bipolar junction transistor - wikipedia, the free encyclopedia

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PNP

NPN

Schematic symbols for

PNP- and NPN-type

BJTs.

Bipolar junction transistorrom Wikipedia, the free encyclopedia

bipolar junction transistor (BJT or bipolar transistor) is a type of transistor that relies

n the contact of two types of semiconductor for its operation. BJTs can be used as amplifiers,

witches, or in oscillators. BJTs can be found either as individual discrete components, or in

rge numbers as parts of integrated circuits.

ipolar transistors are so named because their operation involves both electrons and holes.

hese two kinds of charge carriers are characteristic of the two kinds of doped semiconductor

material. In contrast, unipolar transistors such as the field-effect transistors have only one kind

f charge carrier.

harge flow in a BJT is due to bidirectional diffusion of charge carriers across a junction

etween two regions of different charge concentrations. The regions of a BJT are called

mitter , collector , and base. A discrete transistor has three leads for connection to these

egions. By design, most of the BJT collector current is due to the flow of charges injected

om a high-concentration emitter into the base where there are minority carriers that diffuse toward the collector, and

o BJTs are classified as minority-carrier devices.

Contents

1 Introduction

1.1 Voltage, current, and charge control

1.2 Turn-on, turn-off, and storage delay

1.3 Transistor parameters: alpha (α) and beta (β)

2 Structure

2.1 NPN

2.2 PNP

2.3 Heterojunction bipolar transistor

3 Regions of operation

3.1 Active-mode NPN transistors in circuits

3.2 Active-mode PNP transistors in circuits

4 History

4.1 Germanium transistors

4.2 Early manufacturing techniques

4.2.1 Bipolar transistors

5 Theory and modeling

5.1 Large-signal models5.1.1 Ebers–Moll model

5.1.1.1 Base-width modulation

5.1.1.2 Punchthrough

5.1.2 Gummel–Poon charge-control model

5.2 Small-signal models

5.2.1 hybrid-pi model

5.2.2 h-parameter model

5.2.2.1 Etymology of hFE

6 Applications

6.1 Temperature sensors

6.2 Logarithmic converters7 Vulnerabilities

8 See also

9 References

10 External links



NPN BJT with forward-biased E–B

junction and reverse-biased B–C

junction

ntroduction

JTs come in two types, or polarities, known as PNP and NPN based on the

oping types of the three main terminal regions. An NPN transistor comprises

wo semiconductor junctions that share a thin p-doped anode region, and a PNP

ansistor comprises two semiconductor junctions that share a thin n-doped

athode region.

n typical operation, the base–emitter junction is forward biased, which means

hat the p-doped side of the junction is at a more positive potential than the-doped side, and the base–collector junction is reverse biased. In an NPN

ansistor, when positive bias is applied to the base–emitter junction, the

quilibrium is disturbed between the thermally generated carriers and the

epelling electric field of the n-doped emitter depletion region. This allows thermally excited electrons to inject from the

mitter into the base region. These electrons diffuse through the base from the region of high concentration near the

mitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers

ecause the base is doped p-type, which makes holes the majority carrier in the base.

o minimize the percentage of carriers that recombine before reaching the collector–base junction, the transistor's base

egion must be thin enough that carriers can diffuse across it in much less time than the semiconductor's minority carrierfetime. In particular, the thickness of the base must be much less than the diffusion length of the electrons. The

ollector–base junction is reverse-biased, and so little electron injection occurs from the collector to the base, but

ectrons that diffuse through the base towards the collector are swept into the collector by the electric field in the

epletion region of the collector–base junction. The thin shared base and asymmetric collector–emitter doping is what

ifferentiates a bipolar transistor from two separate and oppositely biased diodes connected in series.

Voltage, current, and charge control

he collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the

ase–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter

unction, which is just the usual exponential current–voltage curve of a p-n junction (diode).[1]

he physical explanation for collector current is the amount of minority carriers in the base region.[1][2][3]

Due to low

vel injection (in which there are much fewer excess carriers than normal majority carriers) the ambipolar transport

ates (in which the excess majority and minority carriers flow at the same rate) is in effect determined by the excess

minority carriers.

etailed transistor models of transistor action, such as the Gummel–Poon model, account for the distribution of this

harge explicitly to explain transistor behaviour more exactly.[4]

The charge-control view easily handles

hototransistors, where minority carriers in the base region are created by the absorption of photons, and handles the

ynamics of turn-off, or recovery time, which depends on charge in the base region recombining. However, because

ase charge is not a signal that is visible at the terminals, the current- and voltage-control views are generally used in

rcuit design and analysis.

n analog circuit design, the current-control view is sometimes used because it is approximately linear. That is, the

ollector current is approximately times the base current. Some basic circuits can be designed by assuming that the

mitter–base voltage is approximately constant, and that collector current is beta times the base current. However, to

ccurately and reliably design production BJT circuits, the voltage-control (for example, Ebers–Moll) model is

equired.[1]

The voltage-control model requires an exponential function to be taken into account, but when it is

nearized such that the transistor can be modelled as a transconductance, as in the Ebers–Moll model, design for

rcuits such as differential amplifiers again becomes a mostly linear problem, so the voltage-control view is oftenreferred. For translinear circuits, in which the exponential I–V curve is key to the operation, the transistors are usually

modelled as voltage controlled with transconductance proportional to collector current. In general, transistor level

rcuit design is performed using SPICE or a comparable analog circuit simulator, so model complexity is usually not of

much concern to the designer.



Simplified cross section of a planar NPN bipolar junction transistor

Turn-on, turn-off, and storage delay

he Bipolar transistor exhibits a few delay characteristics when turning on and off. Most transistors, and especially

ower transistors, exhibit long base-storage times that limit maximum frequency of operation in switching applications.

ne method for reducing this storage time is by using a Baker clamp.

Transistor parameters: alpha (α) and beta (β)

he proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy

oping of the emitter region and light doping of the base region causes many more electrons to be injected from themitter into the base than holes to be injected from the base into the emitter. The common-emitter current gain is

epresented by βF or hFE; it is approximately the ratio of the DC collector current to the DC base current in forward-

ctive region. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for

igh-power applications. Another important parameter is the common-base current gain, αF. The common-base current

ain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a

alue close to unity; between 0.98 and 0.998. It is less than unity due to recombination of charge carriers as they cross

he base region. Alpha and beta are more precisely related by the following identities (NPN transistor):

Structure

BJT consists of three differently doped semiconductor regions,

he emitter region, the base region and the collector region. These

egions are, respectively, p type, n type and p type in a PNP

ansistor, and n type, p type and n type in an NPN transistor. Each

emiconductor region is connected to a terminal, appropriately

beled: emitter (E), base (B) and collector (C).

he base is physically located between the emitter and the collector

nd is made from lightly doped, high resistivity material. The

ollector surrounds the emitter region, making it almost impossible

or the electrons injected into the base region to escape beingollected, thus making the resulting value of α very close to unity,

nd so, giving the transistor a large β. A cross section view of a BJT

ndicates that the collector–base junction has a much larger area

han the emitter–base junction.

he bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This means that

nterchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in

everse mode. Because the transistor's internal structure is usually optimized for forward-mode operation, interchanging

he collector and the emitter makes the values of α and β in reverse operation much smaller than those in forward

peration; often the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios

f the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large

everse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is

everse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection

fficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the

arriers injected into the emitter–base junction must come from the emitter.



Die of a KSY34 high-frequency

NPN transistor, base and emitter

connected via bonded wires

The symbol of an

NPN BJT. The

symbol is "not

pointing in."

The symbol of a

PNP BJT. The

symbol " pointsin proudly."

The low-performance "lateral" bipolar transistors sometimes used in CMOS

processes are sometimes designed symmetrically, that is, with no difference

between forward and backward operation.

Small changes in the voltage applied across the base–emitter terminals causes the

current that flows between the emitter and the collector to change significantly.

This effect can be used to amplify the input voltage or current. BJTs can be thought

of as voltage-controlled current sources, but are more simply characterized as

current-controlled current sources, or current amplifiers, due to the low impedance

at the base.

Early transistors were made from germanium but most modern BJTs are made from

silicon. A significant minority are also now made from gallium arsenide, especially

for very high speed applications (see HBT, below).

NPN

PN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor

he "base") between two N-doped layers. A small current entering the base is amplified to produce

large collector and emitter current. That is, when there is a positive potential difference measured

om the emitter of an NPN transistor to its base (i.e., when the base is high relative to the emitter)s well as positive potential difference measured from the base to the collector, the transistor

ecomes active. In this "on" state, current flows between the collector and emitter of the transistor.

Most of the current is carried by electrons moving from emitter to collector as minority carriers in

he P-type base region. To allow for greater current and faster operation, most bipolar transistors

sed today are NPN because electron mobility is higher than hole mobility.

mnemonic device for the NPN transistor symbol is "not pointing in," based on the arrows in the

ymbol and the letters in the name.[5]

NP

he other type of BJT is the PNP, consisting of a layer of N-doped semiconductor between two

yers of P-doped material. A small current leaving the base is amplified in the collector output.

hat is, a PNP transistor is "on" when its base is pulled low relative to the emitter.

he arrows in the NPN and PNP transistor symbols are on the emitter legs and point in the

irection of the conventional current flow when the device is in forward active mode.

mnemonic device for the PNP transistor symbol is " pointing in ( proudly/ permanently)," based on

he arrows in the symbol and the letters in the name.[6]

Heterojunction bipolar transistor

he heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high

equencies up to several hundred GHz. It is common in modern ultrafast circuits, mostly RF systems.[7][8]

eterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is

omposed of a larger bandgap material than the base. The figure shows that this difference in bandgap allows the barrier

or holes to inject backward into the base, denoted in figure as ∆φp, to be made large, while the barrier for electrons to

nject into the base ∆φn is made low. This barrier arrangement helps reduce minority carrier injection from the base

hen the emitter-base junction is under forward bias, and thus reduces base current and increases emitter injection

fficiency.

he improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower

esistance to access the base electrode. In the more traditional BJT, also referred to as homojunction BJT, the efficiency

f carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and

ase, which means the base must be lightly doped to obtain high injection efficiency, making its resistance relatively



Bands in graded heterojunction NPN

bipolar transistor. Barriers indicatedfor electrons to move from emitter to

base, and for holes to be injected

backward from base to emitter; Also,

grading of bandgap in base assists

electron transport in base region; Light

colors indicate depleted regions

Applied voltagesB-E Junction

Bias (NPN)

B-C Junction

Bias (NPN)Mode (NPN)

E < B < C Forward Reverse Forward-activeE < B > C Forward Forward Saturation

E > B < C Reverse Reverse Cut-off

E > B > C Reverse Forward Reverse-active

Applied voltagesB-E JunctionBias (PNP)

B-C JunctionBias (PNP)

Mode (PNP)

E < B < C Reverse Forward Reverse-active

E < B > C Reverse Reverse Cut-off

E > B < C Forward Forward Saturation

E > B > C Forward Reverse Forward-active

The relationship between , and .

high. In addition, higher doping in the base can improve figures of merit like the

Early voltage by lessening base narrowing.

The grading of composition in the base, for example, by progressively increasing

the amount of germanium in a SiGe transistor, causes a gradient in bandgap in

the neutral base, denoted in the figure by ∆φG, providing a "built-in" field that

assists electron transport across the base. That drift component of transport aids

the normal diffusive transport, increasing the frequency response of the

transistor by shortening the transit time across the base.

Two commonly used HBTs are silicon–germanium and aluminum gallium

arsenide, though a wide variety of semiconductors may be used for the HBT

structure. HBT structures are usually grown by epitaxy techniques like MOCVD

and MBE.

Regions of operation

Bipolar

ansistors have five distinct regions of operation,

efined by BJT junction biases.

he modes of operation can be described in terms of the

pplied voltages (this description applies to NPN

ansistors; polarities are reversed for PNP transistors):

Forward-active: base higher than emitter,

collector higher than base (in this mode the

collector current is proportional to base current

by ).

Saturation: base higher than emitter, but collector

is not higher than base.Cut-Off: base lower than emitter, but collector

is higher than base. It means the transistor is

not letting conventional current go through

from collector to emitter.

Reverse-active: base lower than emitter,

collector lower than base: reverse

conventional current goes through transistor.

n terms of junction biasing: ('reverse biased

ase–collector junction' means Vbc < 0 for NPN,

pposite for PNP)

Forward-active (or simply, active): The

base–emitter junction is forward biased and

the base–collector junction is reverse biased.

Most bipolar transistors are designed to afford

the greatest common-emitter current gain, βF,

in forward-active mode. If this is the case, the collector–emitter current is approximately proportional to the base

current, but many times larger, for small base current variations.

Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region,

a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles.

Because most BJTs are designed to maximize current gain in forward-active mode, the βF in inverted mode is

several (2–3 for the ordinary germanium transistor) times smaller. This transistor mode is seldom used, usually

being considered only for failsafe conditions and some types of bipolar logic. The reverse bias breakdown voltage

to the base may be an order of magnitude lower in this region.

Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current

conduction from the emitter to the collector (or the other direction in the case of NPN, with negatively charged



Structure and use of NPN transistor. Arrow

according to schematic.

carriers flowing from emitter to collector). This mode corresponds to a logical "on", or a closed switch.

Cutoff : In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is

very little current, which corresponds to a logical "off", or an open switch.

Avalanche breakdown region

lthough these regions are well defined for sufficiently large applied voltage, they overlap somewhat for small (less than

few hundred millivolts) biases. For example, in the typical grounded-emitter configuration of an NPN BJT used as a

ulldown switch in digital logic, the "off" state never involves a reverse-biased junction because the base voltage never

oes below ground; nevertheless the forward bias is close enough to zero that essentially no current flows, so this end of

he forward active region can be regarded as the cutoff region.

Active-mode NPN transistors in circuits

he diagram shows a schematic representation of an NPN transistor

onnected to two voltage sources. To make the transistor conduct

ppreciable current (on the order of 1 mA) from C to E, V BE must be above

minimum value sometimes referred to as the cut-in voltage. The cut-in

oltage is usually about 650 mV for silicon BJTs at room temperature but

an be different depending on the type of transistor and its biasing. This

pplied voltage causes the lower P-N junction to 'turn-on' allowing a flow of

ectrons from the emitter into the base. In active mode, the electric field

xisting between base and collector (caused by V CE) will cause the majority

f these electrons to cross the upper P-N junction into the collector to form

he collector current I C. The remainder of the electrons recombine with

oles, the majority carriers in the base, making a current through the base

onnection to form the base current, I B. As shown in the diagram, the

mitter current, I E, is the total transistor current, which is the sum of the

ther terminal currents, (i.e., I E = I B + I C).

n the diagram, the arrows representing current point in the direction of

onventional current – the flow of electrons is in the opposite direction of he arrows because electrons carry negative electric charge. In active mode,

he ratio of the collector current to the base current is called the DC current

ain. This gain is usually 100 or more, but robust circuit designs do not depend on the exact value (for example see

p-amp). The value of this gain for DC signals is referred to as , and the value of this gain for small signals is

eferred to as . That is, when a small change in the currents occurs, and sufficient time has passed for the new

ondition to reach a steady state is the ratio of the change in collector current to the change in base current. The

ymbol is used for both and .[9]

should also be noted that the emitter current is related to exponentially. At room temperature, an increase in

by approximately 60 mV increases the emitter current by a factor of 10. Because the base current is approximatelyroportional to the collector and emitter currents, they vary in the same way.

Active-mode PNP transistors in circuits

he diagram shows a schematic representation of a PNP transistor connected to two voltage sources. To make the

ansistor conduct appreciable current (on the order of 1 mA) from E to C, must be above a minimum value

ometimes referred to as the cut-in voltage. The cut-in voltage is usually about 650 mV for silicon BJTs at room

mperature but can be different depending on the type of transistor and its biasing. This applied voltage causes the

pper P-N junction to 'turn-on' allowing a flow of holes from the emitter into the base. In active mode, the electric field

xisting between the emitter and the collector (caused by ) causes the majority of these holes to cross the lower p-n

unction into the collector to form the collector current . The remainder of the holes recombine with electrons, the

majority carriers in the base, making a current through the base connection to form the base current, . As shown in

he diagram, the emitter current, , is the total transistor current, which is the sum of the other terminal currents (i.e.,

E = I B + I C).

n the diagram, the arrows representing current point in the direction of conventional current – the flow of holes is in the



Ebers–Moll Model for an NPN

transistor.[23]

* I B, I C, I E: base, collector and

emitter currents * I CD, I ED: collector and

emitter diode currents * αF, αR: forward and

reverse common-base current gains

Drift-field transistor – high speed bipolar junction transistor. Invented by Herbert Kroemer[19][20]

at the

Central Bureau of Telecommunications Technology of the German Postal Service, in 1953.

Spacistor – circa 1957.

Diffusion transistor – modern type bipolar junction transistor. Prototypes[21]

developed at Bell Labs in

1954.

Diffused-base transistor – first implementation of diffusion transistor.

Mesa transistor – Developed at Texas Instruments in 1957.

Planar transistor – the bipolar junction transistor that made mass-produced monolithic integrated

circuits possible. Developed by Dr. Jean Hoerni[22] at Fairchild in 1959.

Epitaxial transistor (http://www.computerhistory.org/semiconductor/timeline/1960-Epitaxial.html) – abipolar junction transistor made using vapor phase deposition. See epitaxy. Allows very precise control of

doping levels and gradients.

Theory and modeling

ransistors can be thought of as two diodes (P–N junctions) sharing a common region that minority carriers can move

hrough. A PNP BJT will function like two diodes that share an N-type cathode region, and the NPN like two diodes

haring a P-type anode region. Connecting two diodes with wires will not make a transistor, since minority carriers will

ot be able to get from one P–N junction to the other through the wire.

oth types of BJT function by letting a small current input to the base control an amplified output from the collector.

he result is that the transistor makes a good switch that is controlled by its base input. The BJT also makes a good

mplifier, since it can multiply a weak input signal to about 100 times its original strength. Networks of transistors are

sed to make powerful amplifiers with many different applications. In the discussion below, focus is on the NPN bipolar

ansistor. In the NPN transistor in what is called active mode, the base–emitter voltage and collector–base voltage

are positive, forward biasing the emitter–base junction and reverse-biasing the collector–base junction. In the

ctive mode of operation, electrons are injected from the forward biased n-type emitter region into the p-type base

here they diffuse as minority carriers to the reverse-biased n-type collector and are swept away by the electric field in

he reverse-biased collector–base junction. For a figure describing forward and reverse bias, see semiconductor diodes.

Large-signal models

n 1954 Jewell James Ebers and John L. Moll introduced their mathematical model of transistor currents:

bers–Moll model

he DC emitter and collector currents in active mode are well modeled by

n approximation to the Ebers–Moll model:

he base internal current is mainly by diffusion (see Fick's law) and

here

is the thermal voltage (approximately 26 mV at 300 K ≈ room temperature).

is the emitter current

is the collector current



Ebers–Moll Model for a PNP transistor.

Approximated Ebers–Moll Model for an

NPN transistor in the forward active mode.

The collector diode is reverse-biased so I CD

is virtually zero. Most of the emitter diode

current (αF is nearly 1) is drawn from the

collector, providing the amplification of the

base current.

is the common base forward short circuit current gain (0.98 to

0.998)

is the reverse saturation current of the base–emitter diode (on

the order of 10−15

to 10−12

amperes)

is the base–emitter voltage

is the diffusion constant for electrons in the p-type base

W is the base width

he and forward parameters are as described previously. A reverse

sometimes included in the model.

he unapproximated Ebers–Moll equations used to describe the three

urrents in any operating region are given below. These equations are based

n the transport model for a bipolar junction transistor.[24]

here


is the base current

is the emitter current

is the forward common emitter current gain (20 to 500)is the reverse common emitter current gain (0 to 20)

is the reverse saturation current (on the order of 10−15

to 10−12

amperes)

is the thermal voltage (approximately 26 mV at 300 K ≈ room temperature).

is the base–emitter voltage

is the base–collector voltage

ase-width modulation

Main article: Early Ef fect

s the collector–base voltage ( ) varies, the collector–base depletion region varies in size. An

ncrease in the collector–base voltage, for example, causes a greater reverse bias across the collector–base junction,

ncreasing the collector–base depletion region width, and decreasing the width of the base. This variation in base width

ften is called the "Early effect" after its discoverer James M. Early.

arrowing of the base width has two consequences:

There is a lesser chance for recombination within the "smaller" base region.

The charge gradient is increased across the base, and consequently, the current of minority carriers injected across

the emitter junction increases.

oth factors increase the collector or "output" current of the transistor in response to an increase in the collector–base

oltage.

n the forward-active region, the Early effect modifies the collector current ( ) and the forward common emitter

urrent gain ( ) as given by:[citation needed ]



Top: NPN base width for low collector-base

reverse bias; Bottom: narrower NPN basewidth for large collector-base reverse bias.

Hashed regions are depleted regions.

Hybrid-pi model

here:

is the collector–emitter voltage

is the Early voltage (15 V to 150 V)

is forward common-emitter current gain when = 0 V

is the output impedance


unchthrough

When the base–collector voltage reaches a certain (device specific) value,

he base–collector depletion region boundary meets the base–emitter

epletion region boundary. When in this state the transistor effectively has

o base. The device thus loses all gain when in this state.

Gummel–Poon charge-control model

he Gummel–Poon model[25]

is a detailed charge-controlled model of BJT dynamics, which has been adopted and

aborated by others to explain transistor dynamics in greater detail than the terminal-based models typically do [2]

http://ece-www.colorado.edu/~bart/book/book/chapter5/ch5_6.htm#5_6_2). This model also includes the dependence

f transistor -values upon the direct current levels in the transistor, which are assumed current-independent in the

bers–Moll model.[26]

mall-signal models

ybrid-pi model

Main article: hybrid-pi model

he hybrid-pi model is a popular circuit model used for analyzing

he small signal behavior of bipolar junction and field effect

ansistors. Sometimes it is also called Giacoletto model because it

as introduced by L.J. Giacoletto in 1969. The model can be quite

ccurate for low-frequency circuits and can easily be adapted for

igher frequency circuits with the addition of appropriate inter-

ectrode capacitances and other parasitic elements.

-parameter model

nother model commonly used to analyze BJT circuits is the "h-parameter" model, closely related to the hybrid-pi

model and the y-parameter two-port, but using input current and output voltage as independent variables, rather than

nput and output voltages. This two-port network is particularly suited to BJTs as it lends itself easily to the analysis of

rcuit behaviour, and may be used to develop further accurate models. As shown, the term "x" in the model represents

different BJT lead depending on the topology used. For common-emitter mode the various symbols take on the



Generalized h-parameter model of an NPN BJT.

Replace x with e , b or c for CE, CB and CC

topologies respectively.

pecific values as:

x = 'e' because it is a common-emitter topology

Terminal 1 = Base

Terminal 2 = Collector

Terminal 3 = Emitter

ii = Base current (ib)

io = Collector current (ic)

V in = Base-to-emitter voltage (V BE)

V o = Collector-to-emitter voltage (V CE)

nd the h-parameters are given by:

hix = hie – The input impedance of the transistor

(corresponding to the base resistance r pi).

hrx = hre – Represents the dependence of the transistor's I B–V BE curve on the value of V CE. It is usually very

small and is often neglected (assumed to be zero).

hfx = hfe – The current-gain of the transistor. This parameter is often specified as hFE or the DC current-gain

( β DC) in datasheets.

hox = 1/ hoe – The output impedance of transistor. The parameter hoe usually corresponds to the output admittance

of the bipolar transistor and has to be inverted to convert it to an impedance.

s shown, the h-parameters have lower-case subscripts and hence signify AC conditions or analyses. For DC conditions

hey are specified in upper-case. For the CE topology, an approximate h-parameter model is commonly used which

urther simplifies the circuit analysis. For this the hoe and hre parameters are neglected (that is, they are set to infinity

nd zero, respectively). It should also be noted that the h-parameter model as shown is suited to low-frequency, small-

gnal analysis. For high-frequency analyses the inter-electrode capacitances that are important at high frequencies must

e added.

tymology of hFE

he 'h' refers to its being an h-parameter, a set of parameters named for their origin in a hybrid equivalent circuit model.' is from forward current amplification also called the current gain. 'E' refers to the transistor operating in a common

mitter (CE) configuration. Capital letters used in the subscript indicate that hFE refers to a direct current circuit.

Applications

he BJT remains a device that excels in some applications, such as discrete circuit design, due to the very wide

election of BJT types available, and because of its high transconductance and output resistance compared to

MOSFETs. The BJT is also the choice for demanding analog circuits, especially for very-high-frequency applications,

uch as radio-frequency circuits for wireless systems. Bipolar transistors can be combined with MOSFETs in an

ntegrated circuit by using a BiCMOS process of wafer fabrication to create circuits that take advantage of the

pplication strengths of both types of transistor.

Temperature sensors

Main article: Silicon bandgap temperature sensor

ecause of the known temperature and current dependence of the forward-biased base–emitter junction voltage, the

JT can be used to measure temperature by subtracting two voltages at two different bias currents in a known ratio [3]

http://www.maxim-ic.com/appnotes.cfm/appnote_number/689).

Logarithmic converters

ecause base–emitter voltage varies as the log of the base–emitter and collector–emitter currents, a BJT can also be

sed to compute logarithms and anti-logarithms. A diode can also perform these nonlinear functions, but the transistor

rovides more circuit flexibility.



Vulnerabilities

xposure of the transistor to ionizing radiation causes radiation damage. Radiation causes a buildup of 'defects' in the

ase region that act as recombination centers. The resulting reduction in minority carrier lifetime causes gradual loss of

ain of the transistor.

ower BJTs are subject to a failure mode called secondary breakdown, in which excessive current and normal

mperfections in the silicon die cause portions of the silicon inside the device to become disproportionately hotter than

he others. The doped silicon has a negative temperature coefficient, meaning that it conducts more current at higher

mperatures. Thus, the hottest part of the die conducts the most current, causing its conductivity to increase, whichhen causes it to become progressively hotter again, until the device fails internally. The thermal runaway process

ssociated with secondary breakdown, once triggered, occurs almost instantly and may catastrophically damage the

ansistor package.

the emitter-base junction is reverse biased into avalanche or Zener mode and current flows for a short period of time,

he current gain of the BJT will be permanently degraded.

See also

Bipolar transistor biasingGummel plot

Technology CAD (TCAD)

References

^ a

b

c

Paul Horowitz and Winfield Hill (1989). [[The Art of Electronics]] (http://books.google.com

/books?id=bkOMDgwFA28C&pg=PA113&dq=bjt+charge+current+voltage+control+inauthor:horowitz+inauthor:hill) (2nd

ed.). Cambridge University Press. ISBN 978-0-521-37095-0. Wikilink embedded in URL title (help)

1.

^ Juin Jei Liou and Jiann S. Yuan (1998). Semiconductor Device Physics and Simulation (http://books.google.com /books?id=y343FTN1TU0C&pg=PA166&dq=charge-controlled+bjt+physics). Springer. ISBN 0-306-45724-5.

2.

^ General Electric (1962). Transistor Manual (6th ed.). p. 12. "If the principle of space charge neutrality is used in the

analysis of the transistor, it is evident that the collector current is controlled by means of the positive charge (hole

concentration) in the base region. ... When a transistor is used at higher frequencies, the fundamental limitation is the time it

takes the carriers to diffuse across the base region..." (same in 4th and 5th editions)

3.

^ Paolo Antognetti and Giuseppe Massobrio (1993). Semiconductor Device Modeling with Spice (http://books.google.com

/books?id=5IBYU9xrGaIC&pg=PA96&dq=gummel-poon+charge+model#PPA98,M1). McGraw–Hill Professional.

ISBN 0-07-134955-3.

4.

^ Alphonse J. Sistino (1996). Essentials of electronic circuitry (http://books.google.com/books?id=lmcHKS1lkrQC&

pg=PA64&dq=%22not+pointing%22+PNP). CRC Press. p. 64. ISBN 978-0-8247-9693-8.

5.

^ Alphonse J. Sistino (1996). Essentials of electronic circuitry (http://books.google.com/books?id=lmcHKS1lkrQC&

pg=PA64&dq=%22pointing+in%22+PNP). CRC Press. p. 102. ISBN 978-0-8247-9693-8.

6.

^ D.V. Morgan, Robin H. Williams (Editors) (1991). Physics and Technology of Heterojunction Devices

(http://books.google.com/books?id=C98iH7UDtzwC&pg=PA210&dq=%22SIGe+heterojunction%22#PPA201,M1). London:

Institution of Electrical Engineers (Peter Peregrinus Ltd.). ISBN 0-86341-204-1.

7.

^ Peter Ashburn (2003). SiGe Heterojunction Bipolar Transistors (http://worldcat.org/isbn/0470848383). New York: Wiley.

Chapter 10. ISBN 0-470-84838-3.

8.

^ Paul Horowitz and Winfield Hill (1989). The Art of Electronics (2nd ed.). Cambridge University Press. pp. 62–66.

ISBN 978-0-521-37095-0.

9.

^ Third case study – the solid state advent (http://hm-treasury.gov.uk/media

/B/C/queen_mary_ip_research_institute_p5_043_762kb.pdf) (PDF)

10.

^ Transistor Museum, Historic Transistor Photo Gallery, Bell Labs Type M1752 (http://semiconductormuseum.com

/PhotoGallery/PhotoGallery_M1752.htm)

11.

^ Morris, Peter Robin (1990). "4.2". A History of the World Semiconductor Industry. IEE History of Technology Series 12.

London: Peter Peregrinus Ltd. p. 29. ISBN 0-86341-227-0.

12.

^ Transistor Museum, Historic Transistor Photo Gallery, RCA TA153 (http://semiconductormuseum.com/PhotoGallery

/PhotoGallery_TA153.htm)

13.

^ High Speed Switching Transistor Handbook (2nd ed.). Motorola. 1963. p. 17.[1] (http://groups.google.com/group

/sci.electronics.components/tree/browse_frm/month/2003-04/c97c04dc783ab61e?rnum=21&

14.



_done=%2Fgroup%2Fsci.electronics.components%2Fbrowse_frm%2Fmonth%2F2003-04%3F)

^ Transistor Museum, Historic Transistor Photo Gallery, Western Electric 3N22 (http://semiconductormuseum.com

/PhotoGallery/PhotoGallery_3N22.htm)

15.

^ The Tetrode Power Transistor (http://ieeexplore.ieee.org/iel5/16/31551/01472171.pdf?arnumber=1472171) PDF16.

^ Transistor Museum, Historic Transistor Photo Gallery, Philco A01 (http://semiconductormuseum.com/PhotoGallery

/PhotoGallery_A01.htm)

17.

^ Transistor Museum, Historic Transistor Photo Gallery, Surface Barrier Transistor (http://semiconductormuseum.com

/PhotoGallery/PhotoGallery_SurfaceBarrier.htm)

18.

^ Herb’s Bipolar Transistors IEEE Transactions on Electron Devices, vol. 48, no. 11, November 2001

(http://ieeexplore.ieee.org/iel5/16/20748/00960370.pdf) PDF

19.

^ Influence of Mobility and Lifetime Variations on Drift-Field Effects in Silicon-Junction Devices (http://ieeexplore.ieee.org /iel5/16/31630/01474625.pdf?isnumber=&arnumber=1474625) PDF

20.

^ Transistor Museum, Historic Transistor Photo Gallery, Bell Labs Prototype Diffused Base Triode

(http://semiconductormuseum.com/PhotoGallery/PhotoGallery_Prototype_DiffusedBase.htm)

21.

^ Transistor Museum, Historic Transistor Photo Gallery, Fairchild 2N1613 (http://semiconductormuseum.com/PhotoGallery

/PhotoGallery_2N1613.htm)

22.

^ Adel S. Sedra and Kenneth C. Smith (1987). Microelectronic Circuits, second ed . p. 903. ISBN 0-03-007328-6.23.

^ A.S. Sedra and K.C. Smith (2004). Microelectronic Circuits (5th ed.). New York: Oxford. Eqs. 4.103–4.110, p. 305.

ISBN 0-19-514251-9.

24.

^ H. K. Gummel and R. C. Poon, "An integral charge control model of bipolar transistors," Bell Syst. Tech. J., vol. 49,

pp. 827–852, May–June 1970

25.

^ A.S. Sedra and K.C. Smith (2004). Microelectronic Circuits (5th ed.). New York: Oxford. p. 509. ISBN 0-19-514251-9.26.

External links

Simulation of a BJT in the Common Emitter Circuit (http://courses.ece.illinois.edu/ece110/simulation/bjt/)

Lessons In Electric Circuits – Bipolar Junction Transistors (http://www.faqs.org/docs/electric/Semi/SEMI_4.html)

(Note: this site shows current as a flow of electrons, rather than the convention of showing it as a flow of holes, so

the arrows may appear the other way around)

EncycloBEAMia – Bipolar Junction Transistor (http://encyclobeamia.solarbotics.net/articles

/bip_junct_trans.html)

Characteristic curves (http://www.st-and.ac.uk/~www_pa/Scots_Guide/info/comp/active/BiPolar/bpcur.html)The transistor (http://www.play-hookey.com/semiconductors/transistor.html) at play-hookey.com

How Do Transistors Work? (http://amasci.com/amateur/transis.html) by William Beaty

ENGI 242/ELEC 222: BJT Small Signal Models (http://ux.brookdalecc.edu/fac/engtech/andy/engi242

/bjt_models.pdf)

Transistor Museum, Historic Transistor Timeline (http://semiconductormuseum.com

/HistoricTransistorTimeline_Index.htm)

ECE 327: Transistor Basics (http://www.tedpavlic.com/teaching/osu/ece327/lab1_bjt

/lab1_bjt_transistor_basics.pdf) – Summarizes simple Ebers–Moll model of a bipolar transistor and gives several

common BJT circuits.

ECE 327: Procedures for Output Filtering Lab (http://www.tedpavlic.com/teaching/osu/ece327/lab7_proj

/lab7_proj_procedure.pdf) – Section 4 ("Power Amplifier") discusses design of a BJT-Sziklai-pair-based class-ABcurrent driver in detail.

BJT Operation description (http://nanohub.org/resources/5083) for undergraduate and first year graduate students

to describe the basic principles of operation of Bipolar Junction Transistor.

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