Bruce DeVries, age 13, of Mundelein, Illinois, for his question:
How do semiconductors work?
Everybody's favorite semiconductor devices are radio transistors. They put music in your pocket, go where you go and ignore regular plug in house current. Such devices are smaller and neater, more durable and economical than old style radio and TV tubes. They are remarkable because they drive energy by making ordinary electrons perform extraordinary feats.
Wherever there is any sort of electrical energy, the key particles behind the scenes are moving electrons. Generators create house current by pushing electrons through copper wire circuits. Conductors and insulators are used to keep the power rolling. Copper is a conductor material because its atoms have loosely attached electrons that are prone to roam. Silicon is an .insulator because its crystalline lattice structure puts a stop to roaming electrons.
Our mighty generator systems depend upon conductors to carry the current and upon insulators to barricade the leakages. Semiconductors discard this old system and get electrical energy from insulators. Rectifiers, amplifiers, oscillators and dozens of other electrical devices prove that they work. But at present, scientists are not sure what makes these semiconductors work.
The secret is in the chemical recipe and what it does to an insulator's crystalline lattice. The main ingredient may be silicon but it must be infiltrated with a trace of some selected impurity. An N type semiconductor may be silicon with a trace of arsenic. When boron is the impurity in the silicon, it creates a P type material. Various N types and P types are molded in thin sheets. Electrical energy is created when opposite types are sandwiched together in slim wafers.
A semiconductor works because at the atomic level, negative and positive charges attract each other somewhat like the opposite poles of two magnets. Electrons are negative charges. In one of our sample recipes, atoms of arsenic added electrons to the silicon lattice. This made an N for negative type material. In.the other recipe, boron atoms removed electrons from the silicon lattice leaving holes. For reasons unknown, the holes in a P type material act like positive charges.
In wafers of P layers and N layers, opposite teams of potential energy face each other across junction zones. A boost from a small battery can start the action. Positive and negative charges attract each other across the junction, where their mobile breezes can be drawn off as electric current.
The most widely used insulator ingredients are silicon and germanium. Traces of arsenic, phosphorous or antimony may provide the mobile negative electrons in N type materials. Traces of aluminum, gallium or boron may add the positive holes in P type materials. Pure silicon may be a billion times better as an insulator and pure copper a billion times better as a conductor. But those mixed up semiconductors have mysterious talents of their own.