Electric field effects associated with the backside Ge profile inSiGe HBTs

Gang Zhang a, John D. Cressler a,*, Lou Lanzerotti a,b, Rob Johnson a,b,Zhenrong Jin a, Shiming Zhang a, Guofu Niu a, Alvin Joseph a,b, David Harame a,b

a Department of Electrical and Computer Engineering, Alabama Microelectronics Science and Technology Center, Auburn University,

200 Broun Hall, Auburn, AL 36849, USAb IBM Microelectronics, Essex Junction, VT 05452, USA

Received in revised form 18 October 2001; accepted 25 October 2001

Abstract

A comprehensive investigation of electric field effects associated with the backside Ge profile in SiGe heterojunction

bipolar transistors (HBTs) is conducted using calibrated simulations. We show for the first time that the backside Ge

retrograde can alter the local electric field distribution in the base–collector space-charge region near the SiGe to

Si heterojunction, thereby affecting the impact ionization and the apparent neutral base recombination (NBR)

ðIBðVCBÞ=IBðVCB ¼ 0ÞÞ of SiGe HBTs. The changes in the electric field induced by the Ge-induced band offsets con-

tributes to a decrease of the observed impact ionization between comparably doped SiGe HBTs and Si bipolar junction

transistors (BJTs), as well as an improved VCB dependence of IB (apparent decrease in NBR). Experimental data on

SiGe HBTs with various Ge profiles and a Si BJT control are used to support our claims. � 2002 Elsevier Science Ltd.

All rights reserved.

Keywords: SiGe heterojunction bipolar transistor; Electric field effects; Impact ionization; Neutral base recombination

1. Introduction

The introduction of Ge into the base region of the Si

bipolar junction transistor (BJT) to form the SiGe het-

erojunction bipolar transistor (HBT) significantly im-

proves transistor performance at a modest increase in

processing complexity, and thus SiGe HBT technology

has generated worldwide interest for emerging digital,

analog, and RF applications. Previous work shows that

the characteristics of SiGe HBTs are highly sensitive to

the Ge profile shape. For instance, it is well known

that the backside Ge profile strongly influences high-

injection heterojunction barrier effects, which produce

premature roll off of b and fT at high current density [1].

In this paper, we show for the first time, that the

backside Ge profile also alters the electric field distri-

bution in the base–collector space-charge region, and

thereby affects both impact ionization and apparent

neutral base recombination (NBR) ðIBðVCBÞ=IBðVCB ¼0ÞÞ in SiGe HBTs. The impact ionization and apparent

NBR are critical to the breakdown voltage and output

conductance of SiGe HBTs, and are thus key consider-

ations for circuit designers.

First, we use 2D simulations to show how the

backside Ge profile shape influences the collector–base

electric field distribution and how this effect is coupled to

impact ionization and apparent NBR in SiGe HBTs. We

then use experimental data on advanced SiGe HBTs to

validate our claims.

2. Simulation approach

We constructed a 2D MEDICI [2] model based on

the transistor layout and measured SIMS data. The

Solid-State Electronics 46 (2002) 655–659

*Corresponding author. Address: Department of Electrical

and Computer Engineering, Auburn University, 420 Broun

Hall, Auburn, AL 36849, USA. Tel.: +1-334-844-1872; fax: +1-

334-844-1888.

E-mail address: cressler@eng.auburn.edu (J.D. Cressler).

0038-1101/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

PII: S0038-1101 (01 )00333-1

model parameters were tuned by precise calibration to

measured dc and ac device data from a self-aligned,

deep- and shallow-trench isolation, polysilicon emitter,

graded UHV/CVD epitaxial SiGe base technology [3,4].

Table 1 shows the resultant MEDICI parameter values

used in this investigation. The SiGe profiles in the pre-

sent simulations are hypothetical Ge profiles in the

shape of a trapezoid, where the Ge starts at the metal-

lurgical EB junction, grades across the base, peaks at

10%, and then falls to zero at the BC metallurgical

junction over a distance of 6 nm (Fig. 1). This profile is

labeled ‘‘0 nm Ge’’ in the following figures. To explore

the effect of the backside Ge profile shape, we then

varied the SiGe retrograde distance and retrograde lo-

cation in the base–collector junction from this hypo-

thetical 0 nm Ge profile. The doping profile, the Ge

front-side profile, its ramp rate, and the Ge peak content

were all kept identical to facilitate unambiguous com-

parisons. A Si-only case was also simulated to clearly

distinguish heterojunction effects, and used identical

doping profiles, except for the absence of Ge. All sim-

ulations were performed in low injection to distinguish

these effects from previous reported high-injection effects

in SiGe HBTs [1].

3. Simulation results and discussion

For the strained SiGe/Si structure, the band offset in

the SiGe film predominantly resides in the valence

band and its value is proportional to the Ge content:

DEV ¼ 0:74x eV, where x is Ge content [5]. In the

Ge retrograde region at the base–collector junction, the

varying Ge content produces an abrupt change in

the valence band, which is shown in Fig. 1 for the 0 nm

SiGe profile. This change in the valence band cre-

ates a heterojunction-induced electric field, which can

be evaluated for a given valence band grading as

Ge ¼ �qð0� DEVÞ=Dr ¼ 0:74x=Dr, for a linear Ge ret-

rograde, where Dr is the retrograde distance. In our

device model, the value of this electric field is approxi-

mately Ge ¼ 1:23� 105 V/cm for x ¼ 10% and Dr ¼ 6

nm, which is even greater than the peak field formed by

the doping-induced charge in the base–collector space-

charge region (Fig. 2). The impact of Ge retrograde lo-

cation on the resultant electric field in the base–collector

junction is shown in Fig. 2, where the 100 and 160 nm

Ge profiles have the Ge retrograde locations 100 and 160

nm deeper, respectively, than that of the 0 nm Ge (the

retrograde distance is fixed).

It is clear that as the Ge backside profile moves to-

ward the neutral collector, the electric field peak moves

in the same direction and the magnitude of the peak field

drops. This decrease of the peak field reduces the impact

ionization rate, as is measured by the multiplication

factor (M-1) in our simulations (Fig. 3). As Fig. 3 sug-

gests, with a sufficiently deep location of the Ge retro-

grade into the collector, the M-1 of a SiGe HBT can be

even lower than that of a comparably constructed Si

Table 1

MEDICI parameters tuned to measured data in the simulation

Tuned MEDICI parameters Value

NSRHN (cm�3) 1� 1017

NSRHP (cm�3) 1� 1017

TAUN0 (s) 3� 10�5

TAUP0 (s) 1� 10�5

V0.BGN (eV) Si 6:92� 10�3

N0.BGN (cm�3) Si 1:3� 1017

CON.BGN Si 0.5

V0.BGN (eV) SiGe 4� 10�3

N0.BGN (cm�3) SiGe 2� 1019

CON.BGN SiGe 0

Fig. 1. Valence bands and backside Ge profile for the SiGe

control and Si BJT control.

Fig. 2. Simulated electric field distribution in the base–collector

junction for three different Ge retrograde locations.

656 G. Zhang et al. / Solid-State Electronics 46 (2002) 655–659

BJT. This is physically the result of the decrease in the

field across the bulk of the region on the base side of the

base–collector space charge region which produces most

of the impact ionization, as can be seen in Fig. 2. This

phenomenon occurs when the Ge retrograde location is

beyond 170 nm in our device model, as shown in Fig. 3.

Since the Ge retrograde field Ge is reciprocally

proportional to the retrograde distance Dr, it is obvious

that we can also increase Dr to reduce Ge and hence

M-1, as shown in Fig. 4. Subsequently, the M-1 of the

SiGe HBT drops as the peak field is reduced by the Dr

increase (Fig. 5). We observe again that the M-1 of the

SiGe HBT can be reduced even lower than that of Si

BJTs, if Dr is sufficiently large.

The heterojunction-induced electric field in the base–

collector space-charge region should also impact the bias

dependence of the base current. We investigated this by

examining the relative base current change with VCB and

the hole distribution in the base–collector region, for

various backside Ge profiles. Fig. 6 shows that the IB of

the SiGe HBTs becomes less dependent on VCB as the

retrograde distance Dr increases. We refer to this as an

‘‘apparent’’ NBR decrease since the VCB dependence of

IB decreases (a classical signature of improved NBR).

Note, however, that the trap density across the base is

held fixed in all the simulations, and hence this effect is

clearly a field effect, not a recombination effect. This

improvement of IB(VCB) can be explained by the hole

distribution near the boundary of the base–collector

junction, as shown in Fig. 7. When Dr increases, the

electric field at the boundary decreases (Fig. 4), and thus

the space-charge region consumes less neutral base re-

gion for a fixed VCB (Fig. 7). The location of the Ge

retrograde also affects the base hole density distribution

through the influence of the field in the base–collector

Fig. 3. Simulated M-1 vs. the location of Ge retrograde.

Fig. 4. Simulated electric field vs. Ge retrograde distance.

Fig. 5. Simulated M-1 vs. Ge retrograde distance.

Fig. 6. Relative IB change vs. VCB for different Ge retrograde

distance.

G. Zhang et al. / Solid-State Electronics 46 (2002) 655–659 657

space-charge region, as shown in Fig. 8. The con-

sequence of pushing the Ge backside deeper into the

collector is also a reduction of the relative IB drop with

increasing VCB, which is shown in Fig. 9.

This improvement in IB(VCB) is clearly important for

circuit applications because it affects the output con-

ductance, and particularly the difference in VA between

forced voltage and forced current drive operation, as

discussed at length in Ref. [6]. This observation also

potentially provides a means to minimize this enhanced

(worse) IC(VCB) dependence commonly observed in SiGe

HBTs compared to Si BJTs [6].

4. Experiment

To validate our simulation results, SiGe HBTs were

fabricated with Ge backside profile extended 90 and 150

nm, respectively, deeper into the collector than that of

an identical-processed SiGe control profile. A compa-

rable Si-only device was also included for comparison.

The front-side EB Ge grading was kept identical for all

the Ge profiles and all Ge backside profiles have the

same retrograde distance Dr. The collector profiles for

all four wafers were identical. The measured multipli-

cation factors (M-1) for the SiGe HBTs, SiGe control,

and Si BJT are shown in Fig. 10 [7]. Both the 90 and 150

nm Ge profiles have lower M-1 than the SiGe control,

and all of the SiGe profiles have lower M-1 than the Si

BJT, qualitatively consistent with our simulations. This

improvement in M-1 results in a BVCEO improvement

0.25 and 0.50 V over the SiGe control for the 90 and 150

nm profiles, respectively. As shown in Fig. 11, the VCBdependence of IB is also improved as the Ge backside

Fig. 7. Simulated hole concentrations for different Ge retro-

grade distance.

Fig. 8. Simulated hole concentrations for different Ge retro-

grade location.

Fig. 9. Relative IB change vs. VCB for different Ge retrograde

distance.

Fig. 10. Measured M-1 vs. VCB for different Ge retrograde lo-

cation.

658 G. Zhang et al. / Solid-State Electronics 46 (2002) 655–659

location increases, again consistent with our simula-

tions.

5. Summary

The impact of Ge backside profile on impact ion-

ization and apparent neutral base recombination has

been investigated using calibrated 2D simulations, and

are confirmed experimentally. The heterojunction-

induced electric field associated with the Ge backside

location and retrograde distance has a strong impact on

both impact ionization and apparent NBR, and can in

principle be used to improve SiGe HBT breakdown

characteristics and output conductance.

Acknowledgements

This work was supported by an IBM University

Partner Award. The wafers were fabricated at IBM

Microelectronics at Essex Junction, VT. The authors

would like to thank B. Meyerson, D. Herman, and the

IBM SiGe team for their support of this work.

References

[1] Joseph AJ, Cressler JD, Richey DM, Niu G. Optimization

of SiGe HBTs for operation at high current densities. IEEE

Trans Electron Dev 1999;46(5):1347–54.

[2] User’s Manual for MEDICI, 2D Semiconductor Device

Simulator, version 4.0, Avant! Inc., 1997.

[3] Harame DL, Comfort JH, Cressler JD, Crabb EF, Sun

JY, Meyerson BS, et al. Si/SiGe epitaxial-base transis-

tors––Part I: materials, physics, and circuits. IEEE Trans

Electron Dev 1995;429(3):455–68.

[4] Harame DL, Comfort JH, Cressler JD, Crabb EF, Sun

JY, Meyerson BS, et al. Si/SiGe epitaxial-base transis-

tors––Part II: Process integration and analog applications.

IEEE Trans Electron Dev 1995;42(3):469–82.

[5] People R. Physics and applications of GexSi1�x/Si strained

layer heterostructures. IEEE J Quant Electron 1986;

QE22(6):1696–710.

[6] Joseph AJ, Cressler JD, Richey DM, Jaeger RC, Harame

DL. Neutral base recombination and its influence on

the temperature dependence of early voltage and current

gain––early voltage production in UHV/CVD SiGe hetero-

junction bipolar transistors. IEEE Trans Electron Dev

1997;44(3):404–13.

[7] Niu G, Cressler JD, Zhang S, Usha G, Ahlgren DC.

Measurement of collector–base junction avalanche multipli-

cation effects in advanced UHV/CVD SiGe HBTs. IEEE

Trans Electron Dev 1999;46(5):1007–15.

Fig. 11. Relative IB change vs. VCB for different Ge retrograde

location.

G. Zhang et al. / Solid-State Electronics 46 (2002) 655–659 659