Research of organic field effect transistors based on semiconducting single-walled carbon nanotubes

The metal-conducting single-walled carbon nanotubes(m-SWNTs)with small diameters(0.7 nm-1.1 nm)are selectively removed from the single-walled carbon nanotubes(SWNTs)by using HNO 3 /H 2 SO 4 mixed solution.Semiconducting single-walled carbon nanotubes(s-SWNTs)can be separated efficiently from the SWNTs with high controllability and purity based on this novel method,and the outcome is characterized by Raman spectrum.Moreover,the organic field effect transistors(OFETs)are fabricated based on the poly(3-hexylthiophene-2,5-diyl)(P3HT),and untreated SWNTs and separated SWNTs(s-SWNTs)are mixed with P3HT,respectively.It could be found that the P3HT/s-SWNT device exhibits a better field effect characteristic compared with the P3HT device.The current on/off ratio is increased by 4 times,the threshold voltage is also increased from-28 V to-22 V,and the mobility is increased from 3×10-3 cm2/Vs to 5×10-3 cm2/Vs.     

Low frequency noise in junction field effect transistors

Low frequency noise in four-terminal JFETs has been measured as a function of substrate (second gate) bias with temperature and drain current as additional variables. Sharp peaks of noise have been observed at some values of gate bias. The mechanism of low frequency noise caused by Schockley-Read-Hall (SRH) centres and the significance of charge capture processes in and near the channel are discussed. The experimental results show that most of the excess low frequency noise is caused by SRH centres situated in the transition region between the channel and the fully depleted region of the gate-channel junctions. In JFETs with gate-widths of 1000 μm or less the noise caused by unit electronic charge fluctuations at invidual SRH centres can be readily observed.     

Design of active phase shifters based on multichannelheterojunction field effect transistors (MCHFET's)

Design criteria of active phase shifters based on GaAs/AlGaAs multichannel (MC) HFET in the frequency range 4-60 GHz are presented. The phase characteristics of MCHFET devices were studied using the computer aided design program TOUCHSTONE. The dependence of transmission phase on various intrinsic elements in the equivalent circuit model as a function of control gate bias was also studied. There are limited gate bias ranges which correspond to the active regions of the two conducting wells for which a quasi-linear continuous phase shift for analog applications was achieved. Continuously varying the gate bias from V_gs=-1.9 V to V_gs=-0.6 V results in a quasilinear phase shift of 10°, 15°, 21°, and 29° at f=12, 20, 30, and 60 GHz, respectively. Similarly, varying the gate bias from V_gs =-0.4 V to V_gs=0.7 V a quasi-linear phase shift of 21°, 26°, 27°, and 23° at f=12, 20, 30, and 60 GHz, respectively, was achieved. The gain variation was less than 3 dB in these bias regions. With digital applications in mind, a maximum differential phase shift of around 50° was obtained by switching the gate bias discretely. The transmission phase of single gate MCHFET mostly depends on variation of gate source capacitance with gate bias rather than on other intrinsic elements. The dependence of phase shift on various geometrical and structural parameters is also presented. To test the practicality of the device, other scattering parameters (e.g., S₁₁, S₂₂, S₁₂) and the noise figure (NF) were finally studied     

Radiation sensitive detector based on field-effect transistors

The possibility of developing radiation detectors based on field-effect transistors (FET) is investigated. Transistor saturation current is chosen as an informative parameter for modeling. Experimental results show that the drain saturation current of the FET with p-n junction as a gate is decreasing after irradiation. In metal-oxide-semiconductor (MOS) FETs during radiation-induced defect formation two effects are competing, therefore the result of radiation influence is highly dependent on the electro-physical properties of transistors before irradiation and on the absorbed radiation dose. It is shown that saturation current increases with absorbed radiation dose for all the transistors with low electron concentration in a channel above certain levels of absorbed radiation. While the opposite effect is observed for high electron concentration in a channel, i.e. the saturation current drops. Obtained dependences of the drain saturation current of FET on the irradiation dose facilitated development of simple detector design for low levels of radiation. The bridge circuit is used in the radiation sensor to minimize the effect of temperature fluctuations. The sensitivity of the detector is enhanced several times with the help of two pairs of FETs with the opposite sign of radiation sensitivity.     

Low frequency noise in SiGe-base heterojunction bipolar transistors and SiGe-channel metal oxide semiconductor field effect transistors

We show that low frequency noise (LFN) in SiGe-base heterojunction bipolar transistors and SiGe-channel pMOSFETs may be made significantly lower than that in their all-Si counterparts and indicate how this can be done. Optimization of LFN in SiGe channel pMOSFETs follows from a calculation involving a novel analytical model, which accounts for both static and LFN characteristics of SiGe-channel devices.     

基于CORDIC算法的GPS载波跟踪环鉴相器的设计

提出了一种实现GPS载波跟踪环鉴相器的方法,该方法采用CORDIC算法来实现用于鉴相的arctan函数.同时,给出了这种基于CORDIC算法的硬件实现的结构和相应的仿真结果.这种算法结构简单,只需要采用加法和移位操作即可,非常易于硬件实现,并且其仿真结果可以达到GPS的要求.     

A simple precharged CMOS phase frequency detector

We propose a simple precharged CMOS phase frequency detector (PFD). The circuit uses 18 transistors and has a simple topology. Therefore, the detector, in a 0.8-μm CMOS process, works up to clock frequencies of 800 MHz according to SPICE simulations on extracted layout. Further, the detector has no dead-zone in the phase characteristic which is important in low jitter applications. The phase and frequency characteristics are presented and comparisons are made to other PFDs. The phase offset of the detector is sensitive to differences of the duty-cycle between the inputs. Mixed-mode simulations are presented of the lock-in procedure for a phase-locked loop (PLL) where the detector is used. Measurements on the detector are presented for a test-chip with a delay-locked loop (DLL) where the phase detection ability of the detector has been verified     

Noise response of tri-state phase frequency detector

An analytical expression describing the characteristics of a tri-state phase frequency detector is presented. Using this relation, uniform phase jitter and additive noise responses of the said system are analytically studied and the results are compared with simulated responses     

基于CPLD的低功耗爆炸场温度测试系统

为了测量爆炸场等恶劣环境下温度的动态变化,分析炸药或相关弹药的爆炸参数,设计了基于CPLD的低功耗温度存储式测试系统;运用钨铼热电偶温度传感器匹配先进的电源管理模块,并结合动态存储测试技术,能够应用于环境条件比较差的恶劣环境中,在可靠可信、微功耗的基础上能得到较好的实验数据。     

Solution processable quinoxaline based molecular materials for organic field effect transistors

Three new solution processable quinoxaline based donor–acceptor–donor (D–A–D) type molecules have been synthesized for application in field effect transistors. These molecules were characterized by UV–visible spectroscopy, thermal gravimetric analysis, differential scanning calorimetry and cyclic voltammetry. DFT calculation gives deeper insight into the electronic structure of these molecules. The crystallinity and morphology features of thin film were investigated using X-ray diffraction. These molecules show liquid crystalline phase confirmed by DSC and optical polarizing microscopy. Investigation of their field effect transistor performance indicated that these molecules exhibited p-type mobility up to 9.7 × 10^?4 cm² V^?1 s^?1 and on/off ratio of 10⁴.     

A process for high yield and high performance carbon nanotube field effect transistors

Carbon nanotube field effect transistors (CNTFETs) have been considered as one of the potential candidates for nanoelectronics beyond Si CMOS. However, it is not easy to have high performance CNTFETs with high yield currently. In this work, we proposed a local bottom-gate (LBG) CNTFETs combined with a novel device concept and optimized process technologies. High performance of CNTFET with low subthreshold swing of 139 mV/dec, high transconductance of 1.27 μS, and high I_on/I_off ratio of 10⁶ can be easily obtained with Ti source/drain contact after a post annealing process. Record high yield of 74% has been demonstrated. On the basis of the proposed process, lots of high performance CNTFETs can be obtained easily for advanced study on the electrical characteristics of CNTFETs in the future.     

Low energy and area efficient quaternary multiplier with carbon nanotube field effect transistors

In this study, new multiplier and adder method designs with multiplexers are proposed. The designs are based on quaternary logic and a carbon nanotube field-effect transistor (CNTFET). The design utilizes 4 × 4 multiplier blocks. Applying specific rotational functions and unary operators to the quaternary logic reduced the power delay produced (PDP) circuit by 54% and 17.5% in the CNTFETs used in the adder block and by 98.4% and 43.62% in the transistors in the multiplier block, respectively. The proposed 4 × 4 multiplier also reduced the occupied area by 66.05% and increased the speed circuit by 55.59%. The proposed designs are simulated using HSPICE software and 32 nm technology in the Stanford Compact SPICE model for CNTFETs. The simulated results display a significant improvement in the fabrication, average power consumption, speed, and PDP compared to the current best-performing techniques in the literature. The proposed operators and circuits are evaluated under various operating conditions, and the results demonstrate the stability of the proposed circuits.     

Epitaxially-grown GaN junction field effect transistors

Junction field effect transistors (JFETs) are fabricated on a GaN epitaxial structure grown by metal organic chemical vapor deposition (MOCVD). The dc and microwave characteristics of the device are presented. A junction breakdown voltage of 56 V is obtained corresponding to the theoretical limit of the breakdown field in GaN for the doping levels used. A maximum extrinsic transconductance (g_m ) of 48 mS/mm and a maximum source-drain current of 270 mA/mm are achieved on a 0.8 μm gate JFET device at V_GS=1 V and V_DS=15 V. The intrinsic transconductance, calculated from the measured g_m and the source series resistance, is 81 mS/mm. The f_T and f_max for these devices are 6 GHz and 12 GHz, respectively. These JFET's exhibit a significant current reduction after a high drain bias is applied, which is attributed to a partially depleted channel caused by trapped hot-electrons in the semi-insulating GaN buffer layer. A theoretical model describing the current collapse is presented, and an estimate for the length of the trapped electron region is given     

Theory of organic field effect transistors

Field effect transistors with an organic material as active layer are at present essentially used to determine the mobilities in these materials. Until now, in analysing the measured current characteristics, only the simplest (Shockley) model has been used which accounts neither for this type of thin film transistor (TFT), which operates in depletion and accumulation, nor for the nature of the carriers. Starting from two-dimensional simulations for the analogous silicon TFT, we have developed an analytical model for the TFT that accounts for several peculiarities of the current characteristics of this type of transistor. In addition, a first modification has been developed which describes the situation when the charged states are polarons and bipolarons, as is the case in organic materials. Applications to published experimental current characteristics show that a general reanalysis is needed. Copyright © 1998 John Wiley & Sons, Ltd.     

基于AlGaN/GaN场效应晶体管的太赫兹焦平面成像传感器

太赫兹波成像技术在人体安检、医学成像、无损检测等领域具有广泛的应用前景。文中面向高速、高灵敏度和便携式太赫兹成像应用需求,设计实现了一种基于AlGaN/GaN高电子迁移率晶体管自混频检测机制的太赫兹焦平面成像传感器。该焦平面成像传感器由探测器阵列芯片和CMOS读出电路通过倒装互连实现,阵列规模达到3232。探测器阵列中具有对管差分功能的像元设计通过提高探测器的电压响应度和抑制共模电压噪声,提高了焦平面成像的灵敏度。焦平面成像传感器的输出模拟信号通过片外的模数转换（ADC）芯片转化为数字信号,由现场可编程门阵列（FPGA）采集后通过Camera Link图像数据与通信接口发送到计算机。利用该焦平面成像传感器,演示实现了太赫兹光斑、太赫兹干涉环和太赫兹光照下的旋转塑料叶片的视频成像,帧频达到30 Hz。     

Low-power all-optical gate based on sum frequency mixing in APEwaveguides in PPLN

We present an all-optical gate implemented in periodically poled lithium niobate (PPLN) (LiNbO₃). Efficient mixing is achieved by using a phase-matched guided-wave interaction. A control wave at 1.537 μm is used to gate a signal at 1.552 μm, where a control power of 185 mW is sufficient to achieve 96% depletion of a low-power signal. A simple switch configuration is described whereby high-contrast low-power all-optical switching can be performed     

Memory modes of ferroelectric field effect transistors

Effects of traps on the memory characteristics of ferroelectric field effect transistors with the metat-ferroelectric-insulator-semiconductor structure were theoretically analyzed. Various modes of operation, i.e. the polarization-limited mode, trap-limited mode, critical field-limited mode, MNOS mode and probability-limited mode, were derived depending on the device parameters. Experimental devices with gate insulator BaTiO₃ or PLZT were fabricated by using thin film SnO₂ and thin film Te. The memory characteristics of these devices were interpreted in terms of the trap-limited mode and MNOS mode, respectively.     

Spin cast self-assembled monolayer field effect transistors

Top-contact self-assembled monolayer field-effect transistors (SAMFETs) were fabricated through both spin-coating and solution assembly of a semiconducting phosphonic acid-based molecule (11-(5?-butyl-[2,2′;5′,2″;5″,2?;5?,2?]quinquethiophen-5-yl)undecylphosphonic acid) (BQT-PA). The field-effect mobilities of both spin-cast and solution assembled SAMFETs were 1.1-8.0 × 10⁻⁶ cm² V⁻¹ s⁻¹ for a wide range of channel lengths (between 12 and 80 μm). The molecular monolayers were characterized by atomic force microscopy (AFM), attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. It was found that the BQT-PA monolayer films exhibit dense surface coverage, bidentate binding, and tilt angles of ∼32° and ∼44° for the thiophene rings and alkyl chain, respectively. These results indicate that rapid throughput of fabricating SAMFETs is possible even by spin-coating.     

Design calculations for MOS field effect transistors

Design equations for the Metal-Oxide-Semiconductor Field Effect Transistor are developed. Approximate solutions for static characteristics, transconductance, and frequency cutoff are presented for the case of a very high resistivity substrate. Specific sets of static characteristics from computer calculations are presented graphically to illustrate the effects of oxide thickness and various substrate resistivities.     

Optimum semiconductors for power field effect transistors

The optimization of semiconductor material properties for high voltage field effect transistors is discussed. The on-resistance of these devices is shown to be inversely proportional to the carrier mobility and inversely proportional to the cube of the energy band gap. Based upon this, the on-resistance of GaAs FETs is predicted to be at least twelve times smaller than that of present silicon FETs. Comparison of the projected GaAs FET power switching performance with competing silicon devices (MOSFETs, FCTs, GTOs, and bipolar transistors) indicates that the GaAs FET will have better switching efficiency at all operating frequencies for devices designed with breakdown voltages ranging from 200 to 1000 volts.