I have found lots of papers on AlGaN/GaN HEMT, but I am still searching for a paper which have reported all aspects of the device in detail. I need to cross check my calculations and simulations. Any recent paper would be more useful.
This paper by Umesh Mishra is I think the best paper for an overview of AlGaN HEMTs: http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=1021567
And I really like this paper by Gaudenzino Meneghesso that gives an overview of the breakdown properties and aids understanding of device operation: http://iopscience.iop.org/1347-4065/53/10/100211/article
This paper is more recent and gives a detailed exploration into the band structure and polarisation charges: http://cpb.iphy.ac.cn/fileup/PDF/2015-6-067301.pdf
maybe, my recently published book, titled "Basic Properties of III-V Devices - Understanding Mysterious Trapping Phenomena" (2014) may be useful. Construction and electrical performance of AlGaN/GaN HEMTs are discussed in detail as well as polarization properties in GaN based power HEMTs.
the book has been published by Kassel University Press. Also Amazon offers the book. Please, enter the title in GOOGLE, and you will find further details, e.g. contents, introduction, and you can also search for special topics in the book ("GOOGLE book search").