Quadrant silicon avalanche photodiodes (APDs) were fabricated by standard 0.18µm CMOS process, and were characterized at 405nm wavelength for Blu-ray applications. The size of each APD element is 50×50µm2. The dark current was 10pA at low bias voltage, and low crosstalk of about -80dB between adjacent APD elements was achieved. Although the responsivity is less than 0.1A/W at low bias voltage, the responsivity is enhanced to more than 1A/W at less than 10V bias voltage due to avalanche amplification. The wide bandwidth of 1.5GHz was achieved with the responsivity of more than 1A/W, which is limited by the capacitance of the APD. We believe that the fabricated quadrant APD is a promising photodiode for multi-layer Blu-ray system.

nMOS-type and pMOS-type silicon avalanche photodiodes (APDs) were fabricated by standard 0.18µm CMOS process, and the current-voltage characteristic and the frequency response of the APDs with and without guard ring structure were measured. The role of the guard ring is cancellation of photo-generated carriers in a deep layer and a substrate. The bandwidth of the APD is enhanced with the guard ring structure at a sacrifice of the responsivity. Based on comparison of nMOS-type and pMOS-type APDs, the nMOS-type APD is more suitable for high-speed operation. The bandwidth is enhanced with decreasing the spacing of interdigital electrodes due to decreased carrier transit time and with decreasing the detection area and the PAD size for RF probing due to decreased device capacitance. The maximum bandwidth was achieved with the avalanche gain of about 10. Finally, we fabricated a nMOS-type APD with the electrode spacing of 0.84µm, the detection area of 10×10µm2, the PAD size for RF probing of 30×30µm2, and with the guard ring structure. The maximum bandwidth of 8.4GHz was achieved along with the gain-bandwidth product of 280GHz.

We designed and fabricated 4H-SiC PIN avalanche photodiodes (APD) for UV detection. The thickness of an intrinsic layer in a PIN structure was optimized in order to achieve the highest quantum efficiency at the wavelength of interest. The optimized 4H-SiC PIN APDs exhibited a maximum external quantum efficiency of >80% at the wavelength of 280 nm and a gain greater than 40000. Both electrical and optical characteristics of the fabricated APDs were in agreement with those predicted from simulation.

This paper describes the recent advances in semiconductor photodiodes for use in ultra-high-speed optical systems. We developed two types of waveguide photodiodes (WG-PD) -- an evanescently coupled waveguide photodiode (EC-WG-PD) and a separated-absorption-and-multiplication waveguide avalanche photodiode (WG-APD). The EC-WG-PD is very robust under high optical input operation because of its distribution of photo current density along the light propagation. The EC-WG-PD simultaneously exhibited a high external quantum efficiency of 70% for both 1310 and 1550 nm, and a wide bandwidth of more than 40 GHz. The WG-APD, on the other hand, has a wide bandwidth of 36.5 GHz and a gain-bandwidth product of 170 GHz as a result of its small waveguide mesa structure and a thin multiplication layer. Record high receiver sensitivity of -19.6 dBm at 40 Gbps was achieved. Additionally, a monolithically integrated dual EC-WG-PD for differential phase shift-keying (DPSK) systems was developed. Each PD has equivalent characteristics with 3-dB-down bandwidth of more than 40 GHz and external quantum efficiency of 70% at 1550 nm.

The performance of avalanche photodiodes with deep guard rings for Geiger mode operation is studied. The electric field distribution is calculated using the finite element method and the carrier multiplication characteristic is calculated along typical lines in the device. The nonlinear dependence of the ionization rates on the electric field strength can make a guard ring less effective in Geiger mode operation. The maximum single photon detection efficiency that can be obtained without breakdown at the guard ring is calculated for several structure parameters. It is shown that the single photon detection efficiency strongly depends on the guard ring design.

Competition of two-photon and one-photon absorption in Si-APD was studied. Device should be cooled down in order to clearly observe two-photon absorption at low illumination intensity. Passive Geiger mode operation was studied to sensitively detect small number of carriers generated by two-photon absorption. The illumination intensity dependence of the photocurrent pulse count number is well explained by taking into account the two absorption mechanisms and a dead time period that depends on bias voltage.

The performance of an indium-gallium-arsenide avalanche photodiode serving as a 1550 nm single-photon detector is investigated. Quantum efficiency is evaluated for laser pulses with an average of < 0.015 photons per pulse, which are important for the demonstration of unconditionally secure quantum key distribution [G. Brassard et al.: Phys. Rev. Lett. 85, 6, p.1330 (2000)]. An operating temperature of 243 K is achieved by thermo-electrical cooling, yielding a quantum efficiency of 18% with a dark-count probability per gate of 2.8 10-5. The results obtained here guarantee unconditionally secure fiber-optic quantum key distribution over 50 km.

Detection efficiency and dark count of a Geiger mode single photon detection avalanche photodiode was studied by a numerical simulation. The ionization process triggered by a single hole injection was simulated at a bias voltage slightly greater than the avalanche breakdown voltage for calculation of the detection efficiency. Tunneling effect in the multiplication layer was taken into account for the dark count simulation. In the gated-mode operation, the avalanche build-up time also affects on the signal to noise ratio. The multiplication layer thickness is a key parameter for the device performances.