The base resistor value can be calculated using the formula: You can see when a small external voltage is applied to the base, the transistor switches ON and conducts heavier current across the collector emitter terminals, switching on a bigger load. This is the standard method of using any transistor like a switch for controlling a given load.
How to use Transistors like a Switchīipolar transistors are generally a three lead active electronic component which fundamentally work as a switch for either switching ON or switching OFF power to an external load or an associated electronic stage of the circuit.Ī classic example can be seen below, where a transistor is connected as a common emitter amplifier: Here we’ll study the functions and the way of handling and implementing bipolar transistors into practical circuits. New electronic hobbyists usually find it difficult to handle these useful components and configure them as circuits for an intended application. Even ICs are made up of 1000s of closely knit transistors which constitute the features of the particular chip. If there wouldn’t be transistors there wouldn’t be any ICs or any other semiconductor component. Transistors (BJTs) are the building blocks of all semiconductor devices found today. A low power signal across Gate/Source terminal allows the transistor to switch a comparatively high power load across its collector terminal.įor the sake of simplicity we will discuss BJTs here, since their charcaeritics is less complex compared to MOSFETs. A low power signal across base/emitter terminal allows the transistor to switch a comparatively high power load across its collector terminal.įor MOSFETs these are designated as Gate, Source, Drain. Transistors are basically of two types: bipolar junction transistor (BJT), and metal–oxide–semiconductor field-effect transistor ( MOSFET)įor a BJT, the 3 terminals are designated as base, emitter, collector. Therefore, it can be considered that the emitter current is the summation of the base and the collector current.Transistors are 3 terminal semiconductor devices which is able to conduct relatively high power across their two terminals, in response to a significantly low power input at the third terminal. In other words, the emitter current actually flows through the collector circuit. This current constitutes the collector current (I C). Rest of the holes cross the thin depletion region and reach the collector region. While crossing the N-type material, the electrons tend to combine with electrons, generally very few, and constitute the base current (I B). This condition formulates the emitter current (I E). Forward bias on the emitter to base junction causes the holes to flow from P type emitter towards the bias.
I E = I B + I C Working of PNP TransistorĪs shown in the following figure, the emitter to base junction is forward biased and the collector to base junction is reverse biased. Therefore, it can be considered that the emitter current is the summation of the base and the collector current. This current constitutes collector current (I C). Rest of the electrons cross the thin depletion region and reach the collector region. While crossing the P-type material, electrons tend to combine with holes, generally very few, and constitute the base current (I B). Forward bias on the emitter to base junction causes the electrons to flow from N type emitter towards the bias. Working of NPN TransistorĪs shown in the above figure, the emitter to base junction is forward biased and the collector to base junction is reverse biased. One of the junctions of a transistor must be forward biased and other must be reverse biased when it operates. The depletion region of the device is reduced and majority current carriers are injected towards the junction. This condition causes a current to flow through the circuit. In order to function, bipolar transistors must have both junctions biased. In semiconductor circuits, the source voltage is called as the bias voltage.
The base passes most of the charge carriers to the collector as it is comparatively lightly doped than emitter and the collector.įor a proper functioning of the transistor, the emitter-base region must be forward-biased and collector-base region must be reverse-biased. The emitter is heavily doped so that it can inject large number of charge carriers for current conduction. The base is much thinner than the emitter, and the collector is comparatively wider than both.
Transistors have three sections namely – the emitter, the base, and the collector.