The Design of SiGe pnp HBTs

The Design of SiGe pnp HBTs

At present, SiGe technology development is almost exclusively centered on npn SiGe HBTs.

However, for high-speed analog and mixed-signal circuit applications, a complementary (npn+pnp) bipolar technology offers signification performance advantages over an npn-only technology .

for example : Push-pull circuits

Push-pull circuits

Iv

-VCC

+VCC

RL

OvClass B output stage

NPN and PNP of Si BJTs

Performance : npn Si BJTs > pnp Si BJTs

BJT) Si pnp a of base type(Nholeμ

BJT) Sinpn an of base type-(Pele

μ todue n

NPN SiGe HBT

Valance band offset in SiGe strained layers translates

into an induced conduction band offset

Enhance minorty electron transport

Base

Fermi-level 被拉平

PNP SiGe HBT

The valance band offset directly results in a valance band barrier , even at low injection.

strongly degrades minority hole transport and limits the frequece response.

Base

Simplistic hypothetical npn and pnp SiGe profiles

With const. E,B,C doping, and a Ge content not subject to thermodynamic stability constraint.

This artificial assumption on constant doping yields ac performance numbers (eg. fT) that are lower than what would be expected for a real complementary.

Profile Optimization Issues

Npn without any Ge retrograding into the collector(i.e. an abrupt transition from the peak Ge content to zero Ge content in the CB junction).

C-B junction

Abrupt transition

E B C

Ge profile


pnp without any Ge retrograding into the collector

An obvious valence band barrier even for low Ge content

Valence band barrier in pnp

acts to block minority holes transiting the base.

The pileup of accumulated holes produces a retarding electric field in the base , which compensates the Ge-grading-induced drift field.

jc

increasesdensity current theas

rsenseffect wo this

f

Jc

Decreasing

T

Retrograding Ge into the collector

retrograding of the Ge edge into the collector can “smooth” this valence band offset in the pnp SiGe HBT, although at the expense of film stability.

For an increase of the Ge retrograde from 0 to 40nm

doping. equalat eperformanc BJT Si pnp over the

Tfpeak in increase 2 aroughly yielding

FT and Current gain

Ge retrograding in PNP “smooth” valence band barrier

40-50 nm of Ge retrograding in the pnp SiGe HBT is sufficient to “ smooth” the valence band barrier

The box Ge retrograde (back side) on PNP

The box Ge retrograde is not effective in improving the pnp SiGe HBT performance , since it does not smooth the Ge barrier, but rather only pushes it deeper into collector

The effects of Ge retrograding on NPN

The effects of Ge retrograde on the npn SiGe HBT performance ------- minor, while the film stability is worse due to the additional Ge content.

So, we know using one Ge profile design for both npn and pnp SiGe HBTs is not optimum for high peak Ge content values.

Stability Constraints in PNP SiGe HBTs

The total amount of Ge that can be put into a given SiGe HBT -----limited by the thermodynamic stability.

Above the critical thickness the strain in

the SiGe film relaxes generating defects.

The empirical critical thickness of a SiGe multilayer

with a top-layer Si cap

----- approximately 4x the theoretical

stability result of Matthews and Blakeslee

Stability Constraints in PNP SiGe HBTs

The peak Ge content

The Ge retrograde distance trade off

The best design point in PNP

Similar exercise for the npn

For npn SiGe HBT , the ac performance is not sensitive to the SiGe profile shapes used.

So the same Ge profile may be used for both pnp and npn SiGe HBTs.

being advantageous from a

fabrication viewpoint.

Low-Injection Theory

Minority carrier transport in an npn SiGe HBT

The minority carrier base transit time ----- determined by the net force acting on electron

resulting from the induced electric field.

b

BEBE

W

ibnb

b

kTqVkTqV

C

xnxDdxxP

eeqJ

02

//

)()()(

)]()[(

B

The net force on the electrons

Two components :

(1). The quasi-electric field due to the gradient

induced by conduction band offset.

(2). The built-in electric field.

The hole current density with a nonuniform bandgap :

dx

dEp

dx

dpqDJ V

ppp


offset band valence SiGe

][

) (doping potential 的起引不均勻所造成

dxVEd

dx

dq

bandvalenceactual

dxVdE

).q

kT

μ

Drelation(Einsteinclassical theand, )dx

dfield(εelectric

inbuiltthe0),p

(Jion approximat Webster theApplying

dx

Ed

qdx

dp

qp

kT

dx

d V ][1

The net force acting on the electrons becomes

finally,


dxVEd

dx

dp

q

kT

dxCEd

fieldelectricinbuilt

dx

dq

fieldelectricquasi

dxCEd

nF

][][

][

dx

GEd

dx

dp

q

kT

nF

][


So the base transit time is determined only by the total band offset across the neutral base.

--------and is independent of its distribution between conduction and valence bands for low-injection operation(i.e. p=Nab).

BGE

Impact of High Injection

To shed light on this issue, we consider the following four representative band offset distributions(band alignments) :

. off-rollfrequency cutoff affect thecan

VE

CE

Four representative band offsets (with the same total band offset)

gVC EEE 0 gCV EEE 0

gVgC EEEE 2gCgV EEEE 2

(1) This case is closest to strained SiGe on Si

(2) This case is applicable to strained Si on relaxed SiGe

(3) This case is applicable to published SiGeC bandgap predictions

(4)

Simulation

2-D dc and ac simulations were performed for a 0.5µm emitter width npn SiGe HBT.

FT V.S. JC

)1(

)2()3(

)4(

The conduction band edge of simulation

The explantation of the physics

This case gives the highest(best) FT and the highest(best) Jcritical , because it has the largest valence band offset , which acts to effectively prevent hole injection into the collector.

gCgV EEEE 2

(4)


(3) This case is applicable to published SiGeC bandgap predictions

gVgC EEEE 2

This case gives the lowest FT and Jcritical, because it has the largest conduction band offset which serves as a barrier to electron, and thus results in excess charge storage.


For FT and Jcritical

(1) This case is closest to strained SiGe on Si

gVC EEE 0gCV EEE 0

(2) This case is applicable to strained Si on relaxed SiGe

this coduction band barrier

height

this conduction band barrier height resulting from the pileup of holes

Because

SiGe HBTs under High –Current density operation

From the viewpoint of improving the ac characteristics of SiGe HBTs under high-current density operation, a large positive valence band offset together with a negative conduction band offset is the most desirable bandgap offset distribution.(as condition (4) ).

It is worth noting that these offsets(condition (1) ) are in fact different from those produced by strained SiGe on Si (i.e. mostly EV, with a small EC.)


One way to minimize high-injection barrier effects in SiGe HBTs is to retrograde the mole fraction deep into

the collector.

Low-C-content SiGeC

Low-C-content SiGeC layers can provide better thermodynamic stability than strained SiGe

- So we can allow a higher average Ge mole fraction for a deeper grading.

Ge-Induced Collector-Base Field Effects

The specifics of the backside Ge profile(i.e. on the CB side of the neutral base) strongly influence high-injection heterojunction barrier effects, which produce premature roll-off of and FT at high current density.

Here we will show that the backside Ge profile also alters the electric field distribution in the CB space-charge region, and thereby indirectly affects impact ionization in SiGe HBTs.

Influence on Impact Ionization

For a strained SiGe layer on Si, the band offset in the SiGe film predominantly resides in the valence band and its value is proportional to the Ge content.

- according to ΔEV =0.74x (eV), where x is Ge

fraction (i.e. 10% Ge=0.10) .

Valence band edge for Strained SiGe on Si (NPN)(NPN)

This SiGe “control” profile is labeled “0nm Ge”.

(i.e. the location of the SiGe-Si heterointerface is referenced to the metallurgical CB junction)

B-C junction

6nm

Ge profile

Heterojunction-Induced Quasi-Electric Field

This change in the valence band creates a heterojunction-induced quasi-electric field

For the present SiGe control profile

. distance retrograde theis D , 74.0)0(

rrr

VGe D

x

D

Eq

6nMD , % 10 for x , V/cm 10 23.1 r5 Ge

Which is larger than the peak field formed by the doping-induced charge in the CB space charge region.

Heterojunction-Induced Quasi-Electric Field

It is clear that as the backside Ge retrograde location moves toward the neutral collector, the peak electric field moves in the same direction and the magnitude of the peak electric field drops.

絕對

值

Avalanche multiplication factor

This decrease of the peak field reduces the impact ionization rate , as reflected in the avalanche multiplication factor (M-1) .

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The Design of SiGe pnp HBTs