High-quality metamorphic HEMT grown on GaAs substrates by MBE

Metamorphic high electron mobility transistor (M-HEMT) structures have been grown on GaAs substrates by molecular beam epitaxy (MBE). Linearly graded and the step-graded InGaAs and InAlAs buffet layers hal e been compared, and TEM, PL and low-temperature Hall have been used to analyze the properties of the buffer layers and the M-HEMT structure. For a single-delta-doped M-HEMT structure with an In0.53Ga0.47As channel layer and a 0.8 mum step-graded InAlAs buffer layer, room-temperature mobility of 9000 cm(2)/V s and a sheet electron density as high as 3.6 x 10(12)/cm(2) are obtained. These results are nearly equivalent to those obtained for the same structure grown on an InP substrate. A basic M-HEMT device with 1 mum gate was fabricated, and g(m) is larger than 400 mS/mm. (C) 2001 Elsevier Science B.V. All rights reserved.

Observation of N-Shaped Negative Differential Resistance in GaAs-Based Modulation-Doped Field Effect Transistor with InAs Quantum Dots

N-shaped negative differential resistance (NDR) with a high peak-to-valley ratio (PVR) is observed in a GaAs-based modulation-doped field effect transistor (MODFET) with InAs quantum dots (QDs) in the barrier layer (QDFET) compared with a GaAs MODFET. The NDR is explained as the real-space transfer (RST) of high-mobility electrons in a channel into nearby barrier layers with low mobility, and the PVR is enhanced dramatically upon inserting the QD layer. It is also revealed that the QD layer traps holes and acts as a positively charged nano-floating gate after a brief optical illumination, while it acts as a negatively charged nano-floating gate and depletes the adjacent channel when charged by the electrons. The NDR suggests a promising application in memory or high-speed logic devices for the QDFET structure.

Experimental Study on the Subthreshold Swing of Silicon Nanowire Transistors

SOI based wrap-gate silicon nanowire FETs are fabricated through electron beam lithography and wet etching. Dry thermal oxidation is used to further reduce the patterned fins cross section and transfer them into nanowires. Silicon nanowire FETs with different nanowire widths varying from 60 nm to 200 nm are fabricated and the number of the nanowires contained in a channel is also varied. The on-current (I-ON) and off-current (I-OFF) of the fabricated silicon nanowire FET are 0.59 mu A and 0.19 nA respectively. The subthreshold swing (SS) and the drain induced barrier lowering are 580 mV/dec and 149 mVN respectively due to the 30 nm thick gate oxide and 1015 cm(-3) lightly doped silicon nanowire channel. The nanowire width dependence of SS is shown and attributed to the fact that the side-gate parts of a wrap gate play a more effectual role as the nanowires in a channel get narrower. It seems the nanowire number in a channel has no effect on SS because the side-gate parts fill in the space between two adjacent nanowires.

Microelements in Potato and Lily Samples Studied by Laser-Induced Breakdown spectroscopy Technology

A LIBS setup was built in the Institute of Modern Physics. In our experiments, LIBS spectra produced by infrared radiation of Nd : YAG nanosecond laser with 100 and 150 mJ pulse energy, respectively, were measured by fiber optic spectrometer in the ranges of 230-430 run and 430-1080 nm with a delay time of 1.7 and gate width of 2 ms for potato and lily samples prepared by vacuum freeze-dried technique. The lines from different metal elements such as K, Ca, Na, Mg, Fe, Al, Mn and Ti, and nonmetal elements such as C, N, O and H, and some molecular spectra from C-2, CaO, and CN were identified according to their wavelengths. The relative content of the six microelements, Ca, Na, K, Fe, Al, and Mg in the samples were analyzed according to their representative line intensities. By comparison we found that there are higher relative content of Ca and Na in lily samples and higher relative content of Mg in potato samples. The experimental results showed that LIBS technique is a fast and effective means for measuring and comparing the contents of microelements in plant samples.

多路信号处理与QDC门信号产生逻辑插件的研制

本论文描述了多路信号处理与QDC（电荷数字转换器）门信号产生逻辑插件的研制过程。该逻辑插件的逻辑电路和模拟电路都经过了精心的设计和调试。核物理研究沿着激发能、核自旋及核同位旋三个自由度深入开展，产生了核物理研究中许多新的前沿领域。随着入射能量的增加，开放的反应道增多，多重探测器投入使用，使核物理实验的数据参数已经从几十路上升到上千路，即需要电子学工作者研制和开发一些高速触发、高密度、高稳定性的电子学硬件系统。本论文描述的多路信号处理与QDC门信号产生逻辑插件，主要用于产生QDC的开门信号。该电路系统早十六路并行、独立的高速放大单元电路、高速比较单元电路、延迟和脉宽成形单元电路、逻辑信号扇出单元电路、触发指示单元电路及多路模拟信号求和单元电路构成。该逻辑插件可接收≥ 20mv的小辐度输入信号而产生32路标准ECL（高电平-0.8V，低电平-1.6V）互补输出逻辑信号。输出ECL逻辑信号用于产生QDC的门控信号。该插件的多路模拟信号求和单元电路对每路输入信号可产生幅度为50mv的输出信号。该输出信号可用于多重符衙事件的鉴别和产生trigger触发信号。总的来说，多路辐度信号处理与QDC门信号产生逻辑插件不仅适用于重离子核物理实验中产生QDC开门信号，也可用于其它核物理实验的多重符合事件的鉴别。该插件的研制成功为多参数获取系统提供了一个良发的前端硬件设备。

Fabrication of an integrated pixel with an organic light-emitting diode driven by copper phthalocyanine organic thin film transistors

An organic integrated pixel with organic light-emitting diodes (OLEDs) driven by organic thin film transistors (OTFTs) is fabricated by a greatly simplified processing. The OTFTs are based on copper phthalocyanine as the active medium and fabricated on indium-tin-oxide (ITO) glass with top-gate structure, thus an organic integrated pixel is easily made by integrating OLED with OTFT. The OTFTs show field-effect mobility of 0.4 cm(2) /Vs and on/off ratio of 10(3) order. The OLED is driven well and emits the brightness as large as 2100cd/m(2) at a current density of 14.6 mu A/cm(2) at -19.7 V gate voltage. This simple device structure is promising in the future large-area flexible OLED displays.

Very low hysteresis organic thin-film transistors

Very low hysteresis vanadyl-phthalocyanine/para-sexiphenyl thin-film transistors (TFTs) have been fabricated using benzocyclobutenone (BCBO) derivatives/tantalum pentoxide (Ta2O5)/BCBO triple gate dielectrics. The field effect mobility, on/off current ratio and threshold voltage of organic TFTs are 0.45 cm(2) V-1 s(-1), 3.5 x 10(4) and -6.8 V, respectively. To clarify the mechanism of hysteresis, devices with different dielectrics have been studied. It is found that the bottom BCBO derivatives (contact with a gate electrode) block the electron injection from a gate electrode to dielectrics.

Potassium-Lead-Switched G-Quadruplexes: A New Class of DNA Logic Gates

A cation-driven allosteric G-quadruplex DNAzyme (PW17) was utilized to devise a conceptually new class of DNA logic gate based on cation-tuned ligand binding and release. K+ favors the binding of hemin to parallel-stranded PW17, thereby promoting the DNAzyme activity, whereas Pb2+ induces PW17 to undergo a parallel-to-antiparallel conformation transition and thus drives hemin to release from the G-quadruplex, deactivating the DNAzyme. Such a K+-Pb2+ switched G-quadruplex, in fact, functions as a two-input INHIBIT logic gate. With the introduction of another input EDTA, this G-quadruplex can be further utilized to construct a reversibly operated IMPLICATION gate.

Bilayer Photoresist Insulator for High Performance Organic Thin-Film Transistors on Plastic Films

A novel bilayer photoresist insulator is applied in flexible vanadyl-phthalocyanine (VOPc) organic thin-film transistors (OTFTs). The micron-size patterns of this photoresisit insulator can be directly defined only by photolithography without the etching process. Furthermore, these OTFTs exhibit high field-effect mobility (about 0.8 cm(2)/Vs) and current on/off ratio (about 10(6)). In particular, they show rather low hysteresis (< 1 V). The results demonstrate that this bilayer photoresist insulator can be applied in large-area electronics and in the facilitation of patterning insulators.

Electrical properties in vanadyl-phthalocyanine-based metal-insulator-semiconductor devices

We investigated electrical properties of vanadyl phthalocyanine (VOPc) metal-insulator-semiconductor (MIS) devices by the measurement of capacitance and conductance, which were fabricated on ordered para-sexiphenyl (p-6P) layer by weak epitaxy growth method. The VOPc/p-6P MIS diodes showed a negligible hysteresis effect at a gate voltage of +/- 20 V and small hysteresis effect at a gate voltage of +/- 40 V due to the low interface trap state density of about 1x10(10) eV(-1) cm(-2). Furthermore, a high transition frequency of about 10 kHz was also observed under their accumulation mode. The results indicated that VOPc was a promising material and was suitable to be applied in active matrix liquid crystal displays and organic logic circuits.

Study on the instability of organic field-effect transistors based on fluorinated copper phthalocyanine

The effects of positive and negative gate-bias stress on organic field-effect transistors (OFET) based on tantalum (Ta)/tantalum pentoxide (Ta2O5)/fluorinated copper phthalocyanine (F16CuPc) structure are investigated as a function of stress time and stress temperature. It is shown that gate-bias stress induces a parallel threshold voltage shift (DeltaV(T)) of OFETs without changes of field-effect mobility mu(EF) and sub-threshold slope (DeltaS). The DeltaV(T) is observed to be logarithmically dependent on time at high gate-bias appropriate to OFET operation. More importantly, the shift is directional, namely, be large shift under positive stress and almost do not move under negative stress. The threshold voltage shift is temperature dependent with activation energy of 0.51 eV We concluded that threshold voltage shift of the OFET with F16CuPc as active layer is due to charge trapping in the insulator in which trapped carriers have redistribution.

Organic integrated pixels fabricated by a full evaporation method

An organic integrated pixel consisting of an organic light-emitting diode driven by an organic thin-film field-effect transistor (OTFT) was fabricated by a full evaporation method oil a transparent glass substrate. The OTFT was designed as a top-gate Structure, and the insulator is composed of a double-layer polymer of Nylon 6 and Teflon to lower the operation voltage and the gate-leakage current, and improve the device stability. The field-effect mobility of the OTFT is more than 0.5 cm(2) V-1 s(-1), and the on/off ratio is larger than 10(3). The brightness of the pixel reached as large as 300 cd m(-2) at a driving current of 50 mu A.

Improved performance by a double-insulator layer in organic thin-flim transistors

Organic thin film transistors based on pentacene are fabricated by the method of full evaporation. The thickness of insulator film can be controlled accurately, which influences the device operation voltage markedly. Compared to the devices with a single-insulator layer, the electric performance of devices by using a double-insulator as the gate dielectric has good improvement. It is found that the gate leakage current can be reduced over one order of magnitude, and the on-state current can be enhanced over one order of magnitude. The devices with double-insulator layer exhibit field-effect mobility as large as 0.14 cm(2)/Vs and near the zero threshold voltage. The results demonstrate that using proper double insulator as the gate dielectrics is an effective method to fabricate OTFTs with high electrical performance.

Bottom-contact organic field-effect transistors having low-dielectric layer under source and drain electrodes

An organic thin-film transistor (OTFT) having a low-dielectric polymer layer between gate insulator and source/drain electrodes is investigated. Copper phthalocyanine (CuPc), a well-known organic semiconductor, is used as an active layer to test performance of the device. Compared with bottom-contact devices, leakage current is reduced by roughly one order of magnitude, and on-state current is enhanced by almost one order of magnitude. The performance of the device is almost the same as that of a top-contact device. The low-dielectric polymer may play two roles to improve OTFT performance. One is that this structure influences electric-field distribution between source/drain electrodes and semiconductor and enhances charge injection. The other is that the polymer influences growth behavior of CuPc thin films and enhances physical connection between source/drain electrodes and semiconductor channel. Advantages of the OTFT having bottom-contact structure make it useful for integrated plastic electronic devices.

Ion channel sensor

The ion channel sensor is reviewed. The concept and sensing principle of this kind of sensor are briefly discussed. The fabrication of the sensing membrane and the application of the ion channel sensor in electroanalytical chemistry are evaluated. The future developing direction is also anticipated.