Low-frequency noise in heterojunction FETs with low-temperaturebuffer

Low-frequency noise was measured on heterojunction FETs grown on low-temperature buffer and superlattice buffer layers. A distinct generation-recombination noise source with an activation energy of 0.70 eV was observed in devices built on a low-temperature buffer. Both structures showed a trap with an activation energy (~0.50 eV) that has a gate-to-source voltage dependence and is believed to be due to interactions between electrons in the channel and traps in the AlGaAs supply layer. Computer simulations of the position of the energy levels as a function of the gate voltage support this idea. A trap due to DX centers with an activation energy of ~0.30 eV in AlGaAs was also observed in both devices     

A novel model of the gate current in heterojunction FETs

The authors present a model of the gate current in heterojunction FETs that takes into account two-dimensional electron gas effects at the heterojunction interface. The gate current results from tunnel and thermionic contributions. This model takes into account a number of technological parameters such as heterojunction barrier height, threshold voltage, gate length, and temperature. It has been tested against experimental measurements of gate current in AlGaAs/GaAs MISFETs at various temperatures. The agreement has been found quite satisfactory in a large range of temperatures     

Application of fan-beam antennas for 60 GHz indoor wirelesscommunication

The application of fan-beam antennas in high-speed indoor wireless communication systems operating in the 60 GHz band is investigated. The effects of line-of-sight obstruction as well as antenna pointing deviation on the power link budget are experimentally studied in a typical laboratory environment. The results are contrasted with those obtained with alternative antenna configurations     

DC-to-RF dispersion effects in GaAs- and GaN-based heterostructure FETs: performance and reliability issues

The performance and reliability implications of DC-to-RF dispersion effects are addressed. The proposed physical explanations and technological counteractions are reviewed. GaAs- and GaN-based FET technologies are considered, trying to point out both similar and peculiar aspects.     

Design and fabrication of double modulation dopedInAlAs/lnGaAs/InAs heterojunction FETs for high-speed andmillimeter-wave applications

We report the design, fabrication, and characterization of InP-based double-sided-doped (DSD) MODFETs with InAs-layer-inserted channels. Devices based on optimized structures show a significant improvement in the effective saturation velocity, from 2.4×10⁷ cm/s for lattice-matched MODFETs to 3.1×10⁷ cm/s for InAs MODFETs. This leads to a maximum extrinsic transconductance of 1.95 S/mm and excellent high-speed performance of f_T=265 GHz for 0.13-μm T-gates. A f_max higher than 300 GHz can be achieved by fabricating a wide lateral recess groove, which simultaneously results in an improved breakdown voltage of 6.7 V. The excellent RF performance is primarily due to the reduction of Coulomb scattering from donor layers, especially under the channel, and to the reduction of scattering caused by the interface roughness. This improvement is achieved by inserting a 4-nm InAs layer, which better confines the two-dimensional electron gas (2DEG) at the center of the channel of MODFET's     

30-GHz-band over 5-W power performance of short-channel AlGaN/GaN heterojunction FETs

This paper describes the small-signal characterization through delay-time analysis and high-power operation of the Ka-band of AlGaN/GaN heterojunction field-effect transistors (FETs). An FET with a gatewidth of 100 /spl mu/m and a gate length of 0.09 /spl mu/m has exhibited a current gain cutoff frequency (f/sub T/) of 81 GHz, a maximum frequency of oscillation (fmax) of 187 GHz, and a maximum stable gain of 10.5 dB at 30 GHz (8.3 dB at 60 GHz). Delay-time analysis has demonstrated channel electron velocities of 1.50/spl times/10/sup 7/ to 1.75/spl times/10/sup 7/ cm/s in a gate-length range of 0.09-0.25 /spl mu/m. State-of-the-art performance-saturated power of 5.8 W with a linear gain of 9.2 dB and a power-added efficiency of 43.2%-has been achieved at 30 GHz using a single chip having a gatewidth of 1.0 mm and a gate length of 0.25 /spl mu/m.     

Imaging diversity receivers for high-speed infrared wirelesscommunication

We discuss two modifications to the design of wireless infrared links that can yield significant performance improvements, albeit at the price of increased complexity. In line-of-sight and non-line-of-sight links, replacement of a single-element receiver by one employing an imaging light concentrator and a segmented photodetector can reduce received ambient light noise and multipath distortion. For a fixed receiver entrance area, such an imaging receiver can reduce transmit power requirements by as much as about 14 dB, depending on the link design and the number of photodetector segments. Imaging receivers also reduce co-channel interference, and may therefore enable infrared wireless networks to employ space-division multiplexing, wherein several transmitters located in close proximity can transmit simultaneously at the same wavelength. In nondirected non-line-of-sight links, replacement of the diffuse transmitter by one that projects multiple narrow beams can reduce the path loss, further reducing the transmit power requirement by several decibels. We describe the design of an experimental 100 Mb/s infrared wireless link employing a multibeam transmitter and a 37-pixel imaging receiver     

Multipriority video transmission for third-generation wirelesscommunication systems

This paper presents a robust dual-priority video partitioning method suitable for twin-class unequal protected video transmission over wireless channels. The partitioning is based on a separation of the variable-length (VL) coded discrete cosine transform (DCT) coefficients within each block. The scheme is suitable for constant bit-rate transmission (CBR), where the fraction of bits assigned to each of the two partitions can be adjusted according to the requirements of the unequal error-protection scheme employed. The distribution of the VL-coded (VLC) information between the two partitions is performed adaptively. Subsequently, the partitioning method was applied to the ITU-T H.263 coding standard. It was shown that, for the input video with quarter common intermediate format (QCIF) spatial resolution (or less), the partitioning overhead can be embedded in the B-bit code word representing the group number (GN), thus avoiding transmission of additional bits. As a transport vehicle, we have considered one of the leading third generation cellular radio standards known as wide-band code division multiple access (W-CDMA). A dual-priority transmission system is then invoked on the W-CDMA system where the video data, after being broken into two streams, are unequally protected. We use a very simple error-correction coding scheme for illustration and then propose more sophisticated forms of unequal protection of the digitized video signals. We show that this strategy results in a significantly higher quality of the reconstructed video data when they are transmitted over time-varying multipath fading channels     

Spontaneous and piezoelectric polarization effects on the outputcharacteristics of AlGaN/GaN heterojunction modulation doped FETs

We report on the calculation of electrical characteristics of AlGaN/GaN heterojunction field effect transistors (HFETs). The model is based on the self-consistent solution of the Schrodinger and Poisson equations coupled to a quasi-2D model for the current flow. Both single and double heterojunction devices are analyzed for [0001] or [000-1] growth directions. The onset of a parasitic p-channel for particular growth directions and alloy concentrations is also shown     

MISO CDMA transmission with simplified receiver for wirelesscommunication handsets

The next-generation wireless personal and mobile communication systems are expected to accommodate not only high-quality voice services, but also a broad range of other multirate services. Of the various multiaccess techniques, wide-band code-division multiple access (CDMA) has been regarded as an important part of the third-generation wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multirate services. In this paper, we consider a system with a simplified receiver structure for direct-sequence CDMA (DS/CDMA) wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. We arrive at this system by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. We find that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simple one-finger matched filter is used at the receiver. This system combines the advantages of pre-RAKE diversity and transmit antenna diversity. It is shown that significant system performance and capacity improvements are possible. The numerical results also reveal that this system is not too sensitive to channel estimation errors     

Adaptive equalization and interference cancellation for wirelesscommunication systems

An adaptive equalization and interference cancellation method is proposed. The proposed scheme can cancel both intersymbol interference and cochannel interference, and is blind in the sense that no knowledge of the training sequences of the interfering users is required. In particular, it is a maximum likelihood sequence estimation (MLSE) equalizer that is implemented by the generalized Viterbi algorithm (GVA) with an RLS-based channel estimator. To demonstrate the potential of the proposed method, various simulation results over a frequency selective Rayleigh fading environment in the presence of cochannel interference are presented. In addition, a sequential algorithm is introduced to reduce the computational complexity of GVA     

湿法化学腐蚀在GaN基材料中的应用

文中计算了AlGaN材料在不同温度KOH水溶液中的湿法腐蚀速率,并研究了湿法化学腐蚀GaN基材料对消除干法刻蚀所引入损伤的作用.为了对比湿法化学腐蚀消除干法刻蚀损伤的效果,分别利用扫描电子显微镜(SEM)、原子力显微镜(AFM)和俄歇电子能谱(AES)对比Ar 干法刻蚀后经湿法化学腐蚀处理和未经处理的表面形貌及组分,并制作了单元可见盲器件,测试其反向漏电流,发现在干法刻蚀后引入湿法化学腐蚀工艺可使器件的反向漏电流得到较大程度的减小.     

湿法化学腐蚀在GaN基材料中的应用

文中计算了AlGaN材料在不同温度KOH水溶液中的湿法腐蚀速率，并研究了湿法化学腐蚀GaN基材料对消除干法刻蚀所引入损伤的作用。为了对比湿法化学腐蚀消除干法刻蚀损伤的效果，分别利用扫描电子显微镜（SEM）、原子力显微镜（AFM）和俄歇电子能谱（AES）对比Ar^＋干法刻蚀后经湿法化学腐蚀处理和未经处理的表面形貌及组分，并制作了单元可见盲器件，测试其反向漏电流，发现在干法刻蚀后引入湿法化学腐蚀工艺可使器件的反向漏电流得到较大程度的减小。     

Application of heterojunction FET to power amplifier for cellulartelephone

The high power properties of heterojunction FETs (H-FET have been investigated. The H-FETs are fabricated by using a strained AlGaAs/GaInAs/GaAs/AlGaAs selectively-doped double heterojunction structure. As compared with GaAs MESFETs, the H-FETs show 1.5 dB higher saturation power and 8% higher power-added efficiency than those of the MESFETs, at 950 MHz and 4.7 V. The H-FETs are more suitable for the power amplifier of cellular telephones     

Comparison of low-temperature electrical characteristics of gate-all-around nanowire FETs, Fin FETs and fully-depleted SOI FETs

Low-temperature electrical characteristics of n-type gate-all-around vertically-stacked silicon nanowire (SNW) field-effect transistors (FETs) with high-k/metal gate have been investigated and are compared to those with Fin and fully-depleted silicon-on-insulator (FD SOI) FETs. In particular, the effective electron mobilities behaviors are discussed. Nanowires with a rectangular cross section of 15 nm in width and 19 nm in height have shown a strongly degraded mobility as compared to those with Fin and FD SOI FETs. Low-temperature measurements have revealed that the mobility degradation is due to higher surface-roughness limited mobility. On the other hand, no significant difference in the interface trap densities among the kinds of FETs measured in the study have been observed from the temperature dependence in the subthreshold slope.     

Performance of BLAST over frequency-selective wirelesscommunication channels

Currently one of the most promising techniques for realizing high spectral efficiencies over wireless links is Vertical-Bell Laboratories Layered Space-Time (V-BLAST). This technique employs multi-element antenna arrays at both the transmitter and receiver to increase the spectral efficiency. In contrast to previous work, we consider the performance of V-BLAST in a frequency-selective fading channel. In particular, we investigate the effect of delay spread on V-BLAST with various QAM modulation formats and different numbers of transmit and receive antennas for two types of delay spread distributions. Comparisons with a flat-fading channel are also provided     

Composite performance and availability analysis of wirelesscommunication networks

With the increasing popularity of wireless communication systems, customers are expecting the same level of service, availability, and performance from the wireless communication networks as the traditional wire-line networks. The traditional pure performance model that ignores failure and recovery but considers resource contention generally overestimates the system's ability to perform a certain job. On the other hand, pure availability analysis tends to be too conservative since performance considerations are not taken into account. To obtain realistic composite performance and availability measures, one should consider performance changes that are associated with failure recovery behavior. A review is first given over the advances in composite performance and availability analysis. Thereafter, three techniques for composite performance and availability analysis are discussed in detail through a queueing system in a wireless communication network. Numerical results show that an approximate model based on a framework originally proposed by Bobbie and Trivedi (1990) provides remarkably accurate predictions on system performance     

Variations on optimal and suboptimal handoff control for wirelesscommunication systems

The design of handoff algorithms for cellular communication systems based on signal-strength measurements is addressed. The system is modeled using a hybrid framework: a mixture of continuous state and discrete event systems. The handoff problem is formulated as an optimization problem to control the switchings within the discrete event system. Performance is evaluated as a function of the expected number of handoffs, the expected handoff delay, and the expected number of signal degradations. A signal degradation occurs when the signal level falls below a threshold. The cost of handoff delay is explicitly specified, in contrast to prior work. Various optimization problems are posed to trade off between these quantities. Based on the optimal solutions which are obtained through dynamic programming, suboptimal versions are proposed for ease of implementation. The performance of the suboptimal algorithm which trades off between the expected number of handoffs and the expected number of signal degradations is improved through the use of signal averaging; however, this algorithm suffers from excessive handoff delay. Therefore, the tradeoff between handoff delay and number of handoffs is considered. The corresponding suboptimal algorithm provides nearly one handoff and almost no delay, which is ideal if call quality is also good. Finally, an algorithm which is a combination of the two previous algorithms is explored     

Ku波段振荡用GaAs场效应晶体管

本文讨论GsAs FET振荡应用时的工作原理、设计考虑。Ku波段振荡用FET在15GHz下获得60mW输出功率,在18GHz下获得40mW输出功率。