Observation of multiple high-field domains in a dielectric-surface-loaded GaAs bulk element

By observing potential distributions and corresponding current waveforms of a partially dielectric-surface-loaded GaAs bulk element, it is found that a domain nucleated at the unloaded cathode region fades away under the loaded region, nucleating another new domain at the cathode. This fading decreases with time after the application of a bias voltage, and eventually several domains of the same size can coexist, keeping a constant separation between them.     

Observation of current instabilities in a dielectric-surface-loaded ntype GaAs bulk element

Experiments were performed to investigate the effect of dielectric surface loading on GaAs bulk element. Current instabilities through the element were observed with a sampling oscilloscope while a dielectric sheet moved along a GaAs element. It was found that travelling-domain-mode current oscillations are suppressed and disappear only when the whole length of GaAs surface is covered with the dielectric sheet.     

Simple analysis and computer simulation on lateral spreading of space charge in bulk GaAs

The growth of a space charge in bulk GaAs with a uniform doping profile but with a highly and three-dimensionally non-uniform electric field has been analyzed by introducing a concept of a "carrier current tube." From this analysis it has been shown how the behavior (i.e., growth or decay) of a small space charge depends upon the magnitude of an initial space charge, the nonuniformity of the field, and the dielectric relaxation time and that a space charge grows where the electric field changes along the carrier current even if no initial space charge exists. The "lateral spreading velocity" of a dipole domain has been estimated (its value is the order of 10⁸cm/s) and its dependence upon the magnitude of an initial domain has been made clear. Furthermore, computer simulation have been performed for bulk GaAs devices with two-dimensional structure, such as a planar electrode, a nonuniform doping profile, and an imperfect cathode notch. The process of two-dimensional dipole domain formation in these devices has been made clear.     

Two-dimensional analysis of short-channel delta-doped GaAs MESFETs

Key design parameters for delta-doped GaAs MESFETs, such as delta doping profile, top layer background doping density, and scaling of lateral feature size, are investigated using a two-dimensional numerical simulation. A three-region (delta-doped conducting channel, top layer, and substrate) velocity-field relation is implemented in the model as appropriate for the particular device structure which is simulated. Simulation results show excellent agreement with a fabricated 0.5-μm gate-length delta-doped GaAs MESFETs based on atomic layer epitaxy material. An extrinsic transconductance of 370 mS/mm and a drain-source current of 270 mA/mm are obtained for typical devices, and the maximum transconductance is as high as 400 mS/mm. These are the best DC results yet reported for 0.5-μm gate-length delta-doped GaAs MESFETs. Considerations of design and optimization are discussed in terms of threshold voltage sensitivity, transconductance, current drive capability, and cutoff frequency, based on both simulation and experiment results     

Two-dimensional numerical analysis of stability criteria of GaAs FET's

Stability criteria of GaAs junction-gate FET's are studied by two-dimensional numerical analysis. The analysis covers the wide range of device geometry from the state of the art FET to the so-called Gunn effect digital devices. It is found that a GaAs FET exhibits either of the following three types of characteristics depending upon device geometry and doping concentration. First, for a thin channel with high doping concentration, the device tends to behave as a normal junction-gate FET with saturating current-voltage characteristics. This is even true when the n-l (device length) and n.d (device thickness) products exceed the previously accepted criteria for Gunn oscillation. Second, a stable negative resistance (SNR) is observed in devices with a moderate channel thickness. Third, for a thick channel, the device exhibits a Gunn oscillation with the domain propagating from the gate edge to the drain. These three categories of behavior are mapped on the nd plane with the help of simple analytic considerations. The map is found to compare well with experimental results.     

Computer analysis on the static negative resistance due to the geometrical effect of a GaAs bulk element

Results of computer analysis on the static negative resistance of a GaAs bulk element with a geometrical effect are presented. It is shown that a static negative resistance appears across an overcritically doped GaAs element with a geometry of expanding cross section toward the anode as a result of the development of a stationary high-field domain at the expanding part of the element. These results are in good agreement with the reported experimental results.     

Two-dimensional analysis of surface-state effects on turn-oncharacteristics in GaAs MESFETs

Surface-state effects on gate-lag or slow current transient in GaAs MESFETs are studied by two-dimensional (2-D) simulation. It is shown that the gate-lag becomes remarkable when the deep-acceptor surface state acts as a hole trap. To suppress it, the deep acceptor should be made electron-trap-like, which can be realized by reducing the surface-state density. Device structures expected to have less gate-lag, such as a self-aligned structure with n⁺ source and drain regions and a recessed-gate structure are also analyzed. An analysis of the possible complete elimination of gate-lag in these structures is given     

Two-dimensional analysis of substrate-trap effects on turn-oncharacteristics in GaAs MESFETs

Effects of substrate traps on turn-on characteristics of GaAs MESFETs are studied by two dimensional (2-D) simulation. When the off-state gate voltage is much more negative than the threshold (pinch off) voltage and the surface-state effects are small, abnormal current overshoot and subsequent slow transients are observed for the case with undoped semi-insulating substrate including an electron trap: EL2. Even if the surface-state effects are pronounced to show the large gate-lag, the drain current may show the overshoot-like behavior at relatively early periods. The case of Cr-doped substrate with a hole trap: Cr is also discussed     

Microwave phenomena in bulk GaAs

CW microwave oscillations were generated at room temperature by usingn-GaAs with a carrier concentration 10¹³- 10¹⁴per cc. Fundamental frequencies between 1 - 10 kMc were excited, and harmonic signals up to 21 kMc were detected. The maximum power output and efficiency on CW basis were 56 mW and 5.2 percent, respectively, at 2 - 3 kMc. A study was made of harmonic content, linewidth, circuit and electronic tunability, as well as the effects of sample thickness, bias voltage, and temperature, on the observed signal frequency. For sample length × carrier concentration products between 2×10¹⁰- 2 × 10¹²cm^-2there was a wide diversity in behavior observed in different samples. This was tentatively attributed to the possibility of exciting not only traveling dipole domains but also growing space-charge waves. These growing space-charge waves were found to exist in samples where the condition nl < 2 × 10¹²cm^-2was satisfied, and are also suspected of being responsible for the observed microwave amplification in bulk GaAs. Three different contacts were used successfully: nickeltin, indium-nickel, and indium-gold. These contacts, and the general semiconductor preparation techniques, will be described in detail.     

Two-dimensional numerical analysis of the minimum isolationdistance for GaAs digital large-scale integration

The minimum device isolation distance (L_min) applicable to GaAs digital large-scale integrated circuits is presented. The leakage current between two n-type layers formed in a semi-insulating (SI) substrate is simulated using a two-dimensional numerical model, and the results are compared with measurements. It is found that the leakage current is restricted by a potential hump formed by residual acceptors in the SI GaAs substrate when an isolating layer loses its compensated SI property. L_min is defined as the distance at which there is a leakage current of 1 mA for an isolating layer width of 1 cm. The calculated value of L_min at room temperature is 1.3 μm with a bias voltage of 2 V and an acceptor concentration of 10¹⁵ cm^-3. L_min decreases to 2/3 of this value when the temperature is reduced from 400 to 100 K, to 1/3 when the acceptor concentration is increased by one order, and to 2/3 when the bias voltage is reduced from 5 to 2 V     

Annealing behavior of undoped bulk GaAs

Heat treatments of bulk undoped GaAs in a sealed ampoule (800° C, 0.5 or 1 h) and in a covered LPE boat under H₂ (695° C, 51 h) were performed with different surface conditions,i.e., exposed surface, proximate surface and a Ga(As) solution covered surface. Four characterization techniques, namely Polaron profile, secondary ion mass spectroscopy (SIMS), photoluminescence (PL) and deep level transient spectroscopy (DLTS), were used to detect the changes in impurities and defects in the surface region of the samples. The carrier concentration profiles, the accumulation of impurities (such as C, Si, Mn and Cu), the PL features and the PL profiles of Mn and Cu related emissions, as well as the trap profiles of EL6 and EL2 for the different surface conditions all demonstrate the important role of both impurities and native defects in surface degradation and the influence of native defects on impurity incorporation.     

Two-dimensional computer design of dual-ring thyristors

The main 2-dimensional equations are presented for the basic computer model of dual-ring amplifying gate thyristors (d.r.t.). These criteria ensure the maintenance of the very high di/dt capability of these devices in any possible triggering conditions.     

Temperature effects in bulk GaAs amplifiers

The microwave-gain characteristics of a bulk GaAs amplifier have been investigated experimentally as a function of temperature. The resistivity of the samples showed a strong temperature dependence, hence the results have been interpreted in terms of changing carrier concentration assuming a constant mobility. It has been found that stable amplification only occurs within a narrow range of temperature (carrier concentration). The highestn . Lproduct (carrier density × sample length) for which gain has been observed was about 5 . 10¹¹cm^-2, and this is in agreement with earlier results and theory. Amplification did not occur belown . L = 8 . 10^{10}cm^-2. The frequency band where negative conductance appears was found to be strongly dependent on temperature (carrier density) and bias field. Noise figures, measured at temperatures where gain occurred, lay between 20 and 30 dB. Noise figure appears to be nearly independent of temperature. Gain versus field measurements on a stable amplifier indicate that the peak field of the velocity-field curve is about 4000 V/cm. Gain versus frequency and noise figure have also been measured on a bulk semiconductor amplifier which was operated with a coaxial transformer to increase the gain over a broad microwave frequency range.     

Two-dimensional analysis of emitter-size effect on current gain for GaAlAs/GaAs HBT's

The current-gain dependence on emitter size (emitter-size effect) is theoretically investigated for GaAlAs/GaAs heterojunction bipolar transistors with an ion-implanted extrinsic base region, using a two-dimensional numerical model. It is clarified that the current gain degrades with a decrease in emitter size, depending on the carrier lifetimes in the extrinsic base region. From the calculated results, it is shown that the emitter-size effect is suppressed for a very-thin-base or a graded-base structure. On the other hand, the effect is enhanced for a double-heterostructure. These results are explained by taking account of the electron-density profile in the extrinsic base region. The mechanism of a lateral diffusion of electrons into the extrinsic base region is also discussed.     

雪崩模式下的体结构GaAs光导开关

利用能量较低的脉冲激光二极管,在较高场强下触发GaAs光导开关,使其工作于雪崩模式,从而产生纳秒上升前沿的快脉冲电压。GaAs光导开关采用垂直体结构设计,芯片厚度为2 mm,电极形状分别为圆环和圆面,触发光脉冲从圆环穿过。快脉冲产生由同轴Blumlein脉冲形成线完成。对基于GaAs光导开关的同轴Blumlein脉冲线进行了模拟仿真和实验,当充电电压超过8 kV(40 kV/cm)后,开关开始了雪崩工作模式。当充电电压约为15 kV(75 kV/cm)时,在50 Ω负载上获得了约11 kV的脉冲电压,实验波形与仿真波形一致。对开关抖动进行了测试,其测试结果显示开关充电电压对抖动影响很大,随着开关偏压增加,开关抖动减小,开关获得了最小抖动约700 ps。     

Stimulated emission from bulk field-ionized GaAs

High-field domains passing throughn-type gallium arsenide pulsed above a critical field produce electron-hole pairs by impact ionization, resulting in emission of recombination radiation of band gap energy. Specimens have been made in a configuration that allows intense ionization to be produced at 78°K in material having initial electron densities of the order of5 times 10^{17}cm^-3Above a certain threshold ionization density, stimulated emission occurs; the linewidth is of the order of 30 Å and emission is mainly within an angle of 7 degrees to the normal to the plane parallel polished faces of the specimen. The threshold current varies little with temperature up to 170°K and above this increases linearly. At currents above the threshold, the spectrum broadens somewhat; the output power is measured to be about 3 watts. The near-field pattern shows that lasing occurs over small separated areas that vary in location from pulse to pulse. Overall power efficiency of the device is low; if, however, only the lasing areas are considered, the efficiency comes much nearer to the theoretical maximum which, it is argued, is about 5 percent. Time dependence of the spectral distribution is complicated and evidence is given to show that carrier heating is an important phenomenon. The lasing process is similar to that occurring in a junction diode laser; some points of difference are discussed. The output frequently shows an unexplained strong polarization.     

Two-dimensional computer analysis of a microwave flat antenna arrayfor breast cancer tomography

In this paper, we report a two-dimensional computer simulation of a microwave flat antenna array for breast cancer tomography. This new technology promises reduction of X-ray exposure and easier access to peripheral areas of the breast. Using our version of the Newton algorithm, we studied two simple mathematical objects and a more sophisticated two-dimensional model of the breast that takes into account dielectric properties of different human tissues and malignant tumors. Our calculations show that, operating at 2 GHz, this device may give very reasonable images of tissues located up to 3-4 cm beneath the surface     

Microwave circuit characteristics of bulk GaAs oscillators

The microwave circuit characteristics of bulk GaAs transit-time mode and limited space-charge accumulation (LSA) mode oscillators have been evaluated experimentally and theoretically. Experimental measurements were performed with a waveguide-coaxial microwave circuit having two experimental degrees of freedom in which the circuit radiation impedance at the device contacts was evaluated by a dyadic Green's function method. Experiments conducted in three rectangular waveguide circuits at fixed bias voltage have established that the LSA mode frequency tuning range is determined by the magnitude and variation of the circuit series inductive reactance XLrelative to the device low-field resistance R0. At a bias voltage which is twice threshold the tuning range is given by1/2.4leq X_{L}/R_{0}leq 2.4. No fixed, linear equivalent circuit characterizes the LSA mode. Analysis and experimental results indicate that the device impedance of small-signal transit-time mode oscillators changes from a passive parallel RC impedance well below threshold to an impedance just above threshold which can be approximated by a series RLC circuit. The series L and C decrease linearly with transit-time mode harmonic order number.