Solid-State Devices Semiconductors Solid-state devices began replacing vacuum tube electron  control valves in the late 1950s. Their long life, reliability,  and resilience in harsh environments make them ideal for  use in avionics.  The key to solid-state electronic devices is the electrical  behavior of semiconductors. To understand semiconductors,  a review of what makes a material an insulator or a conductor  follows. Then, an explanation for how materials of limited  conductivity are constructed and some of their many uses is  explained. Semiconductor devices are the building blocks of  modern electronics and avionics. An atom of any material has a characteristic number of  electrons orbiting the nucleus of the atom. The arrangement  of the electrons occurs in somewhat orderly orbits called rings  or shells. The closest shell to the nucleus can only contain  two electrons. If the atom has more than two electrons, they  are found in the next orbital shell away from the nucleus.  This second shell can only hold eight electrons. If the atom  has more than eight electrons, they orbit in a third shell  farther out from the nucleus. This third shell is filled with  eight electrons and then a fourth shell starts to fill if the  element still has more electrons. However, when the fourth  shell contains eight electrons, the number of electrons in the  third shell begins to increase again until a maximum of 18 is  reached.  [Figure 11-13] The outer most orbital shell of any atom’s electrons is called  the valence shell. The number of electrons in the valence shell  determines the chemical properties of the material. When  the valence shell has the maximum number of electrons, it  is complete, and the electrons tend to be bound strongly to  the nucleus. Materials with this characteristic are chemically  stable. It takes a large amount of force to move the electrons  in this situation from one atom valence shell to that of  another. Since the movement of electrons is called electric  current, substances with complete valence shells are known  as good insulators because they resist the flow of electrons  (electricity).  [Figure 11-14] In atoms with an incomplete valence shell, that is, those  without the maximum number of electrons in their valence  shell, the electrons are bound less strongly to the nucleus.  Not all materials are pure elements, that is, substances  made up of one kind  of atom. Compounds occur when two  or more different type s of atoms combine. They create a  new substance with different characteristics than any of the  component elements When compounds form, valence shells  and their maximum number of electrons remain the rule of  physics. The new compound molecule may either share  electrons to fill the  valence shell or free electrons may exist  to make it a good conductor.  Electrons in incomplete valence shells may also move freely  from valence shell to valence shell of different atoms or  compounds. In this case, these are known as free electrons.  As stated, the  movement of electrons is known as electric  current or current flow. When electrons move freely from  atom to atom or compound to compound, the substance is  known as a conductor.  [Figure 11-15] The material is chemically disposed to combine with other  materials or other ide ntical atoms to fill in the unstable  valence configuration and bring the number of electrons  in the valence shell to  maximum. Two or more substances  may share the electrons in their valence shells and form a  covalent bond. A covalent bond is the method by which atoms  complete their valence shells by sharing valence electrons  with other atoms. Figure 11-14.  Elements w ith full valence shells are good insulators.  Most insulators used in  aviation are compounds of two or more  elements that share electrons to fill their valence shells.  Figure 11-15.  The valence shells of elements that are common  conductors have one (or three) electrons.  Figure 11-13.  Maximum number of electrons in each orbital shell  of an atom.  11-7 Felium  Neon Argon Krypton Ne He Ar Kr Au Ag Cu Al Aluminum  Copper Silver Gold Maximum number of electrons Shell or Orbit Number 1 2 3 4 5 2 8 18 32 50