It seems that this is my week to read papers. Well, it's a good exercise... I strongly recommend it!
Another quite interesting paper: Analytical Charge and Capacitance Models of Undoped Cylindrical Surrounding-Gate MOSFETs. This time, it is from Oana Moldovan et al. This is a quite good paper, that continues their work on DG MOSFETs.
Another one from the same journal: Modeling of Surrounding Gate MOSFETs With Bulk Trap States. It is also interesting to read. Anyway, the approach is different to the previous one, and it is good to keep an open mind.
And the last one from this journal: Surface-Potential Solution for Generic Undoped MOSFETs With Two Gates. The topic has been discussed before, and I still think that the approach by Francisco and Adelmo is easier to understand. However, this is only an opinion.