Bipolar transistor
1) Density of current J electronn that it slides in a npn:
In a BJT npn there is a negligible gap flow in the base
and therefore from the equation of continuity 
the long electric field can be deduced the axis x 
that replaced in the equation of continuity for gaps
and becoming simpler gives 
leaving to according
to member only the derivative is obtained differentiates them perfect
therefore integrating along the base region has 
,
replacing the values 
is obtained where 
having supposed almostconstant D n in the base and
therefore we have replaced it with its medium value while 
2) BJT prototype:
Draft of the transistor more close to the formulation of Schottky, in it in fact the concentration of drugging is constant in the various E regions there are abrupt variations of drogaggio to the splices.
3) active Polarization of a BJT:
Splice BE is polarized directly, that reduces the barrier
of upgrades them and therefore the electrons come inettati in the
base, while splice BC is polarized inversely therefore electrons comes
pushed out towards the collector, the base moreover are much grip
therefore can be used the theory of the diode valve to short base in
order to say that the distribution of the minority bearers in excess
will not be exponential but to delineate. Being Vnegative BE, the associated
exponential to it is worth the 1 therefore current that slides in the
BJT is 
therefore depends in exponential way from
the applied tension, graficando this relation on a semilogaritmica
paper obtains a straight one whose slope is 
therefore inversely proporziona them to the temperature that it
concurs to realize sensors of the this largeness, moreover
intersection with axis VBE = 0
gives the value to us of JS and
from this the value can gain us of loads with present Built_In in 
base, dividendola for loads q is obtained
dopanti atom number presents in base, alias Number of Gummel 
it is important because much more the much
largest one is small is the current transported from the BJT.
4) Gain of current in a BJT:
The current that slides between emettitore and following base is substantially due to the three contributions:
to) Recombination of electrons iniettati from the emettitore with present gaps in basethe rB
b) Recombination of electrons iniettati from the emettitore in the region of loads spaces comprised them between emettitore and base
c) gap Flow from the base to emettitorethe pE
Beingof the current that comes iniettata from the emettitore and that it decomposes itself in the members previously indicated and the member whom the collector catches up, come defined the following coefficients:
is the factor of recombination in base
is the emettitore efficiency
in they function defines the parameter toF that relazionathe C toand cioè the C = -toF andthe con toF = gaT replacing 
in the equation of the currents in the BJT
the C =b F is obtainedthe
B where bF is gain of current in continuous.
5) Model of Ebers-Moll:
This model substantially holds account of the currentthe BE that is recombined
previously in splice BE embezzling it to the present currentJ n that it goes from the
emettitore to the collector, in particular replacing the found
expression for Jn has 
and in analogous way 
can be
expressed, for both decomposing 1ª the quadrant and adding and
embezzlingthe S it is obtained
and also 
.
In these largenesses there are 4 incognito however only 3
are independent in how much between they vige the reciprocity
relation 
, the corresponding circuit sees two
disposed diode valves like in a npn constituted from diode valves,
everyone of they then has in parallel a current generator.
6) Early Effect:
For a BJT polarized in active region splice BC is
polarized inversely, it follows some that the correspondent amplitude
of the region of loads spaces them is function of the applied inverse
tension and that implies a variation of the current 
in fact has 
dove 
is the tension of Hearly, can in particular be
observed that when VCB increases,
alsothe C increases.
From a graphical point of view, VTo are the intersection point on the axis of the abscissas of the tangents to the parametric curves ofC according to VCE (… and therefore also of VCB since in active region CE =V CB is had V 0,7V) such tangents goes taken in the leaving point from the saturation, considering instead the tangents to the straight ones of the active region, obtains one value V' more profit for the planning in how much relative one to the region operation assets.
7) Kirk Effect:
We are in conditions of High_Level Injection when the population of the majority bearers to the equilibrium comes heavy altered from the polarization, when this condition is present to the collector has the amplitude of the region of loads spaces stretches them to diminish, and corresponding the nearly-neutral region of base is expanded, of it it turns out that it diminishes the gain in current of the BJT and gets worse the answer in frequency of the same one.
8) Time of transit in base:
