This paper describes a high-power and low-distortion AlGaAs/GaAs HFET amplifier developed for digital cellular base station system. We proved experimentally that distortion characteristics such as IMD (Intermodulation Distortion) or NPR (Noise Power Ratio) are drastically degraded when the absolute value of the drain bias circuit impedance at low frequency are high. Based on the experimental results, we have designed the drain bias circuit not to influence the distortion characteristics. The developed amplifier employed two pairs of pre-matched GaAs chips mounted on a single package and the total output-power was combined in push-pull configuration with a microstrip balun circuit. The push-pull amplifier demonstrated state-of-the-art performance of 140 W output-power with 11.5 dB linear gain at 2.2 GHz. In addition, it exhibited extremely low distortion performance of less than 30 dBc at two-tone total output-power of 46 dBm. These results indicate that the design of the drain bias circuit is of great importance to achieve improved IMD characteristics while maintaining high power performance.

The security of the ElGamal-type signature scheme is based on the difficulty of solving a discrete logarithm problem. If a random value that is introduced in the signing procedure is small, then the time for generating signature can be reduced. This strategy is particularly advantageous when a signer uses a smart card. In this paper, we show that the secret key can be computed efficiently if the random value is less than O(q) where q is the order of the generator.

This paper describes two attacks against blind decryption (decode) based on the commutative random-self reducibility and RSA systems utilizing the transformability of digital signatures proposed in [2]. The transformable digital signature was introduced in [2],[8] for defeating an oracle attack, where the decrypter could be abused as an oracle to release useful information for an attacker acting as a requester of blind decryption. It was believed in [2],[8] that the correctness of a query to an oracle was ensured by the transformable signature derived from an original signature issued by the decrypter in advance, and a malicious query to an oracle could be detected before the blind decryption by the decrypter or would lead to release no useful information to an attacker. The first attack can decrypt all encrypted data with one access to an oracle. The second one generates a valid signature for an arbitrary message selected by an attacker abusing the validation check procedure.

In this paper, two new signature schemes whose security is based on both discrete logarithms and factorization are proposed to enhance the security of the OSS signature scheme. The major benefit of these new signature schemes is that the signer dose not need to know how to factor p-1, thus it is possible for every user to employ the same modulus p, where p is the modulus commonly used in the system. Furthermore, two identification schemes based on this advantage are also developed in this paper.

AlGaAs/GaAs power HBTs for digital cellular phones have been developed. A three-dimensional thermal analysis taking the local-temperature dependence of the collector current into account was applied to the thermal design of the HBTs. The HBTs were fabricated using the hetero-guardring fully selfaligned transistor technique. The HBT with 220µm2 60 emitters produced a 31.7 dBm CW-output power and 46% poweradded efficiency with an adjacent channel leakage power of -49 dBc at the 50kHz offset bands for a 948 MHz π/4-shifted QPSK modulated signal at a low collector-emitter voltage of 3V. Through comparison with the conventional GaAs power FETs, it has been shown that AlGaAs/GaAs power HBTs have a great advantage in reducing the chip size.

The base transit time of an Aluminum-graded-base PNp AlGaAs/GaAs heterojunction bipolar transistor (HBT) was studied in order to clarify the effect of aluminum grading in the base. Theoretical analysis using a classical drift diffusion model with velocity saturation at the base-collector junction and a high base quasielectric field (58 keV/cm) created by 20%-aluminum linear grading in a 400 base, leads to a base transit time (τb) of 0.9 ps. The base transit time is reduced by four times, compared to the base transit time of 3.6 ps without aluminum grading in the base. In order to demonstrate this advantage, we fabricated aluminum-graded-base PNp AlGaAs/GaAs heterojunction transistor which employs a 20%-aluminum linear graded 400 -wide base. The device with a 2 µm 10 µm emitter showed high RF performance with a cut-off frequency (ft) of 37 GHz and a maximum oscillation frequency (fmax) of 30 GHz at a collector current density of 3.4 104 A/cm2. Further analysis using direct parameter extraction of a small signal circuit model under the collector current density of 1.1-9.9104 A/cm2 indicated the intrinsic transit time, which is the sum of the base transit time and the collector depletion layer transit time (τSC), was as low as 2.3 ps under lowinjection level. Subtracting the collector depletion-layer transit time from the intrinsic time leads to a base transit time of 1.1 ps, which is close to the theoretical base transit time and is the shortest value ever reported. The structure is very attractive for pnp-type AlGaAs HBTs combined with Npn HBTs for complementary applications.

An ultra low noise 50-GHz-Band amplifier (LNA) MMIC has been developed using an AlGaAs/InGaAs pseudomorphic HEMT. A noise figure of 1.8 dB with an associated gain of 8.1 dB is achieved at 50 GHz. The noise figure is less than 2.0 dB from 50 GHz to 52.5 GHz. This is the state-of-the-art noise figure for low noise amplifiers around 50 GHz. The success of this LNA development came from the excellent HEMT and MMIC technologies and the accurate modeling of active and passive elements. Good agreement between measured and simulated data over the band from 40 GHz to 60 GHz is obtained.

A design of a high power laser structure is presented which is based on an increase of the cavity length as well as a maximization of the stripe width. This requires a low value for the modal attenuation coefficent and a low optical confinement factor. A model is presented from which the modal gain, the confinement factor, the active region thickness, the stripe width, the length and the reflection coefficients can be calculated. A variant for all design parameters needed to reach 1 W emission in the fundamental lateral mode is given. These values are used to design the epitaxial structure.

We found degraded output power due to discoloration of an abnormal epitaxial layer caused by supercooling of residual melt in liquid phase epitaxy (LPE) process of AlGaAs/GaAs heteroface solar cells developed to improve conversion efficiency of solar cells for satellites. We studied the discoloration mechanism and found effective methods for obtaning a good epitaxial layer. Using these results, we manufactured about 80,000 pieces of solar cells and employed them in the Japanese domestic Communication Satellite-3 (CS-3) launched by National Space Development Agency of Japan (NASDA). Five years after launch, these solar cells are still supplying the output power than predicted. This paper describes reliability improvements for the surface of epitaxial layer and successful results aftes 5 years of space operation.

A one-by-four static frequency divider using AlGaAs/GaAs heterojunction bipolar transistors (HBTs) was designed to operate at a bias condition that gave a maximum cutoff frequency fT and a maximum oscillation freqency fmax. The fT and fmax applied to the divider were 68 GHz and 56 GHz, respectively. As a result of the tests, the circuit operated up to 34.8 GHz at a power supply voltage of 9 V and power dissipation of 495 mW. A low minimum input signal power level of 0 dBm was also achieved.