1. By Squadron Leader Zahid Mir CS&IT Department, Superior University PHY-BE -14 Transistors

2. Transistor Transistor was invented by a team of three men at Bell Laboratories in 1947. There are two basic types of transistors; – Bipolar Junction Transistor (BJT) – Field-Effect Transistor (FET) BJTs are used in two broad areas; – As a linear amplifier to boost or amplify an electrical signal. – As an electronic switch.

3. Basic Design BJT consist of two back-to-back pn-junctions fabricated in a single piece of semiconductor crystal. The term bipolar refers to use of both holes and electrons as carriers in the transistor structure. The two junctions are emitter-base (E/B) junction & collector-base (C/B) junctions.

4. Transistor Symbol The arrow-head is always at emitter & its direction indicates the conventional direction of current flow ( whereas electrons flow against it). For pnp transistor, arrowhead points from emitter to base, meaning emitter is positive with respect to the base (and also with respect to collector). For npn transistor, arrowhead points from base to emitter, meaning the base (as well as collector) is positive with respect to emitter.

5. EmitterIt is more heavily doped than any of the other regions because its main function is to supply majority charge carriers (either electrons or holes) to the base. BaseIt forms the middle section of the transistor. It is very thin (10 -6 m) as compared to emitter and collector and is lightly doped. Collector Its main function is to collect majority carriers coming from emitter and passing through base. Collector region is made physically larger than the emitter because it has to dissipate much greater power. Due to this difference, there is no possibility of inverting transistor, i.e; making its collector to emitter and vice versa.

6. Transistor Biasing Basic Rule – Emitter-Base junction is always forward biased. – Base-Collector junction is always reversed biased. Example For a pnp transistor, the positive terminal of V EE (dc emitter supply voltage) is connected to the p-type emitter in order to push or repel holes into the base. The negative terminal of V CC (dc collector supply voltage) is connected to the collector so it may attract or pull holes through the base.

7. Important Biasing Rule For a pnp transistor, both collector and base are negative with respect to the emitter. Here collector is more negative than base. For a npn transistor, both collector and base are positive with respect to the emitter. Here collector is more positive than base.

8. Transistor Currents According to Kirchhoff’s Current Law; – All currents flowing into a transistor are taken as positive whereas those flowing out of it are taken as negative. I E + (-I B ) + (-I C ) = 0 I E = I B + I C