Question

Relation between the collector current gain and emitter efficiency and transport factor for bjt ??

Answer 1

BJT (Bipolar Junction Transistor)

A BJT is a type of transistor that uses both electron and hole charge carriers. BJTs use two junctions between two semiconductor types, n-type and p-type.

Basic Circuit of BJT

The Basic Circuit of BJT as shown in figure

A bipolar Junction transistor comprises of two back end p-n junctions, who share a thin common region with width, wB. Contacts are made to every one of the three regions, the two outer areas called called the emitter and collector and the middle region called the base. The structure of a npn bipolar transistor is appeared in Figure (a). The device is called “bipolar” since its operation involves both types of mobile carriers, electrons and holes.

Relation Between Collector Current (Ic) and transport factor (ɑ)

The emitter current, IE, equals the sum of the base current, IB, and the collector current, IC: IE =IC + IB The base-emitter voltage and the base-collector voltage are positive if a positive voltage is applied to the base contact relative to the emitter and collector respectively. From the Energy band diagram of a BJT, The emitter current is the sum of the electron diffusion current, IE,n, the hole diffusion current, IE,p and the base-emitter depletion layer recombination current, Ir,d. IE = IE,n + IE,p + Ir,d    The total collector current is the electron diffusion current, IE,n, minus the base recombination current, Ir,B IC = IE,n - Ir,B The base current is the sum of the hole diffusion current, IE,p, the base recombination current, Ir,B and the base-emitter depletion layer recombination current, Ir,d IB =  Ir,B + IE,p +Ir,d The transport factor, a, is defined as the ratio of the collector and emitter current: Therefore Collector current Ic is proportional to Transport factor Relation Between Emitter Efficiency (gE) and transport factor (ɑ) Now, The transport factor, a, as the product of the emitter efficiency, gE, the base transport factor, aT, and the depletion layer recombination factor, dr. a = aT gE dr The emitter efficiency, gE, is defined as the ratio of the electron current in the emitter, IE,n, to the sum of the electron and hole current diffusing across the base-emitter junction, IE,n + IE,p. gE = IE,n / (IE,n + IE,p) The base transport factor, aT, equals the ratio of the current due to electrons injected in the collector, to the current due to electrons injected in the base. aT  = (IE,n - Ir,B ) / IE,n                                                             Therefore Emitter Efficiency is proportional to Transport factor