Si Si Si Si As Si Si Si Si Si Si Si Si As Si Si Si Si Si Si Si Si As Si Si Si Si Si Si Si Si As Si Si Si Si Free electron Si Si Si Si B Si Si Si Si Si Si Si Si B Si Si Si Si Si Si Si Si B Si Si Si Si Si Si Si Si B Si Si Si Si A “hole” exists because there is no electron in the boron to form covalent bond here. Depletion area Diode symbol N P Holes Electrons − + Potential hill 11-9 Figure 11-19.  A potential hill.  Figure 11-18.  The lattice of boron doped silicon contains holes where the three boron valence shell electrons fail to fill in the combined  valence shells to the maximum of eight electrons. This is known as P-type semiconductor material or acceptor material.  Figure 11-17.  Silicon atoms doped with arsenic form a lattice work of covalent bonds. Free electrons exist in the material from the arsenic  atom’s 5th valence electron. These are the electrons that flow when the semiconductor material, known as N-type or donor material, is  conducting.  and the positive terminal is attached to the P-type material,  electricity can flow in the circuit. The negative potential of the  battery forces free electrons in the N-type material toward the  junction. The positive potential of the battery forces holes in  the P-type material toward the other side of the junction. The  holes move by the rebinding of the doping agent ions closer  to the junction. At the junction, free electrons continuously  arrive and fill the holes in the lattice. As this occurs, more  room is available for electrons and holes to move into the  area. Pushed by the potential of the battery, electrons and  holes continue to combine. The depletion area becomes  extremely narrow under these conditions. The potential hill  or barrier is, therefore, very small. The flow of current in  the electrical circuit is in the direction of electron movement  shown in  Figure 11-20 .