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  Concept of Construction and Mechanism of Operation

Construction of BJT (n-p-n):

Fundamentally, two p-n junctions are made back-to-back. At first, a lightly doped n-region, designated n-, is diffused into substrate to build the collector region. It is followed by a moderately doped, very thin p-region to build the base. At last, this is topped with a heavily doped n-type region, designated n+ that forms the emitter region and is abundant in free charge carriers (figure is as shown below). The base region is very thin, of the order of 0.5 - 1.0 μm that makes its width much less than the diffusion length for the minority carriers in this region, that is, Wb << Lb.

The physical model can be made of BJT that treats it as two p-n junctions back-to-back as shown in figure below. At equilibrium, there are two depletion regions build at the junctions that are neglected in first order approximation computations.

Usually:   

Majority of carrier emitter electron concentration: neo ≈ Ne ≈ 1018 cm-3       
Majority of carrier base hole concentration: pbo ≈ Nb ≈ 1016 cm-3

Majority of carrier collector electron concentration: nco ≈ Nc ≈1015 cm-3   
Minority of carrier emitter hole concentration: peo ≈ ni2/Ne ≈ 2.25x102 cm-3

Minority of carrier base electron concentration: nbo ≈ ni2/Nb ≈ 2.25x104 cm-3
Minority of carrier collector hole concentration: pco ≈ ni2/Nc ≈ 2.25x105 cm-3

Electrical circuit symbol for n-p-n bipolar junction transistor is as well shown in figure below.

Operation of n-p-n BJT in Forward Active Mode:

The bipolar junction transistor might, however, be operated with its base-emitter and base-collector junctions biased in either direction. Though, the most general form of bias is that of Forward Active Mode in which the base-emitter junction is forward biased and the base-collector junction is reverse biased. It is the normal mode of operation of BJT as an amplifying device (as shown in figure below).

694_1.jpg
Figure: Construction of an n-p-n Bipolar Transistor

With base-emitter junction forward biased, an electric field is formed across this junction that lowers its potential barrier. This permits electrons to drift across the junction from emitter and diffuse into the base region. Holes as well drift across the junction from the base and diffuse into emitter region. As the free-electron concentration in emitter is far higher than that of holes in the base by virtue of doping (Ne >> Nb), then most of the current flow via the base-emitter junction is due to electrons in case of n-p-n transistor.

With base-collector junction reverse biased, an electric field is formed across this junction that increases its potential barrier. Due to the thermal generation of free carriers in collector and base regions, then there is a small hole current from collector to base and a small electron current from base to collector. When the base-collector junction was in isolation, such currents combined would form the reverse saturation current for this junction. Though, due to much narrow width of the base region and high electric field intensity across the collector-base junction, electrons “emitted” from the emitter region diffuse right across the base region where they meet up the influence of the reverse biased collector field and are then swept across base-collector junction to be “collected” in collector region. The small fraction, usually of the order of 1%, of electrons injected from the emitter into base really rejoin with holes in the base and build the primary component of base current.

In actual fact, the concentration of charge carriers in base region has a profound persuade on the collector current and this can be controlled by differing the external current supplied to the base. Small modifications in the base current can be employed to effect big changes in the collector current and it is this feature which gives the transistor its amplifying property.

1124_1.B.jpg
Figure: Characteristics of BJT Biased in Forward Active Mode

Current Components:

Note that some components of current flow in each and every region of the transistor are as shown in figure below. Though, the principal components are electronic currents in an emitter and the collector regions and hole current in the base region that contributes to recombination.

Important points to note are:

A) The bias voltages applied to device are almost completely developed across the depletion regions and slight across the neutral regions. Therefore, currents flow as drift currents across the junctions and diffusion currents via the neutral regions.

B) There is a slight recombination of carriers in depletion region and this can be ignored.
 
C) Beneath steady-state bias conditions, the common laws govern diffusion in neutral regions and, therefore, the minority carrier concentrations in such regions follow exponential profiles.

D) Minority carrier concentration profile in the base region can be approximated as linear due to much narrow dimensions of this region. The recombination of holes in base with a few of electrons in transit from emitter to collector accounts for departure from linearity in actuality. Though, the approximation of a linear profile very much simplifies analyses of the current flow.

Minority Carrier Profile:

The majority carrier concentrations in each and every region of the transistor are little modified by bias conditions. The minority carrier concentrations, though, are significantly modified and it is such that control the operation of device. The minority carrier profiles for bipolar transistor operating in forward active mode are shown in figure below.

The forward bias on emitter-base junction leads to a raise in the minority carriers on each and every side of the junction above equilibrium levels by a factor of eVBE/VT. This can be seen that there is an exponential profile extending into an emitter region. The reverse bias on collector-base junction forces minority carriers away from the junction and hence the carrier profiles are decreased to near zero at such boundaries. There is, again, an exponential profile extending to the collector region. In base region, the profile is in reality non-linear however to a good first-order approximation can be treated as linear and a correction can be made for non-linearity that is due to recombination whenever analyzing the current flow.

Notation Applying to an n-p-n Bipolar Junction Transistor:

Ne: doping concentration in emitter (i.e., donor atoms): atoms/cm3
Nb: doping concentration in base (i.e., acceptor atoms): atoms/cm3
Nc: doping concentration in collector (i.e., donor atoms): atoms/cm3

peo: equilibrium minority carrier in the concentration emitter peo: ni2/Ne cm-3
nbo: equilibrium minority carrier concentration in the base nbo: ni2/Nb cm-3
pco: equilibrium minority carrier in the concentration collector pco: ni2/Nc cm-3

De: diffusion coefficient for minority carriers in the emitter: cm2/s
Db: diffusion coefficient for minority carriers in the base: cm2/s
Dc: diffusion coefficient for minority carriers in the collector: cm2/s
 
Le: diffusion length for the minority carriers in emitter: μm or cm
Lb: diffusion length for the minority carriers in base: μm or cm
Lc: diffusion length for the minority carriers in collector: μm or cm

Wb: width of the base region excluding depletion regions: μm or cm

b: minority carrier lifetime in base before the recombination: ns
F: forward transit time for the minority carrier to cross base region: ns

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