Depletion Region (Space Charge Region)
When a p-type material and n-type material meet, there’s a rapid diffusion of electrons and holes from one side to the other, due to the excess of holes in one side and excess of electrons in the other side. As you know, electrons and holes typically want to combine. So they do. But, once they combine, there are no majority carriers because all the electrons and holes are tied together and all you have left are charged particles. As all the majority carriers are “depleted”, it’s called the depletion region.
The area near a pn junction on both sides that has no majority carriers.
Electronic Devices : Conventional Current Version, 9th Edition by Thomas L. Floyd
This region of uncovered positive and negative ions is called the depletion region due to the “depletion” of free carriers in the region.
Electronic Devices and Circuit Theory, 11th Edition by Robert L. Boylestad & Louis Nashelsky
A space charge region (SCR), depleted of mobile carriers, describes the region immediately around the metallurgical junction. This region is also often called the depletion region or depletion layer.
Microelectronic Circuit Design, 4th Edition by Richard C. Jaeger & Travis N. Blalock