Photodiode Chip Bandwidth via Distributed Impedance Line
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Solution Overview
Problem
Existing photodiode designs face challenges in achieving bandwidths above 100 GHz without compromising responsivity, particularly due to limitations in transit time and RC limitations, which are exacerbated in advanced integrated designs like balanced detectors.
Innovation Solution
Implementing a high-impedance line with an impedance distributed over its length, at least twice that of the effective load impedance, to control the RC limitation and increase bandwidth, while maintaining a short, low-overshoot pulse response by using distributed line elements instead of discrete inductive elements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If inductive peaking with discrete inductive elements is used to increase bandwidth, then bandwidth is increased, but phase shift becomes highly non-linear and group delay spread increases
Solution Approach 1:
The patent divides the single discrete inductive element into multiple distributed inductive elements arranged along the signal path. This segmentation transforms the concentrated phase shift into a distributed phase shift, reducing non-linearity and group delay spread while maintaining bandwidth enhancement.
Solution Approach 2:
The patent transitions from a lumped element model (single inductor) to a distributed element model (transmission line with continuous inductance). This dimensional change from point-like to line-like structure allows phase shift to be distributed uniformly along the signal path, improving phase linearity.
2Speed
If the photodiode area or length is reduced to increase bandwidth, then RC limitation is reduced, but responsivity decreases
Solution Approach 1:
The patent replaces the mechanical/geometric approach (reducing photodiode area) with an electrical approach (adding distributed inductance). This substitution allows bandwidth enhancement through electrical circuit design rather than physical dimension changes, preserving both responsivity and bandwidth.
3Speed
If a bonding wire is used as inductor for inductive peaking, then bandwidth is increased, but device complexity and manufacturing precision requirements increase
Solution Approach 1:
The patent merges the inductive function with the existing transmission line structure. Instead of adding separate inductive elements, the inductance is distributed along the signal path that already exists in the photodiode package, combining multiple functions into a single structure.
Solution Approach 2:
The transmission line structure serves dual purposes: signal transmission and inductive peaking. The same physical structure that carries the signal also provides the distributed inductance needed for bandwidth enhancement, eliminating the need for separate inductive components.
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 allows for a significant increase in frequency response up to 110 GHz, with minimal overshoot and group delay spread, ensuring good pulse behavior and maintaining responsivity, applicable to both side-illuminated and vertically illuminated photodiodes.
Implementation Method 1
The connection from the photodiode mesa to the output pad is implemented with a high-impedance line with an impedance distributed over its length that is at least as high as the effective load impedance
Implementation Method 2
Implementing a high-impedance line with an impedance distributed over its length, at least twice that of the effective load impedance, to control the RC limitation and increase bandwidth
Data Source
Figure 1~2b
Figure 3a~3c
Figure 4
AI summary
The invention relates to a photodiode chip which has a great limit frequency and a junction from the active photodiode area of a photodiode mesa to the output pad of the high-frequency output of the photodiode chip. The aim of the invention is to further increase the bandwidth factor of photodiode chips. Said aim is achieved by establishing the connection from the photodiode mesa to the output pad by means of a high-resistance wire with impedance (Zleitung) which is spread across the length thereof and is at least as high as the load impedance (Zlast) effective at the output pad.