Takao Kaneda

Impact ionization coefficients of electrons and holes in

The arsenic composition dependences of electron and hole ionization coefficients, α and β, in

Characteristics of germanium avalanche photodiodes in the wavelength region of 1-1.6 µm

Dark current, quantum efficiency, multiplication noise, and pulse response of germanium avalanche photodiodes with n+-p junction were studied to find an optimum structure. The dark current can be separated by graphical means into a leakage current component and a multiplied component which flows through the junction. The dark current components are also evaluated by using diodes with various diameters. The quantum efficiency and the multiplication noise are shown to be strongly affected by the n+ layer thickness. An n+ layer thickness optimized for signal-to-noise ratio is estimated from experimental and calculated results, using a figure of merit for avalanche photodiodes. The response waveform for mode-locked Nd:YAG laser shows a rise time of 100 ps and a half pulsewidth of less than 200 ps.

Tunneling Current in InGaAs and Optimum Design for InGaAs/InP Avalanche Photodiode

Breakdown voltage and dark current density in p+n and n+p In0.53Ga0.47As diodes are compared with theoretical values taking the backward tunneling current into account. Predominant origin of dark current in an InGaAs diode is attributed to the tunneling current. Using these results, optimum design of an InGaAs/InP avalanche photodiode (APD) to obtain low dark current, high multiplication gain, high quantum efficiency and fast response is also discussed.

Ionization rates for electrons and holes in GaAs

Ionization rates in GaAs are determined from the measurement of photocarrier multiplication. Pure electron and hole initiations are achieved by using the novel crater mesa structure and appropriate optical‐injection wavelengths. The ionization rates for holes are greater than that for electrons except at highest fields. This agrees with the studies of Stillman et al., except for the individual values. The ionization rates for electrons and holes are expressed as α=5.6×106 exp(−2.41×106/E) and β=1.5×106 exp(−1.57×106/E), respectively.

Crystal orientation dependence of ionization rates in germanium

Ionization rates in 〈111〉 and 〈100〉 germanium are determined experimentally. The ionization rates obtained are expressed as α=2.72×106 exp(−1.1×106/E), β=1.72×106 exp(−9.37×105/E) for 〈111〉 and α=8.04×106 exp(−1.4×106/E), β=6.39×106 exp(−1.27×106 /E) cm−1 for 〈100〉 where α and β are ionization rates for electrons and holes, respectively, and E is the electric field. Hole‐ to electron‐ionization‐rate ratios of 〈100〉 Ge are found to be greater than those of 〈111〉 Ge. The multiplication noise power of Ge avalanche photodiodes calculated by using the ionization rates obtained shows good agreement with experimental results.

A model for reach‐through avalanche photodiodes (RAPD’s)

A model for silicon RAPD’s, which have an n+–p–π‐p+ structure and are fabricated by using the ion‐implantation techniques for forming the p layer, is studied. Useful relations for both an excess noise factor and a temperature dependence of an avalanche breakdown voltage are derived and are found to be in good agreement with experiments. The diodes discussed are useful in optical communication systems because of a relatively low operating voltage for the higher quantum efficiency.

Improved germanium avalanche photodiodes

New kinds of germanium avalanche photodiodes with n+-n-p and p+-n structures were devised for improved excess noise and high quantum efficiency performance. Multiplication noise, quantum efficiency, and pulse response were studied and compared with those of the conventional n+-p structure diode. Multiplication noise of the new type of diodes were measured in the wavelength range between 0.63 and 1.52 μm. The effective ionization coefficient ratio of the p+-n diode was lower than unity at a wavelength longer than 1.1 μm and 0.6-0.7 at 1.52 μm, and that of the n+-n-p diode was 0.6-0.7 in the whole sensitive wavelength region. Response times were evaluated by using a mode-locked Nd:YAG laser beam and a frequency bandwidth wider than 1 GHz was estimated. Receiving optical power levels were compared with each other using parameters measured in this study.

Avalanche buildup time of silicon reach‐through photodiodes

The avalanche buildup time t, which is related to the multiplication factor (M) by t=τ1 M, where τ1 is the intrinsic response time, is studied for silicon reach‐through avalanche photodiodes (RAPD’s) by a shot‐noise measurement in the GHz region. The dependences of t on both the length of the avalanche region la and the wavelength exciting the avalanche process (λ) are investigated. The τ1 values obtained increase with la in the region la≳1.0 μm and also take larger values for λ∼6300 A than for λ∼8300 A. These values are in good agreement with a calculation using the modified Emmons equation τ1=Nkeffla/vs except for the very narrow avalanche region (la≲0.4 μm), where N is a constant dependent on la, keff is the effective ratio of hole to electron ionization rates, and vs is the carrier saturation velocity. These results are very useful to investigate the frequency response of RAPD’s. The diodes discussed are used extensively in fiber transmission systems.

Planar InP/GaInAsP/GaInAs buried‐structure avalanche photodiode

A new planar InP/GaInAsP/GaInAs buried‐structure avalanche photodiode, which has a buried active region and an n−‐InP guardring region, has been developed. A useful guardring effect and a uniform multiplication are obtained in the multiplication range of up to 30. The diode shows a dark current of 10 nA at a bias of 90% of the breakdown voltage, a maximum avalanche gain of 50 under 1.3‐μm light irradiation, and a gain‐bandwidth product of about 16 GHz.

Shallow‐junction p+‐n germanium avalanche photodiodes (APD’s)

Low‐noise and high‐quantum‐efficiency germanium APD’s have been investigated by a p+‐n structure. Boron implantation was used to form the p+ layer. Shallow p+‐n junctions have a lower excess noise that n+‐p junctions because holes have a higher ionization coefficient in germanium, and these p+‐n diodes have a higher hole‐to‐electron collection efficiency. Excess noise factors F≈7 at a multiplication factor of 10 are obtained at wavelength of ∼1.4 μm, whereas F≈11 for n+‐p diodes. An internal quantum efficiency of ∼80% is obtained at 1.15 μm.