< 1 μm), substrate bias, and drain voltage. Moreover, comparisons between the developed model and the existing experimental data have been made and good agreement has been obtained. The major advantages of the developed model are that no iterations and no adjustable fitting parameters are required. Therefore, this simple and accurate threshold voltage model will become a useful design tool for ultra short channel MOSFETs in future VLSI implementation.     

Analysis of the properties of soft morphological filtering usingthreshold decomposition

We present the properties of soft morphological operations and the new definitions of binary soft morphological operations. It is shown that soft morphological filtering an arbitrary signal is equivalent to decomposing the signal into binary signals, filtering each binary signal with a binary soft morphological filter, and then reversing the decomposition. This equivalence allows problems in the analysis and the implementation of soft morphological operations in real time by using only logic gates for binary signals instead of sorting the numbers. The architectures of logic-gate implementation of soft morphological operations are also presented. Furthermore, unlike standard morphological filters, the soft morphological closing and opening are in general not idempotent. We develop the conditions and properties for a new class of idempotent soft morphological filters     

Dithienocarbazole‐Based Ladder‐Type Heptacyclic Arenes with Silicon,Carbon, and Nitrogen Bridges: Synthesis,Molecular Properties,Field‐Effect Transistors,and Photovoltaic Applications

A new class of ladder‐type dithienosilolo‐carbazole ( DTSC ), dithienopyrrolo‐carbazole ( DTPC ), and dithienocyclopenta‐carbazole ( DTCC ) units is developed in which two outer thiophene subunits are covalently fastened to the central 2,7‐carbazole cores by silicon, nitrogen, and carbon bridges, respectively. The heptacyclic multifused monomers are polymerized with the benzothiadiazole ( BT ) acceptor by palladium‐catalyzed cross‐coupling to afford three alternating donor‐acceptor copolymers poly(dithienosilolo‐carbazole‐alt‐benzothiadiazole) ( PDTSCBT) , poly(dithienocyclopenta‐carbazole‐alt‐benzothiadiazole) ( PDTCCBT), and poly(dithienopyrrolo‐carbazole‐alt‐benzothiadiazole) ( PDTPCBT) . The silole units in DTSC possess electron‐accepting ability that lowers the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of PDTSCBT , whereas stronger electron‐donating ability of the pyrrole moiety in DTPC increases the HOMO and LUMO energy levels of PDTPCBT . The optical bandgaps (E_g^opt) deduced from the absorption edges of thin film spectra are in the following order: PDTSCBT (1.83 eV) > PDTCCBT (1.64 eV) > PDTPCBT (1.50 eV). This result indicated that the donor strength of the heptacyclic arenes is in the order: DTPC > DTCC > DTSC . The devices based on PDTSCBT and PDTCCBT exhibited high hole mobilities of 0.073 and 0.110 cm² V^?1 s^?1, respectively, which are among the highest performance from the OFET devices based on the amorphous donor‐acceptor copolymers. The bulk heterojunction photovoltaic device using PDTSCBT as the p‐type material delivered a promising efficiency of 5.2% with an enhanced open circuit voltage, V_oc, of 0.82 V.     

Rear‐Passivated Ultrathin Cu(In,Ga)Se2 Films by Al2O3 Nanostructures Using Glancing Angle Deposition Toward Photovoltaic Devices with Enhanced Efficiency

In this work, for the first time, the addition of aluminum oxide nanostructures (Al₂O₃ NSs) grown by glancing angle deposition (GLAD) is investigated on an ultrathin Cu(In,Ga)Se₂ device (400 nm) fabricated using a sequential process, i.e., post‐selenization of the metallic precursor layer. The most striking observation to emerge from this study is the alleviation of phase separation after adding the Al₂O₃ NSs with improved Se diffusion into the non‐uniformed metallic precursor due to the surface roughness resulting from the Al₂O₃ NSs. In addition, the raised Na concentration at the rear surface can be attributed to the increased diffusion of Na ion facilitated by Al₂O₃ NSs. The coverage and thickness of the Al₂O₃ NSs significantly affects the cell performance because of an increase in shunt resistance associated with the formation of Na₂Se_X and phase separation. The passivation effect attributed to the Al₂O₃ NSs is well studied using the bias‐EQE measurement and J–V characteristics under dark and illuminated conditions. With the optimization of the Al₂O₃ NSs, the remarkable enhancement in the cell performance occurs, exhibiting a power conversion efficiency increase from 2.83% to 5.33%, demonstrating a promising method for improving ultrathin Cu(In,Ga)Se₂ devices, and providing significant opportunities for further applications.     

Characteristics of large‐scale nanohole arrays for thin‐silicon photovoltaics

Nanostructured crystalline silicon is promising for thin‐silicon photovoltaic devices because of reduced material usage and wafer quality constraint. This paper presents the optical and photovoltaic characteristics of silicon nanohole (SiNH) arrays fabricated using polystyrene nanosphere lithography and reactive‐ion etching (RIE) techniques for large‐area processes. A post‐RIE damage removal etching is subsequently introduced to mitigate the surface recombination issues and also suppress the surface reflection due to modifications in the nanohole sidewall profile, resulting in a 19% increase in the power conversion efficiency. We show that the damage removal etching treatment can effectively recover the carrier lifetime and dark current–voltage characteristics of SiNH solar cells to resemble the planar counterpart without RIE damages. Furthermore, the reflectance spectra exhibit broadband and omnidirectional anti‐reflective properties, where an AM1.5 G spectrum‐weighted reflectance achieves 4.7% for SiNH arrays. Finally, a three‐dimensional optical modeling has also been established to investigate the dimension and wafer thickness dependence of light absorption. We conclude that the SiNH arrays reveal great potential for efficient light harvesting in thin‐silicon photovoltaics with a 95% material reduction compared to a typical cell thickness of 200 µm. Copyright © 2012 John Wiley & Sons, Ltd.     

Solution‐Processed High‐Performance Tetrathienothiophene‐Based Small Molecular Blends for Ambipolar Charge Transport

Four soluble dialkylated tetrathienoacene ( TTAR) ‐based small molecular semiconductors featuring the combination of a TTAR central core, π‐conjugated spacers comprising bithiophene ( bT ) or thiophene ( T ), and with/without cyanoacrylate ( CA ) end‐capping moieties are synthesized and characterized. The molecule DbT‐TTAR exhibits a promising hole mobility up to 0.36 cm² V^?1 s^?1 due to the enhanced crystallinity of the microribbon‐like films. Binary blends of the p‐type DbT‐TTAR and the n‐type dicyanomethylene substituted dithienothiophene‐quinoid ( DTTQ‐11 ) are investigated in terms of film morphology, microstructure, and organic field‐effect transistor (OFET) performance. The data indicate that as the DbT‐TTAR content in the blend film increases, the charge transport characteristics vary from unipolar (electron‐only) to ambipolar and then back to unipolar (hole‐only). With a 1:1 weight ratio of DbT‐TTAR DTTQ‐11 in the blend, well‐defined pathways for both charge carriers are achieved and resulted in ambipolar transport with high hole and electron mobilities of 0.83 and 0.37 cm² V^?1 s^?1, respectively. This study provides a viable way for tuning microstructure and charge carrier transport in small molecules and their blends to achieve high‐performance solution‐processable OFETs.     

Scheduled WiFi using distributed contention in WLANs: algorithms,experiments, and case-studies

The ubiquitous adoption of WiFi introduces large diversity in types of application requirements and topological characteristics. Consequently, considerable attention is being devoted to making WiFi networks controllable without compromising their scalability. However, the main MAC protocol of WiFi, distributed coordination function (DCF), is a contention-based protocol using random backoff. Thus, operating under DCF, the access of channel is hard to control and nonpredictable. In order to provide controllability of channel access in WiFi, we propose Rhythm, a MAC protocol that achieves scheduled WiFi efficiently using distributed contention. By achieving scheduled WiFi, channel access can be controlled by manipulating the schedule decision. We evaluate the performance of Rhythm through analysis, experiments, and case-studies.     

An improved technique and experimental results for the extractionof electron and hole mobilities in MOS accumulation layers

For the first time, experimental results are presented for electron and hole mobilities in the electron and hole accumulation layers of a MOSFET for a wide range of doping concentrations. Also presented is an improved methodology that has been developed in order to enable more accurate extraction of the accumulation layer mobility. The measured accumulation layer mobility for both electrons and holes is observed to follow a universal behavior at high transverse electric fields, similar to that observed for minority carriers in MOS inversion layers. At low to moderate transverse fields, the effective carrier mobility values are greater than the bulk mobility values for the highest doping levels. This is due to screening by accumulated carriers of the ionized impurity scattering by accumulated carriers, which dominates at higher doping concentrations. For lower doping levels, surface phonon scattering is dominant at low to moderate transverse fields so that the carrier mobility is below the bulk mobility value     

High-power broad-area InGaNAs/GaAs quantum-well lasers in the 1200 nm range

High-power broad-area InGaNAs/GaAs quantum-well (QW) edge-emitting lasers on GaAs substrates in the 1200 nm range are reported. The epitaxial layers of the InGaNAs/GaAs QW laser wafers were grown on n⁺-GaAs substrates by using metal-organic chemical vapor deposition (MOCVD). The thickness of the InGaNAs/GaAs QW layers is 70 Å/1200 Å. The indium content (x) of the In_xGa_1−xN_yAs_1−y QW layers is estimated to be 0.35-0.36, while the nitrogen content (y) is estimated to be 0.006-0.009. More indium content (In) and nitrogen content (N) in the InGaNAs QW layer enables the laser emission up to 1300 nm range. The epitaxial layer quality, however, is limited by the strain in the grown layer. The devices were made with different ridge widths from 5 to 50 μm. A very low threshold current density (J_th) of 80 A/cm² has been obtained for the 50 μm × 500 μm LD. A number of InGaNAs/GaAs epi-wafers were made into broad-area LDs. A maximum output power of 95 mW was measured for the broad-area InGaNAs/GaAs QW LDs. The variations in the output powers of the broad-area LDs are mainly due to strain-induced defects the InGaNAs QW layers.