APD, APD fabrication method, detector, and laser radar system

A detector, comprising:

• an avalanche photodiode (APD), comprising:
• a first electrode layer;
• a substrate layer disposed on the first electrode layer;
• a buffer layer disposed on the substrate layer;
• a gain layer disposed above the buffer layer and comprising:
• a first gain unit comprising:
• a first multiplication layer;
• and
• a first charge layer disposed on the first multiplication layer;
• and
• a second gain unit comprising:
• a second multiplication layer disposed on the first charge layer;
• and
• a second charge layer disposed on the second multiplication layer;
• a gradient layer disposed a first distance above the second charge layer and a second distance above the second multiplication layer, wherein the first distance is less than the second distance;
• an absorption layer disposed on the gradient layer;
• a diffusion barrier layer disposed on the absorption layer;
• a contact layer disposed on the diffusion barrier layer;
• and
• a second electrode layer disposed on the contact layer;
• and
• a control circuit coupled to the APD and configured to receive an electrical signal from the detector for imaging.

A laser radar system, comprising:

• a transmitter optical module;
• a laser configured to transmit a laser pulse on a target object using the transmitter optical module;
• a receiver optical module configured to receive an echo pulse, wherein the echo pulse is a reflected signal of the laser pulse from the target object;
• a detector comprising an avalanche photodiode (APD), wherein the detector is configured to convert the echo pulse into an electrical signal, and wherein the APD comprises:
• a first electrode layer;
• a substrate layer disposed on the first electrode layer;
• a buffer layer disposed on the substrate layer;
• a gain layer disposed above the buffer layer and comprising:
• a first gain unit comprising:
• a first multiplication layer;
• and
• a first charge layer disposed on the first multiplication layer;
• and
• a second gain unit comprising:
• a second multiplication layer disposed on the first charge layer;
• and
• a second charge layer disposed on the second multiplication layer;
• a gradient layer disposed a first distance above the second charge layer and a second distance above the second multiplication layer, wherein the first distance is less than the second distance;
• an absorption layer disposed on the gradient layer;
• a diffusion barrier layer disposed on the absorption layer;
• a contact layer disposed on the diffusion barrier layer;
• and
• a second electrode layer disposed on the contact layer;
• and
• a signal processor coupled to the detector and configured to obtain, based on the electrical signal, a point cloud image corresponding to the target object.