Tadao Ishibashi

High-speed and high-output InP-InGaAs unitraveling-carrier photodiodes

The unitraveling-carrier photodiode (UTC-PD) is a novel photodiode that utilizes only electrons as the active carriers. This unique feature is the key for its ability to achieve excellent high-speed and high-output characteristics simultaneously. To date, a record 3-dB bandwidth of 310 GHz and a millimeter-wave output power of over 20 mW at 100 GHz have been achieved. The superior capability of the UTC-PD for generating very large high-bit-rate electrical signals as well as a very high RF output power in millimeter/submillimeter ranges can lead to innovations in various systems, such as broadband optical communications systems, wireless communications systems, and high-frequency measurement systems. Accomplishments include photoreceivers of up to 160 Gb/s, error-free DEMUX operations using an integrated UTC-PD driven optical gate of up to 320 Gb/s, a 10-Gb/s millimeter-wave wireless link at 120 GHz, submillimeter-wave generation at frequencies of up to 1.5 THz, and photonic frequency conversion with an efficiency of -8 dB at 60 GHz. For the practical use, various types of modules, such as a 1-mm coaxial connector module, a rectangular-waveguide output module, and a quasi-optic module, have been developed. The superior reliability and stability are also confirmed demonstrating usefulness of the UTC-PD for the system applications.

High-Speed Response of Uni-Traveling-Carrier Photodiodes.

The photoresponse of a uni-traveling-carrier photodiode (UTC-PD), which is configured with a neutral narrow-gap light absorption layer and a depleted wide-gap carrier collecting layer, is investigated by small-signal analysis. Drift-diffusion model was used for analyzing carrier dynamics in the absorption layer. For accurately predicting the frequency response, a boundary condition at the edge of the absorption layer was carefully treated by taking into account the electron thermionic emission velocity. High electron mobility in the absorption layer and high drift velocity in the carrier collecting layer associated with the velocity overshoot effect are both essential for short response times. Calculations performed on InP/InGaAsP UTC-PDs with the same absorption and carrier collecting layer thicknesses show that the response can be dominated by the electron transport in the absorption layer provided that the significant velocity overshoot occurs in the carrier collecting layer. Furthermore, a UTC-PD with a quasi-field in the absorption layer can generate a several times broader bandwidth than conventional pin PDs, while maintaining a similar internal quantum efficiency.

Continuous THz-wave generation using antenna-integrated uni-travelling-carrier photodiodes

Photonic generation of continuous millimetre- and sub-millimetre waves up to the THz range using antenna-integrated uni-travelling-carrier photodiodes is described. A device integrating a wideband log-periodic antenna exhibits a maximum output power of 2.6 ?W at 1.04 THz with good linearity. A module with a quasi-optical output port fabricated for practical use generates almost the same output power as the chip at around 1 THz and operates at frequencies of up to 1.5 THz. The output power level and the operation frequency are records for wideband photodiodes operating at 1.55 ?m. Devices integrating resonant narrowband dipole antennae have also been fabricated and the output power increases at resonant peak frequencies confirmed. The device having a peak at 1.04 THz exhibits a maximum (detected) output power of 10.9??W at 1.04 THz with good linearity. This output power is the highest value ever directly generated from a photodiode in the THz range, and several times higher than the maximum value reported by the low-temperature-grown GaAs photoconductive switch at around 1 THz.

Photonic generation of continuous THz wave using uni-traveling-carrier photodiode

A uni-traveling-carrier photodiode (UTC-PD) is monolithically integrated with a wideband log-periodic toothed antenna for generating millimeter and submillimeter waves at frequencies of up to terahertz range. A module with a quasi-optical output port fabricated for practical use operates up to 1.5 THz and generates an output power of 2.3 /spl mu/W at 1.04 THz with good linearity. The output power level and the operation frequency are records for wideband PD modules operating at 1.55 /spl mu/m. An investigation of the operational characteristics of the UTC-PD reveals that the effective use of the electron-velocity overshoot in the junction depletion layer is important for maximizing the output power in the terahertz range.

InP-InGaAs uni-traveling-carrier photodiode with improved 3-dB bandwidth of over 150 GHz

Uni-traveling-carrier photodiodes (UTC-PDs) with ultrafast response and high-saturation output are reported. It is experimentally demonstrated that the photoresponse of UTC-PDs is improved by incorporating a step-like potential profile in the photoabsorption layer. The fabricated device shows a peak electrical 3-dB bandwidth of 152 GHz at a low reverse bias voltage of -1.5 V. The output voltage can be increased to as high as 1.9 V at higher reverse bias voltages with the 3-dB bandwidth staying at over three-quarters of the maximum value. To our knowledge, the obtained response is the fastest among those reported for 1.55-/spl mu/m wavelength photodiodes.

Fabrication and characterization of high-performance InP/InGaAs double-heterojunction bipolar transistors

This paper is on high-performance InP/InGaAs double-heterojunction bipolar transistors (DHBTs) utilizing compositionally step-graded InGaAsP layers between the InGaAs base and InP collector to suppress the current blocking effect. These DHBTs exhibit current gains of 200 and excellent breakdown behavior. Moreover, the DHBTs permit collector current density levels J/sub C/ up to 3/spl times/10/sup 5/ A/cm/sup 2/ at V/sub CE/=1.5 V. A current gain cutoff frequency of 155 GHz and a maximum oscillation frequency of 90 GHz have been successfully obtained at J/sub C/=1.6/spl times/10/sup 5/ A/cm/sup 2/. We have also investigated electron transport properties in the InP collector using a set of DHBTs with different injection energies into the InP collector. By increasing the injection energies, electron velocity is found to decrease from 3.5/spl times/10/sup 7/ cm/s to 1.6/spl times/10/sup 7/ cm/s, due to increased population of upper valleys. This result clearly demonstrates the significant role of nonequilibrium /spl Gamma/-valley transport in determining the high-speed performance of InP/InGaAs DHBTs. >

Self-aligned AlGaAs/GaAs HBT with low emitter resistance utilizing InGaAs cap layer

A self-aligned AlGaAs/GaAs heterojunction bipolar transistor (HBT) with an InGaAs emitter cap layer that has very low emitter resistance is described. In this structure, a nonalloyed emitter contact allows the emitter and base electrodes to be formed simultaneously and in a self-aligned manner. The reduction of emitter resistance greatly improves the HBTs transconductance and cutoff frequency. In fabricated devices with emitter dimensions of 2 mu m*5 mu m, a transconductance-per-unit-area of 16 mS/ mu m/sup 2/ and a cutoff frequency of 80 GHz were achieved. To investigate high-speed performance, a 21-stage ECL ring oscillator was fabricated using these devices. Propagation delay times as low as 5.5 ps/gate were obtained, demonstrating the effectiveness of this structure. >

Uni-traveling-carrier photodiodes

A photodiode (PD) that can provide both high speed and high saturation output is of great importance for the development of future large-capacity fiber-optic communication systems and ultrafast measurement systems. A high output PD combined with an erbium-doped fiber amplifier can eliminate the need for a broadband electronic post-amplifier and thus enables us to construct photoreceivers with a much faster response. We recently developed a uni-traveling-carrier photodiode (UTCPD) that satisfies these requirements. We describe the characteristic features and photoresponse performance of the InP/InGaAs UTC-PD, and outline its application to fiber-optic communication systems.

Emitter-Base Junction Size Effect on Current Gain Hfe of AlGaAs/GaAs Heterojunction Bipolar Transistors

A current gain reduction with emitter-base junction size decrease is found in uniform base AlGaAs/GaAs heterojunction bipolar transistors (HBTs). This characteristic is analyzed using a model that takes into account a lateral diffusion of injected electrons, and is shown to be well explained by this model. The results show the importance of excess base leakage current due to minority carrier recombinations in the external bases of uniform base HBTs.

High-frequency characteristics of AlGaAs/GaAs heterojunction bipolar transistors

The fabrication and high-frequency performance of MBE-grown AlGaAs/GaAs heterojunction bipolar transistors (HBTs) is described. The achieved gain-bandwidth product f<inf>T</inf>is 25 GHz for a collector current density J<inf>c</inf>of 1 × 10⁴A/cm²and a collector-emitter voltage V<inf>CE</inf>of 3 V.f<inf>T</inf>continues to increase with the collector current in the high current density region over 1 × 10⁴A/cm²with no emitter crowding effect nor Kirk effect. The limitation on f<inf>T</inf>in fabricated devices is found to be caused mainly by the emitter series resistance.