1 Concepts of electrons and holes in semiconductors

2 Forward and reverse bias in a p-n junction Under forward bias the width of the depletion region decreases. Current increases exponentially. Under reverse bias the width of the depletion region increases. Very low current flow (leakage current I s )

3 Chapter 4: Bipolar Junction Transistor 4.1 Basic Operation of the npn Bipolar Junction Transistor Figure 4.1 The npn BJT Basic Operation in the Active Region: An npn transistor (CE configuration) with variable voltage sources operating in the active region:  V BE ≈ 0.6 V to forward bias the BE junction  V CE >V BE - the base collector junction is reverse biased We will apply the Shockley equation: Here, the emission coefficient n = 1 (usually the case for ideal p-n junctions) ……… (4.1)

4 Basic Operation in the Active Region – Cont’d First-Order Common- Emitter Characteristics Fig. 4.3 Note: The current flowing in a BJT is mostly due to electrons moving from the emitter through the base to the collector Base current consist of two components: (i) holes crossing from the base into the emitter, and (ii) holes recombining with the electrons injected into the base Usually we desire the base current (i/p current) to be very low

5 Factors affecting the Current Gain For designing a BJT with high β the following points should be considered: Emitter doping should be higher than base doping - this gives higher gain, since electron current constituting the collector current will be much more than the hole current constituting the base current The base width should be very small - reduce base recombination so that most of the electron current from emitter flows to collector The geometry of the device should allow quick diffusion of electrons to the collector junction, and the electron lifetime in the base should be large - reduce base recombination current so that most of the electron current from emitter flows to collector Note: The BJT can be considered as a current controlled current source. Input current is I b and output current is I c.

6 Device Equations – Cont’d From Eq. 4.3 and 4.4 we have: