Description

I. Description of InGaAs/InP APD Epitaxial Structure​
(I) Core Structural Design and Function​
InGaAs material can detect infrared radiation with a wavelength exceeding 1.6μm and is often used as the multiplication region of heterojunction diodes. Therefore, InGaAs/InP APD serves as an ideal photodetector for optical fiber communication with wavelengths of 1.3μm and 1.55μm. Its light absorption layer is made of InGaAs material, which has a relatively high absorption coefficient in the 1.3μm and 1.55μm optical bands, enabling efficient capture of optical signals.​
To prevent the breakdown of InGaAs homojunction from occurring before avalanche breakdown, the P-N junction needs to be placed in the InP window layer. Since the hole ionization coefficient of indium phosphide material is higher than the electron ionization coefficient, n-type InP is selected for the avalanche region. However, there is a large valence band barrier at the heterojunction interface between n-InP and n-InGaAs, which tends to cause the trapping of photogenerated holes. Therefore, an InGaAsP transition region with a graded band gap is inserted between them to form an optimized structure with separate absorption, grading, and multiplication, ensuring the efficiency of carrier transport.​
(II) Ideas for Structural Improvement​
Although InGaAs material is suitable for detecting wavelengths above 1μm, APDs with InGaAs structure have the problems of relatively high noise and dark current. To address this issue, an improved APD structure has been developed: materials with a wider band gap are used as the multiplication region, and materials with a narrower band gap are used as the light absorption region. Through the heterojunction design, the doping level of the multiplication region is reduced without affecting the normal operation of the light absorption region, effectively decreasing the tunnel current and thus optimizing the noise performance and operational stability of the device.​
II. About Avalanche Photodiode (APD)​
(I) Basic Structure and Operating State​
APD is a p-n junction type photodetector diode and a core photosensitive component in the field of laser communication. Its basic structure usually adopts the Read diode structure (i.e., N+PIP+ structure, with the P+-side for light reception), which is prone to generating the avalanche multiplication effect. During operation, an appropriate reverse bias voltage needs to be applied to bring the device into the avalanche multiplication state. At this time, its light absorption region and multiplication region are basically consistent, providing favorable conditions for carrier multiplication.​
(II) Working Principle​
The working principle of APD is based on the avalanche multiplication effect: by applying a relatively large reverse bias voltage to the P-N junction, the photogenerated carriers in the depletion layer are accelerated under the action of a strong electric field to obtain sufficiently high kinetic energy. These high-speed moving carriers collide with the crystal lattice and cause ionization, generating new electron-hole pairs; the newly generated carriers are then accelerated in the electric field, triggering a new round of impact ionization, forming an avalanche-like multiplication of carriers. Finally, current gain is achieved, significantly improving the detection sensitivity of optical signals.​
(III) Product Advantages and Application Scenarios​
APDs made of InGaAs/InP epitaxial materials have the notable advantages of fast response, low noise, and high reliability, and possess relatively high sensitivity in the spectral range of 900nm to 1700nm. Within the frequency range of 60MHz, the noise level intensified by the avalanche effect is lower than that generated by the combination of traditional diodes and external electronic amplifiers. Therefore, they are particularly suitable for distance measurement under low-level signals and the field of optical communication, and can provide accurate and stable optical signal detection support for related equipment.​