Avalanche Photodiode Mesa Dark Current Reduction
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Solution Overview
Problem
The inverted APD structure struggles to restrict dark current and tunnel current from mesa surfaces, leading to increased operating voltage and risk of edge breakdown, as the surface band gap level induces electronic channels and electric field concentrations.
Innovation Solution
Incorporating a depletion control region that prevents the p-type electric field control layer from being depleted, achieved by inactivating donors in the n-type electric field control layer or removing the n-type electric field control layer in an encircling portion, maintaining incomplete depletion and reducing electric potential drops in the light absorbing layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the inverted APD structure is used with p-type light absorbing layers, then receiver sensitivity is improved, but dark current from mesa surfaces increases due to surface band gap levels inducing electronic channels
Solution Approach 1:
An electron transit layer with low impurity concentration is introduced as an intermediary between the avalanche multiplier layer and the n-type electrode buffer layer. This layer has a larger band gap than InGaAs (e.g., InP and InAlAs), which suppresses the formation of electronic channels at the mesa surface while allowing electrons to transit from the light absorbing layers to the avalanche multiplier layer, thus reducing dark current without compromising receiver sensitivity
Solution Approach 2:
The electron transit layer is specifically positioned only in the region where electric field concentration occurs at the mesa surface (between the avalanche multiplier layer and n-type electrode buffer layer), providing localized suppression of electronic channel formation where it is most needed, while maintaining the overall device structure and functionality
2Reliability
If the inverted APD structure is used, then avalanche excess noise is reduced, but operating voltage increases and edge breakdown risk increases due to electric field concentration at exposed surfaces
Solution Approach 1:
The electron transit layer acts as a mediator that reduces electric field concentration at the mesa surface by providing a region with larger band gap and lower impurity concentration. This intermediary layer distributes the electric field more evenly, preventing excessive field concentration that would lead to edge breakdown, thereby allowing lower operating voltages while maintaining good avalanche excess noise characteristics
Solution Approach 2:
The electron transit layer is positioned beforehand between the avalanche multiplier layer and n-type electrode buffer layer to preemptively cushion against electric field concentration and edge breakdown before they can occur during device operation, providing protective buffering against harmful electric field effects
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach effectively reduces dark current from mesa surfaces and lowers operating voltage, minimizing the risk of edge breakdown and tunnel current while maintaining excellent receiver sensitivity.
Implementation Method 1
a depletion control region that is provided in layers on the second mesa side relative to the p-type electric field control layer (5) of the first mesa (101), formed in an encircling portion (14) provided inside an outer circumference of the first mesa (101) as viewed from a laminating direction and encircling an outer circumference of the second mesa (102), and prevents the encircling portion of the p-type electric field control layer (5) from being depleted when bias is applied
Implementation Method 2
an avalanche multiplier layer (6)
Data Source
Figure 1(a)~1(b)
Figure 2
Figure 3
AI summary
This invention provides an APD which can reduce a dark current derived from a mesa surface. An APD 301 is provided with the semi-insulating substrate 1, a first mesa 101 having a first laminate constitution in which a p-type electrode layer 2, a p-type light absorbing layer 3A, a light absorbing layer 3B with a low impurity concentration, a band gap inclined layer 4, a p-type electric field control layer 5, an avalanche multiplier layer 6, an n-type electric field control layer 7A, and an electron transit layer 7B with a low impurity concentration are stacked in this order on a surface of the semi-insulating substrate 1, a second mesa 102 having an outer circumference provided inside an outer circumference of the first mesa 101 as viewed from the laminating direction and having a second laminate constitution in which an n-type electrode buffer layer 8A and an n-type electrode layer 8B are stacked in this order on a surface on the electron transit layer 7B side, and a depletion control region 11 that is provided in layers on the second mesa 102 side relative to the p-type electric field control layer 5, formed in an encircling portion 14 provided inside an outer circumference of the first mesa 101 and encircling an outer circumference of the second mesa 102, and prevents the encircling portion of the p-type electric field control layer 5 from being depleted when bias is applied.