Bipolar Junction Transistors - SCU · Bipolar Junction Transistors ... – amplifiers, switch etc. ... B. Basic Features of IC BJT Structures S. Saha HO #4: ELEN 251

Bipolar Junction Transistors

HO #4: ELEN 251 - Bipolar Transistors Page 1S. Saha

• Bipolar junction transistors (BJT) are active 3-terminal devices with areas of applications:– amplifiers, switch etc.– high-power circuits– high-speed logic circuits for high-speed computers.

• BJT structure:– sandwich of alternating type of Si-layers

♦ npn BJT: sequence of n-p-n♦ pnp BJT: sequence of p-n-p

– npn BJTs are most widely used.

N+ P

E B C

N- N+

A. A Typical NPN-BJT Fabrication


Antimony Implant

SiO2

N+ Buried Collector

P−

1. Form buried collector (BC)− grow oxide− mask #1: BC− implant Sb− drive-in

N+

P−

N-Epi 2. Form N− epitaxial collector− strip oxide− clean− grow single crystal epi layer

NPN-BJT Fabrication


Boron

N+

P−

N-Epi P+P+3. Form junction isolation

− grow oxide− mask #2: ISO− implant B− oxidation

N+

P−

N-EpiP+ P+N+

Phosphorus

4. Form deep collector (sinker)− mask #3: Collector− implant P− drive-in P+ and N+− oxidation

NPN-BJT Fabrication


5. Form Base region− mask #4: Base− implant B− oxidationN+

P−

N-EpiP+ P+N+P

B

N+

P−

N-EpiP+ P+N+P N+

As/P

6. Form Emitter region− mask #5: Emitter− implant As/P− drive-in emitter− oxidation

NPN-BJT Fabrication


7. Define contact area− mask #6: Contact− etch oxide

N+P−

N-EpiP+ P+N+P N+

N+

P−

N-EpiP+ P+N+P N+

Contacts

8. Interconnect− deposit Aluminum− anneal− mask #7: Metal

9. Deposit protective dielectric (SiO2 or Si3N4). Mask #8 to open bonding pads.

NPN-BJT: Active Region Doping Profile


a) Final 2d-cross-section of an npn BJT structure.

1.E+15

1.E+16

1.E+17

1.E+18

1.E+19

1.E+20

1.E+21

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Depth (um)

Con

cent

ratio

n (c

m-3

) N+ Emitter

P Base

N- Collector

N+ Burried layer

b) 1d-cross-section along the intrinsic device.

N+

P−

N-EpiP+ P+N+P N+

Contacts

Intrinsicdevice

NPN

NPN-BJT: A Typical Layout


Atypical layout for the vertical npn-BJTs described in the fabrication example:

CONTACTBase Emitter Collector

P+ IsolationN

−E

pita

xial

laye

r N+ Buried layerP+ Base

N+ Emitter

B. Basic Features of IC BJT Structures


1.E+15

1.E+16

1.E+17

1.E+18

1.E+19

1.E+20

1.E+21

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Depth (um)C

once

ntra

tion

(cm

-3) N+ Emitter

P Base

N- Collector

N+ Burried layer

• The base region is non-uniformly doped. This results in a built-in ε-field across the base which aids e− transport from E → C.

– this built-in ε-field is set up to establish an equilibrium between:

♦ mobile carriers attempt to diffuse away from high concentration regions

♦ mobile carriers pulled by the ε-filed (drift) of fixed ionized donors (ND

+) or acceptors (NA−) left behind by mobile carriers.

∴ ε-field is obtained from: diffusion = drift

Basic Features of IC BJT Structures


• The parasitic elements exist in a BJT structure include:– base resistance, RB from base contact to active area– collector resistance, RC (predominantly through n-layer).

• Isolation must be provided between adjacent devices:– reverse biased PN junctions– trench isolation.

• The N- collector region adjacent to the base:– reduces base-collector capacitance, CBC

– improves base-collector break down voltage, BVCB

– decreases base width (WB) modulation by the collector voltage

– but adds series resistance to the device.

C. Basic BJT Operation


B

E C

≈ 0.7 Vforward bias

WB

e− Mostof the

e−

N+ P N- N+

− + − +

5 Vreverse bias

hole

s• In a typical npn-BJT operation:

– an external potential (≈ 0.7 V) is applied across the E-B junction to forward bias it

– e− are injected into the base by the emitter. (Also, holes are injected into the emitter but their numbers are much smaller because of the relative values of NA, ND).

Basic Operation


– if WB << Ln (diffusion length) in the base, most of the injected e− get into the collector without recombining. A few do recombine; the holes necessary for this are supplied as base current.

– the e− reaching the collector are collected across the C-B junction depletion region.

ICIEElectrons flowingemitter to collector

Recombining electronsHoles

into emitter

E B C

IB

ICIB

IE

Since most of the injected e− reach the collector and only a few holes are injected into the emitter, the required IB << IC.

Therefore, the device has a substantial current gain.

Basic Operation - Derivation of Currents


In order to derive the basic relationship for e− current flowing between E → C, we assume that the device current gain is high.

∴ IB ≅ 0

or, Jp ≡ hole current density in the base ≅ 0

or, Jp ≅ 0 = qµppεx − qDp(dp/dx) (1)

For uniformly doped base, dp/dx = 0,∴εx = 0 and the e−

travelling through the base will move by diffusion only.

However, in IC-BJTs dp/dx ≠ 0 and εx ≠ 0. The direction of this field aids e− flow from E → C and retards e− flow from C → E.

dxdp

pqkT

dxdp

pD

p

px

11==∴

µε (2)



The current due to e− flow between E and C is given by: Jn = qµnnεx + qDn(dn/dx) (3)

Substituting the expression for aiding ε-field, εx from (2) in (3) we get:

We integrate (5) over the quasi-neutral base region, WB.

⎥⎦⎤

⎢⎣⎡ +=∴

dxdnp

dxdpnp

DqJ n

n

dxdn

Dqdxdp

pnkTJ nnn += µ (4)

( )dxpnd

pDq

J nn = (5)Or,



BE C

WB

N+ P N-xdbcxdeb

x = 0 x = WB

dxW

dxpnd

DdxW

qp

JB

on

B

on ∫∫

⎟⎠⎞⎜

⎝⎛

=∴ (6)

In (6) Jn is pulled outside the ∫ assuming no recombination of e- in the base region, i.e. Jn = constant.

( ) ( )0pnWpnDdxW

qp

J Bno

n

B

−=∴ ∫ (7)

pn-Product at the Depletion Edge


In thermal equilibrium, we define: npo ≡ # e− in neutral P-region

≅ ni2/NA

pno ≡ # holes in neutral N-region ≅ ni

2/ND

nno ≡ # e− in N region ≅ ND

ppo ≡ # holes in P region ≅ NA

nn

qkT

nNN

qkT

pnp

n

i

AD

bio

olnln:junction- of potential-inBuilt 2 ==−∴ φ

N P+ ++++ +

+ ++++

−−−−−−

−−−−−

nno

pno

ppo

npo

(8)

eq

pp

eq

nn

kTbi

nop

kTbi

pono

o

φ

φ

=

=: thatshowcan weThen,

(9)



Under the applied bias, VA we replace φbi by (φbi − VA) in (8) & (9) to obtain the non-equilibrium carrier density as:

We now assume low-level injection i.e. the injected carrier densities are small compared to the background concentrations so that nn = nno and pp = ppo. Then from (10) and (8),

N P+ ++++ +

+ ++++

−−−−−−

−−−−−

nn

pn

pno

pp

np

npoe

Vpp

eV

nn

kTAbi

q

np

kTAbi

q

pn

⎟⎠⎞⎜

⎝⎛ −

⎟⎠⎞⎜

⎝⎛ −

=

=

φ

φ (10)

(11)

⎟⎟⎠

⎞⎜⎜⎝

⎛==⎟

⎟

⎠

⎞

⎜⎜

⎝

⎛=

⎟⎠⎞⎜

⎝⎛ −

eq

nneVq

nn kTbi

pnkTAbi

pn oo

φφ(12)



Therefore, under low-level injection we get from (12):

Similarly from (9) and (11) we can show under low level injection:

Therefore, for low level injection we get from (13) and (14):

(13) & (14) define the minority carrier densities at the edge of the space charge region under applied bias while (15) defines the pn-product of carriers at the depletion edge.

nininpipip nnnnppnpnnoooo

// and ;// ,know We 2222 ====

(14)eVq

pp kTA

nn o=

(13)eVq

nn kTppA

o=

eVq

npn kTiA2= (15)



Thus, the pn-products at the edge of the depletion regions within the base of an npn-BJT are given by:

Now, the total charge in the un-depleted base region is given by:

( )

( )

(7)) Eq (from

0

2

2

2

∫

⎥⎦⎤

⎢⎣⎡ −

=∴

=

=

W

Dpdx

eenqJ

enWpn

enpn

B

o n

kTV BEq

kTV CBq

i

n

kTV CBq

iB

kTV BEq

i (16)

(17)

(18)

∫=W

pdxqAQB

oB (19)

Basic Operation - Base Gummel Number


This is an extremely important result. Note that:1 Usually, only one of the two exponential terms is important

since one junction is typically reversed biased. When the deviceis in saturation, both junctions are forward biased and both terms must be included.

2 The quantity, is called the base Gummel number.

It is the total integrated base charge (atoms/cm2). Since I ∝ 1/QB, it is important to minimize QB. ∴ IC transistors use low base doping levels.

⎥⎦⎤

⎢⎣⎡ −=∴ eeII kT

V BEqkTV CBq

sn (20)

(21)Q

DnAqIB

nis

222

where, =

∫=W

dxxNqAQ B

oA

B )(

IC Vs. VBE


Since BC junction is reversed biased the eqVBC/kT term is negligible and we can show from (20) and (21):

(22) predicts that IC vs. VBE is related exponentially. Slope = (kT/q)ln(10)

= 60 mV/(decade I) (@ 25 oC) Relationship holds extremely well for IC

transistors over many decades of JC. Generally, QB is obtained by integration

over the base region. Therefore, QB is typically well controlled to ~ 1012 cm-2 to give high IC for a given VBE.

eQ

nDAqI kT

BEVq

B

inn

222

−=∴ (22)

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

0.0 0.2 0.4 0.6VBE (V)

J C (A

/cm

2 )

ExptIdeal

Decadechangein JC

60mVJS

D. Current gain


• Let us consider the factors that can contribute to base current in a BJT: 1 Recombination in the neutral base region2 Hole injection into the emitter3 Recombination in the E-B space charge region.

ICIEElectrons flowingemitter to collector

Recombining electronsHoles

into emitter

E B C

IB

1. Recombination in the Neutral Base Region


Typically, some of the e− traveling the base will recombine with majority carrier holes. (This is not important for modern IC BJTs).

If we assume that the base is uniformly doped so that εx = 0, the e− current and continuity equations are:

N+ Nnp

npo

P

QBpn

pno

pno

02

2

=−

−

=

τ n

ppn

pnn

n pn

xdnd

D

dxnd

DqAI

o

(23)

(24)

Recombination in the Neutral Base Region


Similar to PN-junction, the general solution of (24) is: np − npo = K1e−x/Ln + K2ex/Ln (25)

The appropriate boundary conditions (from Eq 13) are: np(x = 0) = npoe

qVBE/kT

np(x = WB) ≅ 0

Using these boundary conditions we get from (25):

Substituting (26) in (23) we get the emitter (InE) and collector (InC) e− currents.

⎟⎠

⎞⎜⎝

⎛

⎟⎠

⎞⎜⎝

⎛ −

=

LWL

xW

eV

nn

n

B

n

B

kT

q

ppBE

o

sinh

sinh(26)

Base Transport Factor


The ratio of these two currents is defined as the base transport factor and is given by: αT ≡ InC/ InE = sech(WB/Ln) (29)

In modern IC BJTs, WB << Ln and the recombination in the neutral base region is significantly low.

If WB << Ln, we can use series expansion of sech(WB/Ln) from (29) to get: αT ≅ 1 − WB

2/2Ln2

) (@ csc1

0) (@ coth1

Bn

BkT

q

n

n

C

n

BkT

q

n

n

E

Wx L

WheV

L

n pDqAIn

xL

WeV

L

n pDqAIn

BEo

BEo

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛ −=

=⎟⎟⎠

⎞⎜⎜⎝

⎛⎟⎠

⎞⎜⎝

⎛ −= (27)

(28)

Base Transport Factor


In a typical advanced BJT, WB ≤ 1 µm and Ln ≥ 30 µm ∴αT ≅ 0.9994.

This value of αT would imply a forward current gain:

which is higher than the normally observed value of βF in IC BJTs with WB = 1 µm. Thus, αT is NOT usually a limiting factor in current gain.

The base current due to αT is: (InE - αTInE = (1 - αT)InE)

where τn is e− lifetime in the base.

⎟⎠⎞⎜

⎝⎛ −=∴ 1

2

2

eN

WnAqI kT

V BEq

nA

BiEBREC τ

(30)

16001

>−

=−

≅≡T

T

nCnE

nET

B

CF II

III

αααβ

2. Hole Injection into the Emitter


The dominant mechanism in limiting β in modern BJTs is hole injection from B to E. This process must occur because VBE reduces E → B e-flow barrier and also B → E hole flow the barrier.

The injected hole currents in each case come directly from the analysis of long base and short base diodes (E denotes emitter properties):

⎟⎠

⎞⎜⎝

⎛ −=<<

⎟⎠

⎞⎜⎝

⎛ −=>>

1:

1:

2

2

eV

xND

nqAILx

eV

LND

nqAILx

kT

q

EDE

pEipEpEE

kT

q

pEDE

pEipEpEE

BE

BE

(31)

(32)

E B CxE WB

nppn pn

xE >> LpE

xE

xE << LpE

E B C

nppn pn

Emitter Efficiency


The emitter injection efficiency is defined as:

Then from (27) and (32) we get:

(If WB >> Ln or XE >> Lp, then LB diode approximations replace WBand/or XE with Ln or Lp.)

(34) is only approximately correct in IC structures because NA and ND are not constant. Note that γ is made close to 1 by: (1) making NDE >> NAB; (2) WB small; (3) XE large (prevent hole recombination at E contact).

⎟⎟⎠

⎞⎜⎜⎝

⎛+==

+=

II

II

III

nE

pE

TOT

nE

pEnE

nE 1/1γ (33)

DNDN

xW

nBDE

pEAB

E

B+=

1

1γ (34)

3. E-B Space Charge Recombination


αT and γ are independent of VBE imply that the ratio of collector to base current is a constant, independent of VBE i.e. current level.

In practice, the ratio of the two currents is NOT independent of IC. At low levels the dominant reason is recombination in the E-B depletion region.

From PN junction theory, we find that some recombination of the carriers moving through the depletion region will occur, causing a recombination current.

E B Ce−

injectionholeinjection

*recombination

E-B Space Charge Recombination


exnqAI kTVq

o

EiREC

A

22τ

= (35)

where τo is the lifetime in the depletion region.

Note:– This current flows in the EB circuit and does not directly affect IC.

Thus, as IREC becomes important, the ratio of IC/IB will change.– eqVBE/2kT dependence is important at low current levels.

Summarizing our discussion of current gain:

II

III

II

II

nE

REC

nE

nCnE

nE

pE

C

B +−

+≅≡β1

eV

nDxWN

LW

DxNDWN

kT

q

oin

EBA

n

B

nED

PBA BE22

2

221 −++≅

τβ(36)

E. Heavy Doping Effect in BJTs


The modern BJTs are limited in β due to emitter efficiency. From simple analysis we found for SB-diodes Eq (34):

Eq (37) ⇒ γ↑ as NDE↑

∴ β[≅ γ/[1−γ) ]↑ as NDE↑ for the heavily doped emitters

DNDN

xW

nBDE

pEAB

E

B+=

1

1γ(37)

1019 1020 1021

β max

NDE (cm−3)

Simple theory (Eq 37)

Actual

Heavy Doping Effect: Bandgap Narrowing


β↓ in actual devices is due to heavy doping effect in the emitter.

The effective ionization energy for impurities decreases at high doping (> 1018 cm−3) level which can be described as bandgap narrowing (BGN) in the semiconductor. Assume,

∆Eg = reduction in energy gap, Eg due to heavy doping effect Ego = bandgap for the lightly doped value of semiconductors

Due to BGN, the intrinsic carrier concentration is given by: nie

2 = CT3e−(Ego

− ∆Eg)/kT = nio

2e∆Eg)/kT (38)

where, nio2 = CT3e−Ego/kT is the lightly doped value of ni.

BGN is directly responsible for γ↓ as well as β↓ in actual devices.

BGN due to Heavy Doping Effect


For an NPN transistor in the active mode of operation, the injected e− current into base is:

Here, QB = base Gummel number; WB = width of the neutral base region.

Similarly, the injected hole current into emitter is:

Here, QE = emitter Gummel number; LpE = hole diffusion length into E. If width of the neutral E, xE < LpE, we use xE in (40).

eW dxxN

nDqe

QnDq

J kTV BEq

AB

ibnBkT

V BEq

B

ibnBnE

B∫=≅

0

222

)((39)

eL dxxN

nDqeQ

nDqJ kT

V BEq

DE

ienBkT

V BEq

E

ienBpE

pE∫=≅

0

222

)((40)



(41)

∫∫+

=+

=∴

L dxxNnD

W dxxNnDJJ

pE

B

DEibnB

ABiepE

nE

pE

0

2

0

2

)(

)(1

1

1

1γ

Assuming no BGN in the lightly doped (< 1018 cm−3) base region, we get from (38): nie

2/nib2 = e∆Eg/kT

Therefore, If NAB and NDE are constants, we get from (41):

If xE < LpE, replace LpE by xE in (42).

(42)kTE

pE

B

DE

AB

nB

pE

nE

pEg

eLW

NN

DD

JJ ∆

+

=+

=

1

1

1

1γ



(43)kTE

pE

B

DE

AB

nB

pE

nE

pEg

eLW

NN

DD

JJ ∆

+

=+

=

1

1

1

1γ

• Eq (34) shows that γ↓ by the term e∆Eg/kT

• In order to estimate the reduction in emitter efficiency we need to know how ∆Eg varies with NDE.

• Typically ∆Eg ≈ 100 − 200 meV for NDE ≈ 1019 − 1020 cm−3.

• Note the validity of (43):– NDE and NAB are not constants but f(x)– in heavily doped regions, the hole lifetime and hence LpE

depends strongly on doping level.

Home Work 2: Due April 14, 2005


1) The cross-section of a double-poly bipolar transistor fabricated as part of a silicon IC is shown below. Design a plausible process flow to fabricate such a structure following the idea of junction isolated npn-bipolar process flow discussed in class. Your answers should be given in terms of a series of 2d-sketches of the structure after each major process steps.

Briefly explain your reasoning for each step and the order you choose to do things. Clearly, label all the sketches.

Trench-Isolated double-poly NPN



2) An npn-transistor has a cross-sectional area of 10−5 cm2 and a quasi-neutral base region that is uniformly doped with NA = 4x1017 cm−3, Dn = 18 cm2/sec, and WB = 0.5 µm.

(a) If a close emitter contact is used so that the recombination in the neutral emitter can be neglected, find the emitter injection efficiency γ. Assume that the total emitter Gummel number is 8x109 atoms and Dp = 2 cm2/sec.

(b) If the minority-carrier lifetime in the base is 10−6 sec, estimate the base transport factor αT.

(c) What is β for this transistor?

3) Consider a uniformly doped p-base region of an npn-BJT device with base width = WB, emitter area = AE, base doping = NA under active normal bias.

Use electron drift-diffusion Eq. to show that the injected electron current InE from n-emitter at the edge of the emitter depletion region in the p-base region can be approximately given by:

⎟⎠

⎞⎜⎝

⎛ −= 12

eVq

WNnDqA

In kT

A B

inEE

BE



4) A bipolar transistor is fabricated by growing a P-type epitaxial layer on an N-substrate. The P-type base is uniformly doped at 5x1017 cm−3. The epi-layer is 1.0 µm thick. The emitter is formed by ion implantation of As (Q = 5x1015 @ 50 KeV). The emitter depth of ≅ 0.28 µm.

(a) Calculate the base Gummel number.

(b) Estimate the transistor current gain β assuming it is limited by emitter efficiency. Neglect heavy doping effects and assume Dn = 2Dp.

(c) If a reduction in energy gap due to heavily doped emitter is ~ 200 mV, re-calculate β to include heavy doping effects. Assume µn = 1350 cm2/v.sec, τpE =0.2 nsec, and LpE = 0.2 µm. State any assumptions you make.

(d) Comment on your results in part (b) and (c).

Documents

Bipolar Junction Transistors - SCU · Bipolar Junction Transistors ... – amplifiers, switch etc. ... B. Basic Features of IC BJT Structures S. Saha HO #4: ELEN 251