Why can't we take one slab of p-type semiconductor and physically join it to another slab of n type semiconductor to get p-n junction?

Hi Vikas!

Good question!! The answer to this question is as follows: A P-N junction cannot be made by simply joining the two pieces together. A P-N junction must be a continuous crystal structure. When we join the pieces of P- type and N-type semiconductors together, there will be microscopic discontinuity at the boundary of the semiconductors. So, a P-N junction will not form.

Hope this is clear to you.