The figure above shows that the various

**transistor current components**which flow across the forward biased emitter junction and the reverse biased collector junction.- The emitter current I
_{C}component consists of hole current I_{pE}(holes crossing from emitter into base) and electron current I_{nE}(electrons crossing from base into emitter). - The ratio of hole to electron currents I
_{pE}/I_{nE}, crossing the emitter junctions in proportional to the ratio of the conductivity of the p material to that of the n materials. Since the**doping**of the emitter is made much larger than the dopin of the base, emitter current consists almost entirely of holes in p-n-p transistor.

- If the emitter were open circuited, no carriers are injected from emitter into the base and emitter current I
_{E}=0. Under this condition, the collector-base junction J_{C}acts as a reverse-biased diode and therefore , the collector current I_{CO}would equal to the reverse saturation current I_{CO.} - Therefore, when emitter-base junction is forward-biased and collector–base junction is reverse-biased, the total collector current will be sum of two currents . Hence

I

_{C}=I_{PC}+I_{CBO}------> (a)_{}Now we define the various parameters which relate the

*are defined and described below*

**Transistor Current Components**## Emitter Efficiency Î´

It is the ratio of current of injected carriers at emitter-base junction to total emitter current.

Î´ = I

_{pE}/ (I_{pE}+ I_{nE}) = Ip_{E}/I_{E}------> (b)
Where I

_{PE }is the hole diffusion current at emitter junction.
Since emitter is nearly doped I

_{pE}>>I_{nE}and Î´__~__1## Transport Factor Î²

It is the ratio of injected carrier current reaching at collector-base junction J

Î² =I_{C }to injected carrier current at emitter-base junction J_{E.}_{pC}/ I

_{pE}------> (C)

## Large – Signal Current Gain

The ratio of the negative of the collector current, increment to the emitter – current change from zero (cut-off) to I_{E}is called the large –signal current gain of a common base transistor. It is denoted by Î±,

Large signal currne gain Î± = (I

_{C}– I

_{CO}) / I

_{E}------> (d)

Since I

_{C}and I

_{E}have opposite signs, Î± is always positive.

_{}

Since I

_{pC}+ I

_{C}– I

_{CO}.

Î± = I

_{p}

_{C}/ I

_{E}

= (I

_{pC}/ I

_{pE}) x (I

_{pE}/ I

_{E})

Using equation (b) and (C),

Î± = Î² x Î´ ------> (e)

- Hence the transistor alpha is the product of the transport factor and the emitter frequency. This assumes thath the collector multiplication ratio Î±* is unity. Î±*nis the ration of the total currnet crossing collector junction to the hole current ( for p-n-p transistor) arriving at the junction. For most transistors Î±* = 1.

I

_{C}= –Î±I

_{E}+ I

_{CO}------> (f)

- In the active region I
_{C}is independent of collecto voltage V_{C}and depends only upon I_{E}. In order to generalise equation (f) i.e. for any value of V_{C}replaces I_{CO}by the current I=Io(e^{v/Î·v}r-1) in a p-n-p diode.

_{O}by I

_{CO}and V by V

_{C}the complete expression for I

_{C}for any V

_{C}replace I

_{CO}and I

_{E}is given by

I

_{C}= Î±I

_{E}+ I

_{CO}(1-e

^{ve/v}

^{r}) ------> (e)