Reduction of backgating in GaAs/AlGaAs MESFETs by optimisation of active-layer/buffer-layer interface

Backgating measurements made on GaAs MESFETs with abrupt, graded alloy and graded superlattice interface AlGaAs buffer layers were compared to measurements made on conventional GaAs buffer-layer MESFETs. Only the superlattice interface structure showed a reduction in the backgating transconductance (by a factor of 24 compared to the GaAs buffer-layer FET). The lack of reduction in the backgating transconductance for the abrupt and graded alloy interface devices is attributed to traps resulting from GaAs growth on an AlGaAs layer.     

Impact ionization and light emission in AlGaAs/GaAs HEMT's

Impact ionization and light emission phenomena have been studied in AlGaAs/GaAs HEMTs biased at high drain voltages by measuring the gate excess current due to holes generated by impact ionization and by analyzing the energy distribution of the light emitted from devices in the 1.1-3.1 eV energy range. The emitted spectra in this energy range can be divided into three energy regions: (i) around 1.4 eV light emission is dominated by band-to-band recombination between cold electrons and holes in GaAs; (ii) in the energy range from 1.5 to 2.6 eV energy distribution of the emitted photons is approximately Maxwellian; and (iii) beyond 2.6 eV the spectra are markedly distorted due to light absorption in the n⁺ GaAs cap layer. The integrated intensity of photons with energies larger than 1.7 eV is proportional to the product of the drain and gate currents. This suggests recombination of channel electrons with holes generated by impact ionization as the dominant emission mechanism of visible light     

Thermal stability of Al/Ni gate AlGaAs/GaAs HEMT's

The study on the instability of gate contacts of Al/Ni gate AlGaAs/GaAs HEMT's was performed by means of storage tests carried out at three different temperatures: 200°C, 240°C and 300°C. Data from tests as long as 5000 hours were analyzed. At the highest temperature the main failure mode was the reaction between the Al of the gate electrode and the Au of the metallization. At 200°C, 240°C an increase of the barrier height was detected. The activation energy determined and the comparison with the data existing in literature is reported.     

Reduction of backgating in GaAs SISFET's with a low-temperaturebuffer

The use of a low-temperature molecular beam epitaxy (MBE)-grown buffer layer to reduce backgating in GaAs/AlGaAs semiconductor-insulator-semiconductor FETs (SISFETs) is discussed. Comparison with a control wafer having no low-temperature buffer (LTB) reveals an improvement in backgating threshold voltage by a factor of 3, improvement in output conductance and short-channel characteristics, and no significant change in threshold voltage, threshold-voltage spread, and microwave characteristics. The FETs with LTB exhibited increased sensitivity, at 80 K, to trapping of hot electrons     

Analysis of delta-doped and uniformly doped AlGaAs/GaAs HEMT's byensemble Monte Carlo simulations

Transport properties and device performance of delta-doped and uniformly doped AlGaAs/GaAs high electron mobility transistors (HEMTs) with identical threshold voltages and gate capacitors are investigated using two-dimensional self-consistent ensemble Monte Carlo simulations. The model includes the effects of real-space transfer and carrier degeneracy, as well as the influence of DX centers and surface states. A one-to-one comparison of simulation results for the two devices demonstrates superior performance for the delta-doped HEMT and provides a physical basis for the observed improvements. In particular, the delta-doped HEMT maintains its superior device performance as gate bias is increased. Reasons for these improvements are reported     

Trap studies in GaInP/GaAs and AlGaAs/GaAs HEMT's by means oflow-frequency noise and transconductance dispersion characterizations

The presence of traps in GaInP/GaAs and AlGaAs/GaAs HEMT's was investigated by means of low frequency noise and frequency dispersion measurements. Low frequency noise measurements showed two deep traps (E _a1=0.58 eV, E_a2=0.27 eV) in AlGaAs/GaAs HEMT's. One of them (E_a2) is responsible for the channel current collapse at low temperature. A deep trap (E_a1'=0.52 eV) was observed in GaInP/GaAs HEMT's only at a much higher temperature (~350 K). These devices showed a transconductance dispersion of ~16% at 300 K which reduced to only ~2% at 200 K. The dispersion characteristics of AlGaAs/GaAs HEMT's were very similar at 300 K (~12%) but degraded at 200 K (~20%). The low frequency noise and the transconductance dispersion are enhanced at certain temperatures corresponding to trap level crossing by the Fermi-level. The transition frequency of 1/f noise is estimated at 180 MHz for GaInP/GaAs HEMT's and resembles that of AlGaAs/GaAs devices     

Analysis of kink-related backgating effect in GaAs MESFET

Two-dimensional simulation of backgating effect in a GaAs MESFET is made in which impact ionization of carriers and deep donors “EL2” in the substrate are considered. The kink-related backgating is reproduced, which is qualitatively consistent with recent experiments. Based on the simulated results, physical mechanism of kink-related backgating effect is discussed     

Carrier injection and backgating effect in GaAs MESFET's

The relation between the backgating effects on GaAs MESFET's and current conduction in the semi-insulating substrate is studied. The onset voltage of the backgating effect is found to coincide with the trap-fill-limited voltage for the substrate conduction. This observation implies that carrier injection in the substrate is directly related to the backgating effect.     

Temperature dependence of backgating effect in GaAs integrated circuits

The backgating effect in GaAs IC's has been found to be temperature dependent. The threshold voltage for backgating increases with temperature, resulting in lower backgating at higher temperatures. The measured activation energy of the backgating threshold versus temperature is 83 meV, in agreement with the energy difference between the Fermi level and the EL2 level at the surface of semi-insulating GaAs.     

Orientation effect on AlGaAs/GaAs heterojunction bipolartransistors

Orientation effects on N-p-n AlGaAs/GaAs heterojunction bipolar transistors (HBT's) have been demonstrated for the first time. We have observed that the current gains of HBT's fabricated on the same wafer are strongly dependent on the emitter direction. The HBT's with emitter direction of [010] show the highest current gain and the smallest emitter-size effect. This orientation effect could be attributed to the piezoelectric effect, which superposes the piezoelectric charges to the original emitter doping and generates the weak lateral electric field that drifts the injected carriers at the emitter periphery. The difference of the saturation voltage between collector-emitter of those HBT's corresponds to the superposed piezoelectric charges     

High-efficiency millimeter-wave GaAs/GaAlAs power HEMT's

The power, gain, and efficiency of 0.5-µm gate-length, 75- and 50-µm gate-width multiple heterojunction high electron mobility transistors (HEMT's) have been evaluated from 10 to 60 GHz. At 10 GHz, with a source-to-drain voltage as low as 2.4 V, the device delivers a power density of 0.37 W/mm with 13.4-dB gain and 60.8-percent efficiency. At 60 GHz, a 50-µm device gave 0.4 W/mm with 3.6-dB gain and 14-percent efficiency. The power density and efficiency of these 0.5- µm gate-length HEMT's above 40 GHz are the best reported for a three-terminal device. Fundamental frequency oscillations up to 104 GHz were observed when a device was bonded as a free-running oscillator.     

Electron mobility in a AlGaAs/GaAs/AlGaAs quantum well

An oscillatory dependence of the electron mobility on the quantum well (QW) thickness in a AlGaAs/GaAs/AlGaAs heterostructure with double-sided modulation doping has been observed experimentally. A steep decrease in mobility with increasing electron concentration in the QW is established. The conditions for an increase in mobility on introducing a thin barrier into the QW are determined. The first experimental observation of an increase in mobility by a factor of 1.3 in a QW of thickness L=26 nm upon introducing a thin (1–1.5 nm) AlAs barrier is reported.     

Ultra-submicrometer-gate AlGaAs/GaAs HEMTs

Ultra-submicrometer-gate AlGaAs/GaAs high-electron-mobility transistors (HEMTs) that have gate lengths ranging from 25 to 85 nm and were fabricated using electron-beam lithographic techniques on epitaxial wafers grown by molecular beam epitaxy are discussed. These devices show that velocity overshoot and short-gate geometry effects play an important role for the gate lengths less than 100 nm. The maximum intrinsic transconductance is 215 mS/mm, and the effective saturated electron velocity reaches 3×10⁷ cm/s for a 30-nm HEMT     

单量子阱GaAs/AlGaAs半导体激光器

本文介绍了用分子束外延法制作的梯度折射率分别限制式单量子阱GaAs/AlGaAs半导体激光器。该器件具有较低的阈值电流密度和单模运转特性,连续输出功率可达55mw。     

AlGaAs/GaAs/AlGaAs DHBT's for high-temperature stable circuits

High-temperature devices are required for a large number of industrial applications. In order to demonstrate the feasibility of a high temperature operating circuit on GaAs an operational amplifier was designed and fabricated. A corresponding technology for transistors and circuits for operation up to 300°C with AlGaAs/GaAs/AlGaAs DHBT's is presented. For the amplifier circuit an open loop gain of 49.5 dB at room temperature and 35.8 dB at 200°C was measured, which is in good agreement with the circuit simulation. High temperature stability has been proven by a storage test at 400°C over 1000 h for the ohmic contact metallization and 200 h for the transistors     

超亚微米栅AlGaAs/GaAs HEMT

在分子束外延生长的外延晶片上,用电子束刻蚀技术制作了超亚微米栅AlGaAs/GaAs高电子迁移率晶体管(HEMT),其栅长分布为25～85nm。该器件表明,速度过冲和短栅几何效应对栅长小于100nm的器件起着重要的作用。栅长为30nm的HEMT的最大本征跨导为215mS/mm,有效饱和电子速度可达3×10~7cm/s。     

GaInNAs/GaAs量子阱的光致发光谱和光调制反射谱

研究了GaInNAs/GaAs多量子阱在不同温度和激发功率下的光致发光(PL)谱以及光调制反射(PR)谱.发现PL谱主发光峰的能量位置随温度的变化不满足Varshni关系,而是呈现出反常的S型温度依赖关系.进一步测量,特别是在较低的激发光功率密度下,发现有两个不同来源的发光峰,它们分别对应于氮引起的杂质束缚态和带间的激子复合发光.随温度变化,这两个发光峰相对强度发生变化,造成主峰(最强的峰)的位置发生切换,从而导致表观上的S型温度依赖关系.采用一个基于载流子热激发和出空过程的模型来解释氮杂质团簇引起的束缚态发光峰的温度依赖关系.     

Optically induced backgating transients in GaAs FET's

Long-term current transients have been induced with optical pulses in depletion-mode GaAs field-effect transistors (FET's). The millisecond-to-second duration and the bias dependence of the transients are similar to substrate trapping and backgating events initiated by ionizing radiation. A specific region of a FET-the semiconductor region adjacent to the connecting strip between the gate electrode and the gate bonding pad-is particularly sensitive to optical backgating. In this region low-incident optical energies produce a positive current transient; but when the optical intensity exceeds ∼1 mJ/cm², a transient decrease in current is observed. Optical studies promise to be a simple and convenient means of simulating many of the effects of ionizing radiation.     

GaAs/AlGaAs and InGaAs/AlGaAs MODFET inverter simulations

Although MODFET's have exhibited the fastest switching speed for any digital circuit technology, there is as yet no clear consensus on optimal inverter design rules. We therefore have developed a comprehensive MODFET device model that accurately accounts for such high gate bias effects as transconductance degradation and increased gate capacitance. The device model, which agrees with experimental devices fabricated in this laboratory, is used in the simulation of direct-coupled FET logic (DCFL) inverters with saturated resistor loads. Based on simulation results, the importance of large driver threshold voltage not only for small propagation delay times but for wide logic swings and noise margins is demonstrated. Furthermore, minimum delay times are found to occur at small supply voltages as seen experimentally. Both of these results are attributed to the reduction of detrimental high gate bias effects. The major effect of reducing the gate length on delay time is to decrease the load capacitance of the gate. Using 0.25-µm gates, delay times of 5 and 3.6 ps at 300 and 77 K, respectively, are predicted. Finally, the recently introduced In-GaAs/AlGaAs MODFET's are shown to have switching speeds superior to those of conventional GaAs/AlGaAs MODFET's.