Naoki Hara

Surface passivation of InGaP/InGaAs/GaAs pseudomorphic HEMTs with ultrathin GaS film

We report on the successful surface passivation of wide recess InGaP/InGaAs/GaAs pseudomorphic HEMTs with MBE-grown ultrathin GaS film (2 nm) employing a single precursor, tertiarybutyl-galliumsulfide-cubane ([(t-Bu)GaS]/sub 4/). At the recess length of 1.1 /spl mu/m, a GaS-passivated device with a 0.5-/spl mu/m gate length has the maximum transconductance (g/sub m max/) of 347 mS/mm, which is about 40% higher than that of 240 mS/mm for a device without GaS passivation. We found that one of the causes of an increased g/sub m max/ is the decrease of sheet resistance on the recessed surface because GaS passivation has reduced the depletion layer. Meanwhile, the two-terminal gate-to-drain reverse breakdown voltage (BV/sub gd/) was reduced after GaS passivation. The BV/sub gd/ is independent of the recess length between gate and drain (L/sub gd/) for GaS-passivated devices, unlike that for devices without GaS passivation. According to our calculation of the BV/sub gd/ involving the effects of impact ionization and the interface state, the BV/sub gd/ becomes almost independent of the L/sub gd/, when the interface state density (N/sub int/) is below 1/spl times/10/sup 12/ cm/sup -2/. Then, the calculated surface potential at the recess region is less than 0 eV. This result suggests that GaS passivation can remarkably reduce the N/sub int/ at the recess region.

Energy Band Control of GaAsSb-Based Backward Diodes to Improve Sensitivity of Millimeter-Wave Detection

The effect of energy band control with varying carrier concentration in GaAsSb-based backward diodes was investigated and diode parameters were analyzed to enhance voltage sensitivity. The backward diodes consisted of a heterojunction of p-GaAs0.51Sb0.49/i-In0.52Al0.48As/n-In0.63Ga0.37As/n-In0.53Ga0.47As, and they were mostly lattice-matched to an InP substrate. The degree of energy band bending at the depletion layer in n-In0.63Ga0.37As was varied on the basis of the carrier concentration in the n-In0.53Ga0.47As band control layer. Voltage sensitivity depends on carrier concentration since the concentration affects the band bending structure at the junction. The parameter analysis indicated that junction capacitance decreased when the carrier concentration in the band control layer decreased. When the carrier concentration in the band control layer was as low as 5×1018 cm-3, with the diode mesa at a diameter of 2.0 µm, an unmatched voltage sensitivity of 1495 V/W was obtained.

Characterization of interdiffusion coefficients in GaAs‐AlAs superlattices with laser Raman spectroscopy

A new method to estimate interdiffusion coefficients in superlattices (SLs) is proposed. The method is based on measurement of thicknesses of layers which remain without forming an alloy after an annealing that induced interdiffusion. The measurement was done from the frequency of phonons based on the Raman spectra. Values of interdiffusion coefficients obtained by the method were almost in the same order as those reported previously. It is also shown that the gallium atoms in GaAs‐AlAs SLs diffuse more rapidly into AlAs layers than aluminum atoms which diffuse into GaAs layers. Interdiffusion coefficients decreased at first with the annealing time and increased slightly when the annealing was done for more than 1.5 h at 860 °C.

NiAuGeAu ohmic contacts for a planar InP-based high electron mobility transistor structure with suppressed drain conductance frequency dispersion

We have fabricated a thermally stable low-contact-resistance Ni/AuGe/Au contact metal for InP-based high electron mobility transistors (HEMTs) without a heavily doped cap layer. The contact resistance is strongly influenced by the amount of Ni. Minimum contact resistance of 0.19 Ω mm was obtained from a Ni (1 nm)/AuGe (50 nm)/Au (99 nm) contact annealed at 300 °C for 5 min in N2. A smooth surface was obtained after contact formation and excellent thermal stability was achieved during isothermal annealing at 350 °C for 1 h in a N2 ambient. This alloyed ohmic contact satisfies the requirements for device fabrication process steps after the ohmic contact formation. It prevents hole accumulation by eliminating the hole barrier at the carrier-supply layer/channel interface. It also suppresses the kink phenomena and drain conductance frequency dispersion of InP-based HEMTs, and therefore will greatly facilitate the fabrication of ultrahigh-speed integrated circuits.

Suppression of drain conductance dispersion in InP-based HEMTs for broadband optical communication systems

Demonstrated InP-based HEMTs without drain conductance (g/sub d/) frequency dispersion for broadband optical communication systems. It was possible to markedly suppress the g/sub d/ dispersion by using composite channel and double-doped structures rather than a conventional HEMT structure. Furthermore, we clarified that hole generation time by impact ionization determines the frequency range of the g/sub d/ dispersion in a conventional InP-based HEMT by investigating the g/sub d/ dispersion over a wide range of frequencies (100 Hz-20 GHz).

Picosecond Pulse Generators in InP-Based High Electron Mobility Transistor Technology for 10 Gbps Wireless Communication

In this paper, we present simple digital-based pulse generators (PGs) that emit short pulses with a full width at half-maximum (FWHM) of only several picoseconds in order to realize millimeter-wave impulse radio (IR) systems that can transmit signals at 10 Gbps and above. There are distinctive advantages of using digital techniques. Scaling devices and reducing parasitic capacitances can enhance performance, which is examined by a transition analysis of the PGs. Secondly, retiming techniques enable the PGs to suppress jitter, which is essential for maintaining transmission quality. Two types of PGs are demonstrated: Type-I PG with advanced 75 nm InP-based high electron mobility transistors (HEMTs) having a cavity structure generated short pulses with a record FWHM of <4.9 ps at 20 Gbps, realizing sub-THz IR systems. Type-II PG with mature 100 nm InP-based HEMTs without the cavity retimed 6.5 ps pulses, resulting in very small jitter of 256 fs. Type-II PG was used to construct an IR transmitter of over 10 Gbps at millimeter-wave frequencies (75–110 GHz).

93–133 GHz Band InP High-Electron-Mobility Transistor Amplifier with Gain-Enhanced Topology

In this study, we developed a new type of high-frequency amplifier topology using 75-nm-gate-length InP-based high-electron-mobility transistors (InP HEMTs). To enhance the gain for a wide frequency range, a common-source common-gate hybrid amplifier topology was proposed. A transformer-based balun placed at the input of the amplifier generates differential signals, which are fed to the gate and source terminals of the transistor. The amplified signal is outputted at the drain node. The simulation results show that the hybrid topology exhibits a higher gain from 90 to 140 GHz than that of the conventional common-source or common-gate amplifier. The two-stage amplifier fabricated using the topology exhibits a small signal gain of 12 dB and a 3-dB bandwidth of 40 GHz (93–133 GHz), which is the largest bandwidth and the second highest gain reported among those of published 120-GHz-band amplifiers. In addition, the measured noise figure was 5 dB from 90 to 100 GHz.

An 85 GHz Distributed Amplifier with 15.5 dBm Output Saturated Power Using 0.1 µm InP-based High Electron Mobility Transistors

In this paper, we describe an eight-stage distributed amplifier (DA) developed using 0.1-µm-gate-length InP-based high electron mobility transistors (HEMTs) for millimeter-wave broadband systems. To obtain high power in a wide frequency range, a tapering technique in the drain line section and gain-boosting and bandwidth-expanding techniques in the lumped amplifier section were employed. The measured output saturated power (Psat) was 15.5 dBm with a power added efficiency (PAE) of 13.9% at a frequency of 75 GHz, whereas the measured small signal gain and 3 dB bandwidth were 11 dB and 85 GHz, respectively. The Psat value is the highest reported in any DA.

Band Alignment of Molecular-Beam-Epitaxy-Grown GaS/GaAs Structure Using a Single [(t-Bu)GaS]4 Precursor

We report on the band alignment of a molecular-beam-epitaxy (MBE)-grown GaS/GaAs structure using a single tertiarybutyl-gallium sulfide cubane [(t-Bu)GaS]4 precursor. The optical band gap of epitaxial GaS film grown at a growth temperature (TS) of 500°C is estimated by spectroscopic ellipsometry to be 2.7 eV, whereas that of amorphous GaS film grown at a TS of 350°C is only 2.2 eV. X-ray photoemission spectroscopy (XPS) was used to estimate the band discontinuity between GaS and GaAs. Consequently, epitaxial and amorphous GaS/GaAs structures, respectively, reveal straggling type-I and staggered type-II band alignments.

GaAs surface passivation with MBE grown GaS thin film

We report on the successful GaAs surface passivation with GaS thin film grown by MBE employing the single precursor, tertiarybutyl-gallium sulfide-cubane ([(t-Bu)GaS]/sub 4/). GaAs bandgap PL intensity increased by passivating with GaS and has been maintained for a year. Furthermore, we investigated the relationship between the interface state density and the GaAs surface reconstruction before GaS passivation. The PL intensity for c(4/spl times/4)As was largest among the surface reconstructions investigated. Also, a minimum interface state density as low as 5/spl times/10/sup 10/ eV/sup -1/ cm/sup -2/ was obtained for an Al/GaS/n-GaAs MIS structure of c(4/spl times/4)As. In addition, we demonstrated the feasibility of GaS passivation for device applications.