Question

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?

Answer 1

if we physically join them then diffusion of majority charge carriers wont occur as there will be a thick membrane stopping diffusion of majority charge carriers....

Answer 2

p-n junction does not work

Explanation:

p-type semiconductors have holes as majority carriers and n-type semiconductors have electrons as majority. When a p-n junction is formed, the holes from p and electrons from n move to the other type, so the width of the depletion layer widens. A p-n junction works if there is a continuous structure when joined. But if we physically join the two semiconductors, there will always be very small microscopic gap between the two. Hence it won't work.