Transistor Current Components

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.

• Not all the holes crossing the emitter junction J_E reach the collector junction J_C because some of them combine with the electrons in the n-type base. If I_pC is the hole curret at t J_C, there must be a bulk recombination current I_PE-I_PC leaving the base, as indicated in figure.
• 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)

• Types of Semi Conductors

Now we define the various parameters which relate the Transistor Current Components are defined and described below

Emitter Efficiency δ

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

                                       δ = 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   δ ~ 1

Transport Factor β

It is the ratio of injected carrier current reaching at collector-base junction J_C to injected carrier current at emitter-base junction J_E.

                                                  β =I_pC / I_pE   ------> (C)

• Working principle and operation of Trnasistor

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 α,

Large signal currne gain  α = (I_C – I_CO) / I_E    ------> (d)
Since I_C and I_E have opposite signs, α is always positive.
Since I_pC + I_C – I_CO.
                                           α = I_pC / I_E
                                               = (I_pC / I_pE)  x  (I_pE / I_E)

Using equation (b) and (C),
                                              α = β x δ    ------> (e)

• Hence the transistor alpha is the product of the transport factor and the emitter frequency. This assumes thath the collector multiplication ratio α* is unity. α*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 α* = 1.

It is clear that if the emitter is forward-biased and the collector is reverse-biased, the collector current is given by the relation (d), or
                                               I_C= –αI_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/ηvr-1) in a p-n-p diode.

Replacing I_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 = αI_E + I_CO(1-e^ve/vr)   ------> (e)

CSV Jun 19, 2016

SHARES

Share

Pin it

Tweet

Share

Share

Share

Buffer

Print