“On the History of the Numerical Methods Solving the Drift Diffusion Model”, by Bernd Meinerzhagen, is an excellent review article on the history of TCAD simulation methods. It discusses the importance of that Scharfetter-Gummel discretization and the use of box integration methods.
https://doi.org/10.33079/jomm.20030403
Unfortunately, the above link does not work, but the article can be found here:
http://www.jommpublish.org/p/61/
In addition, it contains reference to many of the numerical methods employed in TCAD device simulation in order to achieve robustness in TCAD software. I encourage every TCAD software developer to read this paper from an important contributor to the field.