Silicon is an atomic element that contains four electrons in  its valence shell. It tends to combine readily with itself and  form a lattice of silicon atoms in which adjacent atoms share  electrons to fill out the valance shell of each to the maximum  of eight electrons.  [Figure 11-16]  This unique symmetric  alignment of silicon atoms results in a crystalline structure.  required for electron flow in a semiconductor are discussed  in the following paragraphs. When silicon is doped with an element or compound  containing five electrons in its valence shell, the result is a  negatively charged material due to the excess free electrons,  and the fact that electrons are negatively charged. This is  known as an N-type semiconductor material. It is also known  as a donor material because, when it is used in electronics, it  donates the extra electrons to current flow.  Once bound together, the valence shells of each silicon atom  are complete. In this state, movement of electrons does not  occur easily. There are no free electrons to move to another  atom and no space in the valence shells to accept a free  electron. Therefore, silicon in this form is a good insulator. Silicon is a primary material used in the manufacture of  semiconductors. Germanium and a few other materials are  also used. Doping silicon can also be performed with an element that  has only three valence electrons, such as boron, gallium, or  indium. Valence electron sharing still occurs, and the silicon  atoms with interspersed doping element atoms form a lattice  molecular structure. However, in this case, there are many  valence shells where there are only seven electrons and not  eight. This greatly changes the properties of the material.  The absence of the electrons, called holes, encourages  electron flow due to the preference to have eight electrons  in all valence shells. Therefore, this type of doped silicon is  also semiconductive. It is known as P-type material or as an  acceptor since it accepts electrons in the holes under certain  conditions.  [Figure 11-18] Combining N- and P-type semiconductor material in certain  ways can produce very useful results. A look at various  semiconductor devices follows. Since silicon is an insulator, it must be modified to become  a semiconductor. The process often used is called doping.  Starting with ultra-pure silicon crystal, arsenic, phosphorus,  or some other element with five valence electrons in each  atom is mixed into the silicon. The result is a silicon lattice  with flaws.  [Figure 11-17]  The elements bond, but numerous  free electrons are present in the material from the 5$^{th}$ electron  that is part of the valence shell of the doping element atoms.  These free electrons can now flow under certain conditions.  Thus, the silicon becomes semiconductive. The conditions  A diode is an electrical device that allows current to flow in  one direction through the device but not the other. A simple  device that can be made from N- and P-type semiconductors  is a semiconductor diode. When joined, the junction of these  two materials exhibits unique properties. Since there are  holes in the P-type material, free electrons from the N-type  material are attracted to fill these holes. Once combined, the  area at the junction of the two materials where this happens  is said to be depleted. There are no longer free electrons or  holes. However, having given up some electrons, the N-type  material next to the junction becomes slightly positively  charged, and having received electrons, the P-type material  next to the junction becomes slightly negatively charged.  The depletion area at the junction of the two semiconductor  materials constitutes a barrier or potential hill. The intensity  of the potential hill is proportional to the width of the  depletion area (where the electrons from the N-type material  have filled holes in the P-type material).  [Figure 11-19] The two semiconductors joined in this manner form a diode  that can be used in an electrical circuit. A voltage source  is attached to the diode. When the negative terminal of the  battery is attached to the N-type semiconductor material  11-8 Figure 11-16.  The silicon atoms with just the valence shell electrons  share these valence electrons with each other. By sharing with four  other silicon atoms, the number of electrons in each silicon atom  valence shell becomes eight, which is the maximum number. This  makes the substance stable and it resists any flow of electrons.  Semiconductor Diodes Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Si Valence electrons