High performance avalanche photodiodes were designed and fabricated for the application of high speed optical fiber communication system. Electric fields in the multiplication layer at an arbitrary bias voltage were calculated for the design of InP/InGaAs APD with HI-LO doping structure. From the knowledge of electric field distribution, the solution of avalanche gain equation can be obtained. As a result, charge density of charge sheet layer should lie in the range between $3.0\times10^{12}cm^{-2}$ and $3.5\times10^{12}cm^{-2}$ for the high performance in excess of 60 GHz of gain-bandwidth product. Higher charge density requires more difficult fabrication techniques while lower charge density shows lower performance of APD.
APD was fabricated by the MOCVD-grown InP/InGaAs epitaxial film which have 1.5 ㎛-thick high purity InGaAs absorption layer and n-InP charge sheet layer with charge density of $3.0\times10^{12}cm^{-2}$. Charge plate was formed by $CH_4/H_2$ reactive ion etching (RIE). InP etching of 500~800 A was successfully obtained and controlled. On the top of this etched surface, undoped InP multiplication layer was overgrown by the MOCVD. Crystal defects, which are induced during RIE, are completely removed during MOCVD regrowth. This was confirmed by the very low dark current less than 5 nA at 0.9 $V_B$. Zn diffusion was performed by sealed ampoule method for the pn junction formation. The diffused depth controllability of ±0.1 ㎛ makes it possible to control the multiplication layer thickness of 0.15∼0.4 ㎛
The breakdown voltages of fabricated APDs were 38∼46 V, and dark currents at 0.9 $V_B$ were extremely low. The calculated thickness of multiplication layer was 0.15∼0.3 ㎛ from the breakdown voltage and photocurrent analysis. The -3dB bandwidth at G=20 exceeded 3 GHz. The expected gain-bandwidth products were 80 ∼ 100 GHz. The gain curve obtained from the photocurrent are well fitted with the calculated curve. This is one evidence that APD was well designed. The temperature coefficient(γ) measurement showed that γ is between 0.6 and $1.5\times10^{-3}/\circ C$ and has breakdown voltage dependence. From the analysis of $V_B$-dependence of $\gamma$, we can obtained empirical formula which is applicable for the expectation of $V_B$(T) without measurement. The application of 2.5 Gbps optical receiver with APD-FET type shows sensitivity of -33.7 dBm at $10^{-9}$ BER and PRBS(pseudo random binary sequence)=$2^{23}-1$ conditions.
