Katsuhiko Nishida

Reliability of Light Emitters and Detectors for Optical Fiber Communication Systems

Life test results on light emitters and detectors showed promisingly long lifetimes for short and long wavelength optical fiber communication systems. Data are presented on 0.85 μm AIGaAs LDs and LEDs, Si APDs and p-i-n-PDs, 1.3 μm InGaAsP LDs and LEDs, and Ge APDs and PDs. Degradation mechanisms and counteractions are also discussed.

Room-temperature bonding of heterogeneous materials for near-infrared image sensor

A room-temperature bonding technique using cone-shaped microbumps with the aid of ultrasonic vibration is applied to the fabrication of a near-infrared (NIR) image sensor. The image sensor is fabricated using the chip-on-chip integration of an InGaAs photodiode array on an InP substrate and a Si CMOS readout IC. The pixel pitch is 25 µm to compose quarter-VGA class (320 × 256 pixels) resolution. A high-quality imaging of a heated object is demonstrated. Bonding of the VGA array with 15 µm pitch is attempted to realize a high-resolution image sensor.

Gain-Enhanced InGaAs–InP Heterojunction Phototransistor With Zn-Doped Mesa Sidewall

A gain-enhanced InGaAs-InP heterojunction phototransistor (GE-HPT) with a Zn-doped mesa sidewall designed to reduce both base-emitter recombination currents and base-collector dark currents is realized. The developed GE-HPT has an optical conversion efficiency as large as 24 kA/W at an incident optical power of 29 nW and a wavelength of 1.55 mum, which is the highest gain yet reported in HPTs. The measured dark current in the base-collector junction is comparable to that of planar-type PIN photodiodes. The high gain and low dark current characteristics of the phototransistors make them promising for use in a weak light detection system.

Effects of Zn Doped Mesa Sidewall on Gain Enhanced InGaAs/InP Heterobipolar Phototransistor

The excellent detectability of the gain enhanced InGaAs/InP heterobipolar phototransistor (GE-HPT) is demonstrated and attributed to a reduction in the reverse leakage current at the base-collector junction and the enhancement of current gain at the emitter-base junction achieved by using a current blocking structure with a Zn doped mesa sidewall. The common emitter grounded current gain agrees well with the photo-conversion efficiency of several tens of thousands of A/W at incident optical powers in the hundred nanowatt to sub-picowatt range over several orders of magnitude. The deep mesa structure in the GE-HPT is also effective in ensuring superior isolation of better than 25 dB between adjacent arrays.

Optimization of Multiplication Factor of Avalanche Photodiode

General formulas for the photomultiplication factor Mp are derived by considering the dark current injected into the avalanche region and the space charge effect due to the avalanche current. The numerical calculations were carried out in case of the silicon PN junction. The optimum impurity concentration to maximize Mp in the avalanche region is derived for a given junction area and a given load resistance. The analyses also indicate Mp is much improved with decreasing the dark current. The analytical results are well confirmed in the silicon P+ PNN+ diode with an engraved guard ring. The junction uniformity was improved by gallium diffusion ( P) preceding boron diffusion ( P+). Both the spatial variation of Mp in the junction plane and the avalanche noise were reduced to one tenth of those of the conventional P+ N junction diodes.

Threshold Current Density and Power Saturation in Read Diode

In p+-n-ν-n+ silicon diodes, the oscillation characteristics have been examined by varying both the impurity density Nd and the thickness of the drift region. An empirical relation between the threshold current Ith and the frequency f was Ith=5f2 A/cm2, which has been verified theoretically. A relation between the current density Isat, at which the microwave output power is saturated, and f was also obtained experimentally as Isat=130f A/cm2. This result indicates that the decrease of electron velocity just behind the pulsed conduction current in the drift region is the most probable cause of power saturation. In the diode having a fixed drift length, since a higher Nd allows a larger Isat, a higer output power could result when the Nd was higher. The maximum pulse power of 5W at 8 GHz was obtained when the ratio of the avalanche length to the drift length was approximately 10~20%.

Fabrication of VGA size near-infrared image sensor using room-temperature flip-chip bonding technology

Room-temperature bonding technique using cone-shaped microbumps with the aid of ultrasonic vibration is applied to the fabrication of a near-infrared image sensor. The image sensor is fabricated using the flip-chip bonding of an InGaAs/InP photodiode-array chip and a Si CMOS readout IC chip. The pixel pitch is 15 μm to compose VGA class (640 × 512 pixels) resolution. High-quality imaging of a heated object and blood vessels of human hand is demonstrated.