Continuing Education What Is a Transistor? Another major electronic component used in elevators is detailed. Learning Objectives P-type silicon) are created essentially as they are for diodes, by a process known After reading this article, you should have learned about: ♦ How transistors operate by attracting and repelling charge as doping. Crystalline silicon is not carriers electrically conductive. (Other ♦ The effect of doping on crystalline silicon semiconducting materials have been ♦ Why the doping process is used used, but today the choice is usually ♦ How base, collector and emitter are connected in silicon.) But, when it is exposed to trace electronic circuits amounts of certain other elements, the ♦ How to handle transistors crystalline silicon becomes a semiconductor. This does not mean it by David Herres has a fixed resistance that lies Like diodes, transistors are major players in somewhere between that of an insulator like the electronics arena, notably in elevator motion glass and a conductor like copper. To the controllers and variable-frequency drives contrary, it means that under some conditions, (VFDs) that have an extensive operator’s Continued interface. Also like diodes, the transistor semiconductor mechanism is very simple in that it involves the attraction and repulsion of charge carriers by the bias voltages applied to the device terminals. The electrons and holes migrate toward or away from the semiconductor junctions, thereby regulating higher flows of current between various terminals and ground. The defining difference between diodes and Value: 1 transistors is that in the diode there are two layers (N- and P-type silicon, with a single contact junction between them) where the hour semiconductor action takes place. This article assumes the reader has already understood the (0.1 CEU) author’s piece “What Is a Diode?” (ELEVATOR Approved for WORLD, August 2014) or understands how Continuing these devices work on a subatomic level and in Education by NAEC electrical circuits. Compared to diodes, for CET® and NAESA transistors are somewhat more complex, International for because they have three semiconductor layers Figure 1: Transistor packages in order from top to bottom: QEI. with two junctions. The materials (N- and TO-3, TO-126, TO-92 and SOT-23 (photo by Transisto) December 2014 • ELEVATOR WORLD 93 the crystalline silicon conducts, and under effective as charge carriers and necessary other conditions, it does not conduct. for semiconductor operation. What underlies all of this is that silicon, Crystalline silicon doped with boron or with an atomic number of 14, consists of a gallium becomes P-type silicon. We have nucleus with four electrons orbiting in its seen in the previous article on diodes that valence (outer) shell. (The number of N- and P-type silicon bonded together electrons in the valence shell of any make a junction. With leads attached at the element accounts for its physical far ends, they constitute a useful device properties and the ways in which it that conducts or does not conduct, interacts with other nearby atoms.) Atoms depending upon the biasing. are able to share valence electrons with Transistors also make use of the adjacent atoms. Because it has four semiconducting properties exhibited by electrons in its valence shell, pure silicon N- and P-type crystalline silicon. Today, likes to assume the form of a crystal lattice, bipolar transistors have been largely each atom sharing electrons and, hence, supplanted, first by field-effect transistors tightly bonded to four adjacent silicon (FETs) and, finally, by the ubiquitous atoms. The crystalline silicon is very stable, metal-oxide semiconductor FET Figure 2: Amplifier circuit, common-emitter and, electrically, it is an insulator. (MOSFET), which has much higher input configuration, with voltage-divider bias circuit: When the crystalline silicon is doped impedances. Nevertheless, there are plenty note that the emitter arrowhead points outward, with phosphorous or arsenic (don’t worry, of bipolar transistors around in older indicating that the transistor is of the NPN type (image by Aflafla1). the amount is infinitesimal), each of which equipment, as well as in contemporary has five electrons in its outer shell, the applications. In this piece, we will stick to some configurations, through external situation changes abruptly. There is then bipolar transistors, since they are more circuitry. one free electron for each doped silicon basic and provide a good background for Each of these semiconductors can be atom. The excess free electrons travel in understanding the more exotic varieties. P- or N-type silicon. The emitter and the spaces between the silicon atoms, and As mentioned above, transistors consist collector are always the same type, and the they are known as “charge carriers.” of three semiconductor layers, a base, an base is the other type. Accordingly, Crystalline silicon, doped with emitter and a collector. These terms do not transistors may be either PNP or NPN. The phosphorous or arsenic and thus having have to be taken too seriously, because the PNP version has an N-type silicon base free electrons, is N-type silicon. It is no “base” is not a base, the “emitter” does not with emitter and collector composed of longer an insulator but is capable of emit anything, and the “collector” does not P-type silicon. “NPN” indicates a P-type conducting an electrical current, due to collect anything. Still, that is the base and an N-type collector and emitter. the presence of charge carriers. terminology, and it is quite graphic. The two junctions, like the single junction In a mirror-image process that reverses Each layer has a lead attached so the of a diode, may be forward or reverse the polarities, crystalline silicon can be transistor can be wired to other biased, and so, will conduct or not conduct. In most circuits, NPN and PNP Even when the transistor terminals are not labeled, it is transistors can be interchanged, but all possible to identify them and ascertain the transistor type by polarities (including those provided by the power supply) will be reversed. looking at the arrowhead. A transistor schematic consists of a circle with an internal semiconductor doped by exposing it (so that minute components, such as resistors, capacitors shown, including leads that permit circuit amounts are absorbed) to boron or and coils, to comprise a circuit. The three connections. The base resembles a flat gallium, both of which have three electrons semiconductors are bonded, always with plate like the cathode of a diode. Collector in the outer shell. This process creates a the base in the middle and the emitter and and emitter are lines that connect to the deficiency of electrons in the space collector on either side. But, do not base at an angle. By convention, the base is between the silicon atoms. These empty conclude that the middle lead goes to the shown at left, the collector at top and spots are called “holes” and may be base, because this is not always the case. emitter at bottom, but this may vary if thought of as positively charged particles Since there are three layers, there are two required by the overall schematic layout. with negligible mass, like protons that have junctions. The base has two junctions, and The base, collector and emitter may be positive polarity but are far more massive. the emitter and collector each have a single labeled on the schematic, but not always. Whether these holes actually exist as junction they share with the base. The You can always distinguish emitter and particles or more nearly resemble emitter and collector have no common collector, because the angle line having the conceptual entities, we can leave to the junction and are not electrically arrowhead is always the emitter. Moreover, metaphysicians. The fact is that they are connected, except through the base and, in if the arrow points inward toward the base, 94 www.elevatorworld.com • December 2014 the transistor is PNP, and if it points waveform of any desired frequency. This that feeds the motor. These frequencies outward, the transistor is NPN. To would apply to a wide range of are supplied by the inverter section of summarize, even when the transistor applications: the VFD. terminals are not labeled, it is possible to ♦ Electronic equipment that incorporates The principle types of electronic identify them and ascertain the transistor an audio beep such as a telephone, oscillators are the linear and non-linear type by looking at the arrowhead. video game or fire-alarm system will (relaxation) oscillator. A linear oscillator Transistors have three leads. With one have this signal generated within an produces a pure sine wave. It consists of a lead as common, it is possible to have two oscillator circuit. transistor configured as an amplifier with a two-wire circuits, and this is how transistors are configured. The emitter is It is possible to get a good idea of the condition of the usually the common connection. In this mode, the base emitter is the input, and transistor on a go/no-go basis using just the ohmmeter. the collector emitter is the output. Small ♦ A signal generator produces a range of input changes result in identical output feedback loop, meaning that the output is frequencies and waveforms that changes but at a much higher power level. connected back to the input. When technicians inject into defective This stepped-up output receives its initially powered up, any small amount of equipment for purposes of signal electrical energy from the power supply, so noise generated by atomic motion in the it is not a question of getting free power. tracing and viewing (with an circuitry or within the semiconductor is Nevertheless, the input/output ratio makes oscilloscope) at each stage until the amplified and conducted back through the the devices extremely useful in many types fault is located.