Industrial construction of pn-junction diodes?
Separately processed pieces of semiconductor mechanically joined won't work well as a pn junction because the facing surfaces need to be perfectly joined (as the crystalline structure level), and without and intermediate impurities (oxidation of the surface). In theory it would work, if you could align the crystals perfectly, but in practice it can't be done. 

Generally a pn junction is formed by taking a bulk doped semiconductor (e.g. n-type), and diffusing p-type impurity at the surface. This creates a pn junction beneath the surface of the semiconductor. The p portion is actually counterdoped - it contains n impurities, but more p impurities, giving a new p-type characteristic.