Electronic Switching

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In railroading a switch diverts a train from one track to another. In plumbing, a valve can redirect the flow of water between pipes. In early electronics, directing electrons along a certain path was controlled by what North American scientists called a tube, while British scientists referred to it as a valve. That device controlled the current flow in a high vacuum between electrodes to which a difference in electrical potential had been applied.

The simplest vacuum tube was the diode. It acted much like a check valve that prevents water from flowing in the wrong direction. The electronic check valve featured a heated, electron-emitting cathode and an anode. Electrons could only flow in one direction, from the cathode to the anode. Electronic control grids within the bottle could exert more precise control by varying the voltage on the grid.

“Transistor” was a term coined by John Robinson Pierce of Bell Laboratories. He borrowed parts of two relevant words because it selectively transfers an electrical current across a resistor. Its function is still best understood in that transference and resistance context. Most, but not all, of the vacuum tube’s early functions are now performed by transistors. Audiophiles still prefer what they sometimes call “little tone bottles.”

Many mechanical switches have been replaced by transistorized electronic switches. Transistors are made from silicon. This chemical element is not a good conductor of electricity. It is rather a semiconductor. This means it’s not really a conductor or an insulator. When it is treated with impurities, a process known as doping, we can alter its natural behavior. Doping silicon with the arsenic, phosphorus, or antimony, causes it to gain some extra “free” electrons that can carry an electric current. Electrons have a negative charge so treating silicon in this way puts it in the category of negative or n-type semiconductors.

If we dope silicon with different impurities such as boron, gallium, and aluminum, it will have less than its fair share of free electrons. This causes it to attract electrons from nearby materials. This positive type of silicon is referred to as p-type. Both the n-type and p-type materials are electrically neutral in that the silicon actually has no charge in itself. It is true that n-type silicon has extra “free” electrons that increase its conductivity in one way, and that p-type silicon has fewer of those free electrons thus helping it to increase its conductivity in the opposite way.

The enhanced directional conductivity results from adding neutral atoms of so called impurities. The silicon was also neutral prior to the doping and remains uncharged thereafter. The essential concept is that the presence or absence of extra electrons refers to those free electrons that can move about and help to carry an electric current.

With two different types of silicon, we can make sandwiches by putting them together in layers. Such combinations of p-type and n-type material make all kinds of electronic components possible and such a variety of components can work in many different ways. If we join a piece of n-type silicon to a piece of p-type silicon, and put electrical contacts on either side, things start to happen at the junction between the two materials. If we apply current, we can make electrons flow through the junction from the n-type side to the p-type side and out through the circuit to perform useful work.

Because the p-type side of the junction has fewer electrons, it pulls electrons over from the n-type side. If we reverse the current, the electrons won’t flow at all. We have thereby fulfilled the role of the earlier vacuum tube or valve diode with a more durable solid state component. The diode is also sometimes referred to as a rectifier. Either name refers to an electronic component that lets current flow through it in only one direction. It is especially useful if you want to convert two-way alternating electric current into one-way direct current.

If we join two individual diodes back-to-back, we will have two PN-junctions connected together in series which would share a common Positive, (P) or Negative, (N) terminal. The fusion of two diodes thus produces a three layer, two junction, three terminal device forming the basis of a Bipolar Junction Transistor, or BJT. These three terminal active devices made from different semiconductor materials can then act as either an insulator or a conductor by the application of a small voltage.

In this configuration transistors can act as simple switches. The terminals are defined as a collector on the input or supply side of the circuit and an emitter on the output or demand side of the circuit. There is also a terminal known as the base which acts as a gatekeeper. The absence or presence of a voltage at the base determines whether the gate or valve is open or closed. A transistor conducts current across the collector-emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is off. When base voltage is present, the switch is on.

This Living Crown treatment is a part of the Ascension University’s Guest Lecturer Service.

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