A 0.5-μm-gate GaAs/AlGaAs inverted HEMT IC-multiplier and D/Aconverter

A gate recess process for a 0.5-μm I-HEMT (inverted high electron mobility transistor) has been developed. A drain conductance for the 0.5-μm I-HEMT as small as 2 mS/mm was achieved, indicating a small short-channel effect. The threshold voltage uniformities were studied in microscopic and macroscopic areas in a 2-in wafer. The uniformities are very high, i.e. the standard deviations of microscopic and macroscopic areas are 10 and 30 mV, respectively, at a threshold voltage of 0.1 V. An 8×4 parallel multiplier was fabricated, and a multiplication time of 1.67 ns was obtained at room temperature. An 8-b digital/analog converter (DAC) was fabricated and operated at a clock rate of 1.2 GHz. The DC linearity of the DAC is better than 0.18 LSB. These results confirm that an I-HEMT is very well suited for high-speed integrated circuits     

Determination of AlGaAs/GaAs HEMT parasitic resistances

An improved technique has been developed to measure source and drain parasitic resistances of AlGaAs/GaAs HEMTs. Similar to the measurement technique typically used for MESFETs, a positive d.c. gate crowding current is applied. Because of the structure of the HEMT, this gate current must be kept very small in order to prevent significant leakage into the AlGaAs layer, which would result in current paths not present in normal operation of the device. The small d.c. gate current necessary to limit the current in this leakage path did not yield a usable signal-to-noise ratio of the measured gate-source, gate-drain and drain-source voltages needed to calculate the parasitic resistances. To overcome this problem, modulation of the drain current with a low-frequency a.c. signal coupled with lock-in techniques to measure the desired voltages was implemented. The resulting improvement in signal-to-noise ratio has made the gate crowding technique suitable for measuring the parasitic resistances of AlGaAs/GaAs HEMTs.     

GaAs/AlGaAs环形激光器的量子阱结构优化

为了研究量子阱结构对半导体环形激光器阈值电流的影响,从F-P腔激光器的振荡条件出发,分析了半导体环形激光器的阈值电流密度与量子阱结构参量的函数关系,并推导出最佳量子阱数的表达式。利用器件仿真软件ATLAS建立环形激光器的等效模型,仿真、分析了不同工作温度下,量子阱数、阱厚及势垒厚度对阈值电流的影响。结果表明,阈值电流随量子阱数和阱厚的增加先减小后增大,存在一组最佳值;在确定合适的量子阱数和阱厚后,相对较窄的势垒厚度有助于进一步降低阈值电流;采用GaAs/AlGaAs材料体系和器件结构,其最佳量子阱结构参量为M=3,dw=20nm及db=10nm。     

Comparison of OMVPE Grown GaAs/AlGaAs and GaAs/InGaP HEMT and PHEMT structures

Symmetric δ-doped InGaP and AlGaAs PHEMT structures have been grown by organometallic vapor phase epitaxy with properties that approach those of MBE grown AlGaAs structures. The 300 and 77K carrier concentrations for the InGaP PHEMT were 2.72 and 2.56 × 10¹² cm^{2 −2} and the mobilities were 5,920 and 22,000 cm^{2 2}/V.s. These excellent values suggest that problems associated with switching the anion at the channel heterojunction have been overcome. The corresponding values for the AlGaAs PHEMT were 2.51 and 2.19 × 10¹² cm^{2 −2} and 6,500 and 20,400 cm²/V.s. The uniformity in the indium concentration in the InGaAs layer as determined by photoluminescence, photoreflection, double crystal x-ray diffraction, and Rutherford backscattering was found to be good, but the percent In in the AlGaAs pseudo-morphic high electron mobility transistor (PHEMT) was less than that in the InGaP PHEMT even though the programmed values were the same. The uniformity in the doping distribution as determined by secondary ion mass spectroscopy and electrochemical capacitance-voltage measurements was found to be good, but it decreased with distance from the center of the susceptor. Also, most of the dopants in the δ-doped InGaP and AlGaAs layers were activated.     

Modeling of GaAs/AlGaAs MODFET inverters and ring oscillators

Detailed understanding of the MODFET inverter chains is lacking and present designs are based on ground rules developed by trial and error. We developed a simple and straightforward model for the current-voltage characteristics using results from numerical solutions of the quantum mechanical problem; this model agrees very well with experimental results obtained in our laboratory. High-frequency gate capacitance voltage characteristics for a wide gate voltage range was modeled for the first time in a similar fashion. These models were used to simulate a chain of inverters at 77 and 300 K for a wide range of supply voltage and load current. The maximum device speed is obtained for small supply voltages, ≤ 1 V both at 300 and 77 K, where the effects of transconductance degradation and large gate capacitance are minimized. The devices exhibit under 10-ps switching times both at 300 and 77 K, with 77 K logic swing being much larger. The results are in qualitative agreement with the reported experimental results.     

A new fabrication technology for AlGaAs/GaAs HEMT LSIs using InGaAsnonalloyed ohmic contacts

The authors studied the nonalloyed ohmic characteristics of HEMTs (high electron mobility transistors). At high integration levels, nonalloyed ohmic contacts were found to have two advantages: an extremely short ohmic length with low parasitic source series resistance and direct connection between the source/drain and gate with the same metal. The propagation delay in a ring oscillator with a single-metal source/drain and gate formed simultaneously was 37 ps/gate (L _g=0.9 μm). The very short ohmic metal contacts and just three contact holes made it possible to reduce the memory cell area greatly. The cell is 21.5×21.5 μm², one of the smallest ever reported for a GaAs-based static RAM. Using smaller load HEMTs or resistor loads in the memory cell, combined with nonalloyed ohmic technology with quarter- or subquarter-micrometer-gate HEMTs it is possible to fabricate a very-high-speed LSI such as a 64-kb static RAM with a reasonable chip size     

Dual bridge 6 Gsample/s track and hold circuit inAlGaAs/GaAs/AlGaAs HEMT technology

A T&H circuit with a sampling rate of 6 Gsample/s has been experimentally demonstrated in AlGaAs/GaAs/AlGaAs HEMT technology. The circuit utilises a dual bridge topology suitable for interleaved ADC circuits, doubling the effective sampling rate with an insignificant increase in area compared to the previously reported solutions     

GaAs MESFET ring oscillator on Si substrate

GaAs MESFET ring oscillators were fabricated on a Si substrate and successfully operated. Epitaxial techniques to grow a GaAs layer on a Si substrate were investigated. The device-quality GaAs epitaxial layer was obtained by introducing a Ge layer (by ionized cluster-beam deposition) and alternating GaAs/GaAIAs layers (by MOCVD). The typical transconductance of 140 mS/mm was obtained for the FET with a 0.5 µm × 10 µm gate. The minimum delay time was 66.5 ps/ gate at a power consumption of 2.3 mW/gate.     

基于InP全增强HEMT逻辑的环形压控振荡器研究

A voltage-controlled ring oscillator （VCO） based on a full enhancement-mode InAlAs/InGaAs/InP high electron mobility transistor （HEMT） logic is proposed. An enhancement-mode HEMT （E-HEMT） is fabricated, whose threshold is demonstrated to be 10 mV. The model of the E-HEMT is established and used in the SPICE simulation of the VCO. The result proves that the full E-HEMT logic technology can be applied to the VCO. And compared with the HEMT DCFL technology, the complexity of our fabrication process is reduced and the reliability is improved.     

A voltage-controlled ring oscillator using InP full enhancement-mode HEMT logic

A voltage-controlled ring oscillator (VCO) based on a full enhancement-mode InAIAs/InGaAs/InP high electron mobility transistor (HEMT) logic is proposed. An enhancement-mode HEMT (E-HEMT) is fabricated, whose threshold is demonstrated to be 10 mV. The model of the E-HEMT is established and used in the SPICE simulation of the VCO. The result proves that the full E-HEMT logic technology can be applied to the VCO. And compared with the HEMT DCFL technology, the complexity of our fabrication process is reduced and the reliability is improved.     

High-power broad mesa structure AlGaAs/GaAs single-quantum-well edge-emitting LED

An edge-emitting light-emitting diode (LED) with a wide emitting region for optical sensing and information processing which requires slit-shaped light sources is discussed. Through the use of a high-quantum efficiency single-quantum-well structure grown by molecular beam epitaxy an output power as high as 3 mW at an injected current of 100 mA with a 50- mu m-wide mesa structure is achieved at the 780-nm emission wavelength. A uniform and rectangular intensity profile, which is suitable for practical use, is obtained with a mesa-restricted structure.<>     

Effect of GaAs/AlGaAs quantum-well structure on refractive index

We investigate the refractive index difference between the GaAs/AlGaAs quantum wells (QWs) and bulk AlGaAs. We find the refractive index difference is smaller when the electric field in the nominally intrinsic MQW region is larger, or when the well (GaAs) thickness of the QW's is larger, or when the Al fraction of the QWs is smaller. The maximum refractive index difference between the 100 Å GaAs/100 Å Al_0.2Ga_0.8As QWs at zero electric filed and bulk Al_0.1Ga_0.9As is about 0.044. Even with a small refractive index difference of 0.0132, the OFF-state reflectance of a normally-off MQW modulator at the designed photon wavelength will increase from the desired value of 0% to 90% when the reflectivity of the bottom mirror is 0.99     

Planar structure AlGaAs/GaAs pin photodiode grown by MOCVD

A planar structure AlGaAs/GaAs PIN photodiode was fabricated by first using MOCVD. The p+-region was formed by Zn-diffusion in a high-resistivity AlGaAs window layer. The internal quantum efficiency, close to unity, and the cut-off frequency, as high as 1.2 GHz, have been demonstrated by 280 ?m diameter devices.     

AlGaAs/GaAs visible ridge waveguide laser with multicavity structure

Ridge waveguide lasers of two different cavity configurations are described. First is the multisection ridge laser structure in which the longitudinal mode selection due to the coupled-cavity effect was observed and single-frequency operation was achieved. In this structure, stable single-transverse-mode operation was ensured by the central 5 μm wide section, and the beam waist and power output were enlarged by the outer 10 μm wide sections. Output power of more than 13 mW under pulsed excitation was obtained. The second configuration is the shallow groove ridge laser in which the 5 μm wide shallow groove terminates slightly above the active region and provides an index perturbation that contributes to mode selection. All the lasers are designed to emil in the visible range between 7600 and 7900 Å. The shortest observed wavelength is 7570 Å, and can be seen by the naked eye.     

Suppression of gate leakage current in n-AlGaAs/GaAs power HEMTs

Experiments of gate leakage current in a double-channel n-AlGaAs/GaAs FET with SiN_x film passivation indicated that the gate leakage current and its stability depend critically on the stoichiometry of the surface. The content of NH₄ OH in the etching solution prior to the SiN_x film deposition and the NH ₃/SiH₄ gas content ratio, particularly at the initial stage of plasma-CVD SiN, deposition, should be selected to minimize the surface state density of possible antisite defects. Extremely low gate leakage currents of 50 nA/mm have been obtained with an improved process, which is useful to realize low-distortion FETs     

Electrochemical capacitance-voltage profiling of the free-carrier concentration in HEMT heterostructures based on InGaAs/AlGaAs/GaAs compounds

The depth distribution of free carriers over the HEMT structures with quantum-well layers is studied by electrochemical capacitance-voltage profiling. It is shown that the actual distribution of the concentration of free carriers and their energy spectrum in the HEMT structure channel can be obtained by numerical simulation of the results of profiling based on the self-consistent solution of one-dimensional Schrödinger and Poisson equations.     

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。     

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