Transistor Basics: NPN & PNP Using 2N3904, 2N3906, 2N2222, and 2N2907 Amazingly, the first operational Transistor was declared 70 years ago, on December, 23 1947!1 The Transistor is probably one of the most revolutionary components ever invented. It led the way for the creation of integrated circuits, microprocessors and computer memory. In this article, we'll discuss the following areas; (click the link to skip to any section that suits your needs) What is a transistor? How does a transistor work? Choosing a transistor for your application Transistor circuit examples History Behind the invention of the transistor Reference links for further study What is a transistor? A transistor, also known as a BJT (Bipolar Junction Transistor), is a current driven semiconductor device which can be used to control the flow of electric current in which a small amount of current in the Base lead controls a larger current between the Collector and Emitter. They can be used to amplify a weak signal, as an oscillator or as a switch. They are usually made of silicon crystal where N &  P type semiconductor layers are sandwiched together. See Figure 1 below.   Figure 1: Figure 1a shows a 2N3904 TO-92 cut-away revealing E - Emitter, B - Base, and C – Collector leads tied to Silicon. Figure 1b is taken from a May 1958 Radio-Electronics Magazine2 showing the N & P type layer slices and arrangements (referenced as Germanium material at that time). Transistors are hermetically sealed and encased in plastic or a metal can with three leads (Figure 2).  Figure 2: A size comparison and a variety of popular package types. How does a transistor work? For an example, we will show how an NPN transistor works. A simple way to view its function as a switch is to think of water flowing through a tube controlled by a Valve. Water pressure represents ‘Voltage’ and water flowing through a tube represents ‘Current’ (Figure 3). The large tubes represent the Collector/Emitter junction with a Valve in-between, expressed in the figure as a Gray Oval, like a moveable flap, which is actuated by current from a small tube representing the Base. The valve keeps the water pressure from flowing from Collector to the Emitter. When water flows through the smaller tube (the Base), it opens the valve between the Collector/Emitter junction, allowing water to flow through to the Emitter, and on to Ground (Ground represents the return for all water or Voltage/Current).  Figure 3: This graphical representation illustrates how a transistor functions. When water flows through the smaller tube (Base), it opens the valve between the Collector/Emitter junction, allowing water to flow through the Emitter to Ground. Choosing a transistor for your application If you want to simply turn on a circuit or switch on a load, there are certain things you should consider. Determine if you want to bias or energize your transistor switch with positive or negative current (i.e. NPN or PNP type, respectively). An NPN transistor is driven (or turned on) by positive current biased at the base to control the current flow from Collector to Emitter. PNP type transistors are driven by a negative current biased at the base to control the flow from Emitter to Collector. (Note that polarity for PNP is reversed from NPN.) See Figure 4 below for more details.  Figure 4: Schematic Symbols for each type of transistor. After the bias voltage is determined, the next variable that is needed is the amount of voltage and current the load requires to operate. These will be the minimum voltage and current ratings of the transistor. Tables 1 and 2 below show some popular transistors and key specifications including their voltage and current limits.   Transistors, NPN and PNP, Leaded and Surface Mount