Jun 19, 2016

Transistor Current Components

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 IC component consists of hole current IpE (holes crossing from emitter into base) and electron current InE (electrons crossing from base into emitter).
  • The ratio of hole to electron currents IpE/InE, 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 JE reach the collector junction JC because some of them combine with the electrons in the n-type base. If IpC is the hole curret at t JC, there must be a bulk recombination current IPE-IPC 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 IE=0. Under this condition, the collector-base junction JC acts as a reverse-biased diode and therefore , the collector current ICO would equal to the reverse saturation current ICO.
  • 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
                                                              IC=IPC+ICBO  ------> (a)
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 = IpE / (IpE + InE) = IpE /IE  ------> (b)

Where IPE is the hole diffusion current at emitter junction.

Since emitter is nearly doped IpE >>InE and   δ ~ 1

Transport Factor β

It is the ratio of injected carrier current reaching at collector-base junction JC to injected carrier current at emitter-base junction JE.
                                                  β =IpC / IpE   ------> (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 IE is called the large –signal current gain of  a common base transistor. It is denoted by α,

Large signal currne gain  α = (IC – ICO) / IE    ------> (d)
Since IC and IE have opposite signs, α is always positive.
Since IpC + IC – ICO.
                                           α = IpC / IE
                                               = (IpC / IpE)  x  (IpE / IE)

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
                                               IC= –αIE + ICO    ------> (f)
  • In the active region IC  is independent of collecto voltage VC and depends only upon IE. In order to generalise equation (f) i.e. for any value of VC replaces ICO by the current I=Io(ev/ηvr-1) in a p-n-p diode.
Replacing IO by ICO and V by VC the complete expression for IC for any VC replace ICO and IE is given by
                                            IC = αIE + ICO(1-eve/vr)   ------> (e)


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