Hiroshi Fushimi

Photoresponse characteristics of uni-traveling-carrier photodiodes

Uni-traveling-carrier photodiode (UTC-PD) is a newly developed high-speed photodiode for generating high output current. The high saturation current is realized by its operation mode in which only electrons are used as active carriers. The bandwidth of a UTC-PD increases with increasing photocurrent level, which results from the fact that electron transport changes from diffusive to drift/diffusive motion due to the self-induced field in the absorption layer. In this report, photoresponse characteristics of InP/InGaAs UTC-PDs, which depend on device parameters and operation conditions, are discussed. Theoretical analysis for calculating 3-dB-down bandwidth as a function of operation current is also presented. The charge-control model for photodiodes, used in this study, allows us to predict small-signal bandwidth by numerical calculation.

Low-driving-voltage electro-optic modulator with novel KTa1-xNbxO3 crystal waveguides

We have successfully demonstrated a low-driving-voltage electro-optic (EO) modulator using newly developed KTa1-xNbxO3 (KTN) buried waveguides. We prepared high-quality and large KTN crystals. The crystals exhibited a large quadratic EO coefficient of 4.8×10-15 m2/V2 at 1.55 µm. The KTN crystals also exhibited a very large linear EO effect, for example 600 pm/V at a biased potential of 60 V/mm, which is twenty times larger than the r33 of LiNbO3. The waveguide layers for the core and cladding were formed by liquid phase epitaxy techniques on the KTN crystals. The propagation loss was less than 0.5 dB/cm and the polarization dependent loss was less than 0.1 dB/cm. The EO modulators had a low Vπ of <2.5 V (4.0-V biased potential, l=6 mm) and a fast response of 3 GHz.

High-power photonic microwave generation at K- and Ka-bands using a uni-traveling-carrier photodiode

A K- to Ka-band photonic microwave generator (PMG) consisting of a uni-traveling-carrier photodiode (UTC-PD) and a monolithically integrated bias circuit utilizing a 1/4-wavelength coplanar waveguide is presented. The device exhibits a high-saturation-output power of +14 dBm at 26 GHz for a bias voltage of -4 V. The output power is almost constant within a frequency range from 23 to 29 GHz. The 3-dB down bandwidth of the generator is as wide as 20 GHz, which is in good agreement with the circuit model calculation. Two types of devices, one with and one without a dc-cut capacitor, exhibit almost the same input-output characteristics. The device for the operation in the 38-GHz band is also fabricated, and it shows a maximum millimeter-wave (mm-wave) output power of +9 dBm. The PMG is suitable for the broad-band high-output fiber-radio applications.

Photoresponse nonlinearity of a uni-traveling-carrier photodiode and its application to optoelectronic millimeter-wave mixing in 60 GHz band

Nonlinear photoresponse observed in the pulsed operation of an InP/InGaAs uni-traveling-carrier photodiode (UTC-PD) is analyzed in terms of the dynamic capacitance associated with charge storage in the absorption layer. To model such an effect, a variable carrier traveling delay time which changes with operation voltage and current is introduced. Optoelectronic millimeter-wave mixing for 59.4-GHz and 600-MHz-modulated 1.55 µm optical signals was examined, and found to efficiently generate a 60 GHz up-converted signal. The characteristics are semiquantitatively explained by the dynamic capacitance model.

Influence of Hole Accumulation on Source Resistance, Kink Effect and On-State Breakdown of InP-Based High Electron Mobility Transistors: Light Irradiation Study

The influence of impact-ionized holes accumulating in the device was studied for InP-based InAlAs/InGaAs HEMTs using light irradiation experiments. Two parasitic phenomena arise from the generation of electron-hole pairs and subsequent accumulation of holes in the body of the device: The hole accumulation in the source region causes the decrease in the source resistance, which eliminates the kink effect. The hole accumulation in the gate region shifts the threshold voltage. This could be a positive feedback that increases the drain current at the on-state breakdown of the device.