6 bjt basic properties

8/11/2019 6 BJT Basic Properties

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It is clear that current lE flows into theemitter of a properly biased p-n-p

transistor and that lc flows out at the

collector, since the direction of hole flow isfrom emitter to collector.


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However, the base current IB requires abit more thought. In a good transistor the

base current will be very small since IE is

essentially hole current, and the collectedhole current Ic is almost equal to IE.

There must be some base current,however, due to requirements of electron

flow into the n-type base region .


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We can account for lBphysically by threedominant mechanisms.

(a) There must be some recombination ofinjected holes with electrons in the base,

even with Wb


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(b) Some electrons will be injected from nto p in the forward-biased emitter junction,

even if the emitter is heavily doped

compared to the base. These electronsmust also be supplied by IB.


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(c) Some electrons are swept into thebase at the reverse-biased collector

junction due to thermal generation in the

collector. This small current reduces IB bysupplying electrons to the base


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BJT FABRICATION

The first transistor invented by Bardeenand Brattain in 1947 was thepoint contact

transistor. In this device two sharp metal

wires, or "cat's whiskers," formed an"emitter" of carriers and a "collector" of

carriers.

These wires were simply pressed onto a

slab of Ge which provided a "base" or

mechanical support, through which the

injected carriers flowed


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Let us review a simplified version of howto make a double polysilicon, self-aligned

n-p-n Si BJT. This is the most commonly

used, state-of-the-art technique for makingBJTs for use in an IC


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MINORITY CARRIER DISTRIBUTIONS AND

TERMINALCURRENTS

we examine the operation of a BJT inmore detail. We begin our analysis by

applying the techniques of previous

chapters to the problem of hole injectioninto a narrow n-type base region.

The mathematics is very similar to thatused in the problem of the narrow base

diode.


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We shall at first simplify the calculations bymaking several assumptions:

1. Holes diffuse from emitter to collector;

drift is negligible in the base region.

2. The emitter current is made up entirely

of holes; the emitter injection efficiency is= 1.


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3. The collector saturation current isnegligible.

4. The active part of the base and the two

junctions are of uniform cross sectionalarea A; current flow in the base is

essentially one-dimensional from emitter

to collector.

5. All currents and voltages are steady

state.


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Solution of the Diffusion Equation in the Base

Region

Since the injected holes are assumed toflow from emitter to collector by diffusion,

we can evaluate the currents crossing the

two junctions. Neglecting recombination in the two

depletion regions, the hole current

entering the base at the emitter junction is

the current IE, and the hole current leaving

the base at the collector is Ic.


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We shall consider the simplified geometryof Fig. a, in which the base width is Wb

between the two depletion regions, and

the uniform cross-sectional area is A.


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In equilibrium, the Fermi level is flat, andthe band diagram corresponds to that for

two back-to-back p-n junctions.

But, for a forward-biased emitter and a

reverse-biased collector (normal active

mode), the Fermi level splits up into quasi-Fermi levels, as shown in fig b


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The barrier at the emitter-base junction isreduced by the forward bias, and that at

the collector-base junction is increased by

the reverse bias.


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The excess hole concentration at the edgeof the emitter depletion region pEand the

corresponding concentration on the

collector side of the base pc

are


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If the emitter junction is strongly forwardbiased (VEB>> kTlq) and the collector

junction is strongly reverse biased (VCB


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We can solve for the excess holeconcentration as a function of distance inthe base p(xn) by using the proper

boundary conditions in the diffusionequation


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The solution is very similar to that of thenarrow base diode problem.


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The excess hole distribution is given by

E l ti f th T i l


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Evaluation of the Terminal

Currents

Having solved for the excess holedistribution in the base region, we can

evaluate the emitter and collector currents

from the gradient of the hole concentration

at each depletion region edge


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Similarly, if we neglect the electronscrossing from collector to base in the

collector reverse saturation current, Ic is

made up entirely of holes entering the

collector depletion region from the base.


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Evaluating Eq , xn= Wbwe have thecollector current


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When the parameters C1 and C2 are substituted from the emitter

and collector currents take a form that is most easily written in terms of


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Now we can obtain the value of IBby acurrent summation, noting that the sum of

the base and collector currents leaving the

device must equal the emitter current

entering. If IE= IEpfor =1,


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Current Transfer Ratio

The value of lE calculated thus far in thissection is more properly designated IEp,

since we have assumed that = 1 (the

emitter current due entirely to hole

injection).


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Actually, there is always some electroninjection across the forward-biased emitter

junction in a real transistor, and this effect

is important in calculating the current

transfer ratio.


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The emitter injection efficiency of a p-n-ptransistor can be written in terms of the

emitter and base properties:


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In this equation we use superscripts toindicate which side of the emitter-base

junction is referred to.

The base transport factor B is


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GENERALIZED BIASING

The terminal currents of the transistor, ifthe device geometry and other factors are

consistent with the assumptions.

Real transistors may deviate from these

approximations.


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The collector and emitter junctions maydiffer in area, saturation current, and other

parameters, so that the proper description

of the terminal currents may be more

complicated.


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if the roles of emitter and collector arereversed, these equations predict that the

behavior of the transistor is symmetrical.

Real transistors, on the other hand, are

generally not symmetrical between emitter

and collector.


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This is a particularly importantconsideration when the transistor is not

biased in the usual way.


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Normal biasing (sometimes called thenormal active mode), in which the emitter

junction is forward biased and the collector

is reverse biased.

In some applications, particularly in

switching, this normal biasing rule isviolated.


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We shall develop a generalized approachwhich accounts for transistor operation in

terms of a coupled-diode model, valid for

all combinations of emitter and collector

bias.

This model involves four measurableparameters that can be related to the

geometry and material properties of the

device.


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The Coupled-Diode Model

If the collector junction of a transistor isforward biased, we cannot neglect pc;

instead, we must use a more general hole

distribution in the base region.


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Figure illustrates a situation in which the emitter and

collector junctions are both forward biased, so that pE

andpc are positive numbers.


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One component Fig. accounts for the holes injected by

the emitter and collected by the collector


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The component of the hole distribution illustrated by Fig.

results in currents IEI and ICI which describe injection in

the inverted mode of injection from collector to emitter


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For symmetrical transistor


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These relations were derived by J. J.Ebers and J. L. Moll and are referred to as

the Ebers-Moll equations.

Although we shall not prove it here, it is

possible to show by reciprocity argumentsthat NIES= IICS

even for nonsymmetrical transistors.


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An interesting feature of the Ebers-Mollequations is that IEand Ic are described by

terms resembling diode relations (IEN and

ICI), plus terms which provide coupling

between the properties of the emitter and

collector.


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The Ebers-Moll equations in terms of emitter and collector current


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BJT F b i ti


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ECE 663

BJT Fabrication


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ECE 663

PNP BJT Electrostatics

El


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ECE 663

PNP BJT Electrostatics

NPN Transistor Band


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ECE 663

NPN Transistor BandDiagram: Equilibrium

PNP Transistor Active Bias


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ECE 663

PNP Transistor Active BiasMode

Most holesdiffuse tocollector

Large injectionof Holes

Collector Fields drive holesfar away where they cant

return thermionically

Few recombinein the base

VEB> 0

VCB> 0

PNP Transistor Active Bias


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ECE 663

PNP Transistor Active BiasMode

Most holesdiffuse tocollector

Large injectionof Holes

Collector Fields drive holesfar away where they cant

return thermionically

Few recombinein the base

VEB> 0

VCB> 0

Forward Active minority carrier distribution


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ECE 663

P+ N P

nE(x)

nE0

pB0

pB(x)

nC0

nC(x)

y

PNP Ph si l C nts


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PNP Physical Currents

6 bjt basic properties

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