178CHAPTER 4. BIPOLAR JUNCTION TRANSISTORSﬂow, in different parts of the transistor.• Bipolar transistors consist of either a P-N-P or an N-P-N semiconductor “sandwich” struc-ture.• The three leads of a bipolar transistor are called the Emitter, Base, and Collector.• Transistors function as current regulators by allowing a small current to controla largercurrent. The amount of current allowed between collector and emitter is primarily deter-mined by the amount of current moving between base and emitter.• In order for a transistor to properly function as a current regulator, the controlling (base)current and the controlled (collector) currents must be going in the proper directions:meshing additively at the emitter and going againstthe emitter arrow symbol.4.2The transistor as a switchBecause a transistor’s collector current is proportionally limited by its base current, it can beused as a sort of current-controlled switch. A relatively small ﬂow of electrons sent throughthe base of the transistor has the ability to exert control over a much larger ﬂow of electronsthrough the collector.Suppose we had a lamp that we wanted to turn on and off with a switch. Such a circuitwould be extremely simple as in Figure 187,4.3(a).For the sake of illustration, let’s insert a transistor in place of the switch to show how it cancontrol the ﬂow of electrons through the lamp. Remember that the controlled current througha transistor must go between collector and emitter. Since it is the current through the lampthat we want to control, we must position the collector and emitter of our transistor where thetwo contacts of the switch were. We must also make sure that the lamp’s current will moveagainstthe direction of the emitter arrow symbol to ensure that the transistor’s junction biaswill be correct as in Figure 187,4.3(b).transistorNPNtransistorPNPswitch(a)(b)(c)+++Figure 4.3:(a) mechanical switch, (b) NPN transistor switch, (c) PNP transistor switch.A PNP transistor could also have been chosen for the job. Its application is shown in Fig-ure 187,4.3(c).The choice between NPN and PNP is really arbitrary. All that matters is that the propercurrent directions are maintained for the sake of correct junction biasing (electron ﬂow goingagainstthe transistor symbol’s arrow).