Can we take one slab of p-type semiconductor and physically join it to another n-type semiconductor to get p-n junction?
No, we cannot form a p-n junction by physically joining a p-type semiconductor slab to an n-type semiconductor slab. Any slab, howsoever flat, will have roughness much larger than the inter-atomic crystal spacing (~2 to 3 Å). Therefore, continuous contact at the atomic level will not be possible, and the junction will behave as a discontinuity for the flowing charge carriers.
Explanation
The textbook explicitly answers this in Example 14.3. The key point is that physical joining cannot achieve atomic-level contact because surface roughness (even on polished slabs) exceeds the inter-atomic spacing of 2-3 Å. A proper p-n junction requires continuous atomic contact, which is achieved through doping processes where part of a p-type wafer is converted to n-type, creating a metallurgical junction. This allows diffusion and drift processes to occur properly across the junction.