For high-speed photodetectors, the RC limit constant and carriers' transit time of a PIN-photodiode are still the dominant factors which play a pivotal role in determining the optical bandwidth, therefore a mushroom-like mesa structure was proposed in fabricating the devices. In such a structure, the length of the absorption layer is made shorter compared with the above and underneath highly doped layers, leading to a reduction in the respective capacitance without undesirable influence on the contact resistance of the device. However, this might come at the cost in processing difficulties, in which a particular care is vital important to precisely control the excessive undercut profile. The question in mind is that are there any additional drawbacks would be coming up?
Instead of mashroom-mesa form, the PN junction is set in a dome transparent plastic whose refractive index is between the junction material and air (or the propagated media for light).
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