A bipolar junction transistor looks like two backtoback pn j

A bipolar junction transistor looks like two back-to-back pn junctions, but it behaves differently because the base is \"narrow.\" (a) With respect to what is the base narrow? (b) Why must the base be narrow for the device to act as a transistor? (c) In a npn BJT, what is emitted from the emitter and collected by the collector? For a npn BJT in active mode which type of carrier (electrons or holes) contributes most to the (d) emitter current? (e) base current? (f) collector current? (g) For the BJT in active mode, how does the base current compare in size to the emitter and collector currents? (h) Based on our voltage definitions V_BE > 0 indicates a forward bias on the BE junction and V_CB>0 indicates a reverse bias on the CB junctions. Sketch the band diagram for a n+pn transistor with V_BE > 0 and V_cB >0. Sketch the minority carrier concentrations in the emitter and the base and assume both are short compared to the diffusion length.

Solution

(a) In BJT base is narrow with respect to emitter and collector.

(b) In BJT base is narrow because narrow Base controls number of holes emitted in PNP transistor and narrow Base controls number of electrons emitted in NPN transistor and also narrow Base required to minimize recombination.

(c) In NPN BJT a lot of electrons are emited from emitter and electrons are collected in the collector to constitute a collector current.