Identification of transistor type and its terminals By:Engr.Irshad Rahim Memon

 Objective of this practical is to identify transistor type such as (i) NPN (ii) PNP, and its terminals such as emitter, base and collector.

 Transistor is semiconductor device, it is constructed of three doped semiconductor materials. Either n- type material is sandwiched between two p-type materials or p-type material is sandwiched between two n-type materials and these three materials make three regions of transistor as shown in figure 01

 Three regions of transistor are known as Base (B), Collector (C), Emitter (E), these regions separated by two pn-junctions known as base- emitter and base-collector junction.  A wire lead is connected to each of three regions, and these leads are labeled as E,B,C for emitter, Base, Collector respectively. Emitter is heavily doped, Base is lightly doped, and Collector is moderately doped.  Physically collector is bigger than emitter and base, because collector has to dissipate more power. Transistors are usually used as amplifiers and switches. Schematic symbol of transistor is shown in figure 02.

 Not only forward resistance of base emitter junction is slightly higher than the resistance of base collector junction but also forward voltage drop of base emitter junction is slightly higher than the forward voltage of base collector voltage.

 DC Source  Few LEDs  Breadboard  Multimeter  Connecting Wire etc.

 Transistor terminals can be identified with any of following methods:  (i) ohm meter (ii) case identification

 In transistor there are two pn-junctions, when ohm meter is connected with any of two leads and if that junction of transistor is forward biased then it will show some value of resistance else it will show “OUT of Range”. So find out both junctions with the help of ohm meter, such that both junctions are forward biased.  Now check if common terminal of transistor is connected with positive probe of ohm meter; transistor is said to be npn else pnp.  This common terminal is base. Junction with slightly higher resistance is base emitter, so emitter terminal is also identified.

 Junction with slightly lower resistance is base collector, so collector terminal is also identified.  R BE >R BC very minor difference (forward resistance is round about in the range of kilo ohms)

 Transistor terminal identification with the help of case is very easy as shown in figure 03. But care must be taken, because same case may have different terminal configuration by different manufacturers.

 Transistor type such as npn or pnp and its terminals are identified.