Realizing High-voltage Thin Film Lateral Bipolar Transistors on SOI with a Collector-tub

8/14/2019 Realizing High-voltage Thin Film Lateral Bipolar Transistors on SOI with a Collector-tub

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Realizing High-voltage Thin Film Lateral

Bipolar Transistors on SOI with a Collector-tub

Sukhendu Deb Roy and M. Jagadesh KumarDepartment of Electrical Engineering,

Indian Institute of Technology, Delhi,

Hauz Khas, New Delhi 110 016, INDIA.

Email: [email protected] Fax: 91-11-2658 1264

Abstract

Using two-dimensional process and device simulation, we present in this

paper, a collector-tub lateral bipolar transistor (CTLBT) on silicon-on-insulator (SOI)

to improve the collector-emitter breakdown voltage (BVCEO). We demonstrate that the

presence of the collector-tub, which is realized by etching the buried oxide (BOX)

followed by an N-implantation on the collector NN+-junction side, firstly reduces the

peak electric field at the silicon film-BOX interface and secondly, facilitates the

collector potential to be absorbed both by the collector drift and substrate regions. It is

shown that the BVCEO of CTLBT is enhanced by 2.7 times when compared with a

conventional lateral bipolar transistor (LBT) with identical drift region dopings.

Key Words: Lateral bipolar transistor, breakdown voltage, simulation, and

silicon-on-insulator (SOI)

Sukhendu Deb Roy and M. Jagadesh Kumar, "Realizing High-voltageThin Film Lateral Bipolar Transistors on SOI with a Collector-tub,"Microelectronics International, Vol.22, pp.3-9, March 2005.


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I. Introduction

Thin film lateral bipolar transistors (LBTs) on silicon-on-insulator (SOI) [1-5]

have drawn wide attention in the recent past due to their compatibility with BiCMOS

technology and possibility of integration in smart power ICs. This is particularly true

for applications in medium voltage range (100V-1000V) such as display driver or

ballast circuits where high voltages LBTs are often used [6-8]. Several studies have

been made to improve the breakdown voltage of these LBTs, either by increasing the

critical electric field for breakdown [9-12] or using REdistribution of SURface Field

(RESURF) principle [13-18] or using novel structures [19-23]. However, a major

problem associated with such thin silicon film ( 1m) on SOI, is the fact that the

breakdown voltage is limited by the high electric field at the silicon film-BOX

interface due to difference in dielectric constant between silicon film and BOX. The

breakdown voltages, therefore, do not increase with the increase in drift region length

and saturate at a lower value. This is because the device is operated with the substrate

grounded and hence, the substrate-BOX interface acts as an equi-potential line and the

entire collector voltage is dropped at the BOX and the silicon film on the collector

high-low (NN+) junction side. The present paper highlights the fact that an

enhancement in the breakdown voltage is possible if a collector-tub concept is used to

reduce the peak electric field at the silicon film-BOX interface, which we propose to

achieve with an N-implantation at the NN+-junction side of a conventional N

+PN

-N

+

LBT structure on SOI. Our two-dimensional simulation studies show that the presence

of the collector-tub in a conventional LBT (CTLBT) enhances the collector-emitter

breakdown voltage (BVCEO) by a factor of about 2.7. The paper also presents an


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analysis of the effect of different device parameters on the breakdown characteristics

for optimum device performance of the CTLBT.

II. Simulation methodology

In order to demonstrate the efficacy of the collector-tub in enhancing the

transistor performance, we have used both process as well as device simulation. We

have first created the lateral bipolar transistor structure with and without the collector-

tub using standard experimental parameters available in the literature [24-27] in the 2-

dimensional process simulator ATHENA [28] so that the simulated device structures

are close to that of an experimentally fabricated device in terms of junction

depths/curvatures and impurity distribution. This structure is then imported to a two-

dimensional device simulator ATLAS [29] to evaluate the device characteristics using

appropriate models as discussed in the following sections.

A. Process simulation to realize the device structure

To generate the CTLBT structure, we have chosen a P-type substrate

(NS=5.0x1013

cm-3

) and an N-type silicon film (ND=1.0x1015

cm-3

) on SiO2 as the

starting material in the two-dimensional process simulator ATHENA. The SOI film

and BOX thickness are 1.0m and 1.2m respectively. As illustrated in Fig. 1, the

process flow begins with the formation of the collector-tub by etching the silicon film

and the BOX (Fig. 1(a)) and followed by phosphorus implantation. The energy, dose,

time and anneal temperature are varied to obtain different diffusion junction depths

(2.8m-20m). The etched region is then refilled with in-situ doped N+-polysilicon

having a concentration of 5.01019

cm-3

(Fig. 1(b)). Next the emitter and collector


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regions are implanted with phosphorus at a dose of 5.71015

cm-2

and an energy of

130 keV and annealed at a temperature of 950OC for 20 min (Fig. 1(c)). The base

region is then opened and implanted with boron at an energy of 62 keV and a dose of

5.01012

cm-2

and a drive-in for 120 min at a temperature of 1100OC (Fig. 1(d)). This

is followed by the opening of the base contact window and deposition of 0.3m layer

of in-situ doped P+-polysilicon having a concentration of 5.010

19cm

-3(Fig. 1(e)).

Finally the emitter, base, and collector metalizations are made with Al deposition

(Fig. 1(f)). The Al deposition is extended 5m long [30-31] both at the base-collector

and NN+-junction side for realizing metal plate junction termination with field oxide

thickness of 0.5m. The LBT structure is obtained following the same process

sequence as described for the CTLBT but without the collector-tub. The overall

process sequence gives the emitter/collector and base doping concentration values of

5.01019

cm-3

and 5.01016

cm-3

respectively, and base-emitter junction depth of 7

m. Fig. 2 shows the generated CTLBT and LBT structures and Fig. 3 gives their

common doping profile.

B. Device Simulation

The structures obtained in ATHENA are imported for simulation in the two-

dimensional device simulator ATLAS. The various models activated in simulations

are Fermi-Dirac distribution for carrier statistics, Klaassens unified mobility model

for dopant-dependent low-field mobility, analytical field dependent mobility for high

electric field, Slotboom model for bandgap narrowing, Selberherrs ionization rate

model for impact ionization and Shockley-Read-Hall (SRH) and Klaassen Auger

recombination models for minority carrier recombination lifetime. The SRH

recombination lifetime for silicon is chosen to be 2.0s for a carrier concentration of


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5.01016

cm-3

and for all other concentrations recombination lifetimes are calculated

using Roulstons equation [32]. The collector-emitter breakdown voltage BVCEO is

calculated at a collector current of 1.0A. Figs. 4 and 5 show the Gummel plots andcurrent gain curves for both LBT and CTLBT for VCB=0 V. The simulated peak

common emitter current gain of both the structures is approximately 30 at a collector

current of 0.1 A.

III. Simulation results on breakdown voltage

A. Effect of collector-tub on breakdown voltage

Fig. 6 shows the output characteristics of CTLBT and LBT. Clearly, due to

the presence of the collector-tub, the BVCEO of CTLBT is significantly enhanced when

compared with that of the LBT. We notice that the breakdown voltage has increased

from 93 V (for LBT) to 255 V (for CTLBT) indicating an improvement of 270 %.

The reason for this significant improvement in breakdown voltage can be understood

from Fig. 7, which shows the electric field profile along the drift region length for

every 10V increment in collector-emitter voltage (VCE) and at breakdown. As

expected, the collector-tub allows the electric field build-up to shift from NN+-

junction side to base-collector junction. Also, the applied reverse voltage is now

supported both by the drift and substrate regions. This is illustrated by the potential

contours in Fig. 8 and electric field vector diagram in Fig. 9, both of which show

spreading of electric filed lines. The maximum breakdown voltage of CTLBT,

however, is limited by the peak electric field at the collector-base junction. On the

other hand, as shown in the potential contour lines and electric field vector diagram of

Figs.10 and 11, the LBT structure shows crowding of electric field lines in the BOX


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at the collector NN+-junction and breakdown of silicon film at the silicon-BOX

interface.

B. Effect of device parameters on breakdown voltage

To study how different parameters such as substrate doping, collector-tub

junction depth, drift region doping affect the break down voltage of CTLBT, we have

varied the above parameters and estimated the collector breakdown voltage. Fig. 12

illustrates the effect of substrate doping (NS) on breakdown voltage. We notice that

the breakdown voltage is maximum for an optimum substrate doping. This can be

understood from Fig. 13, which gives the electric field profile at various substrate

dopings. At low substrate dopings, the electric field build-up takes place at the

collector-base junction. The depletion volume in the substrate becomes large and the

substrate leakage current limits the device breakdown. At high substrate dopings,

however, the electric field builds up at the NN+

-junction and breakdown takes place at

the substrate and collector-tub junction interface. A low substrate doping and deep

collector-tub junction depth makes the electric field build-up at the drift region and

the device breakdown is then limited both by the drift region length and its doping.

Fig. 14 shows the effect of collector-tub junction depth (Xj) on the breakdown

voltage and Fig. 15 gives the electric field profile at various Xj. The breakdown

voltage increases with increasing Xj, reaches a maximum and then decreases. This is

because a deep Xj makes the device to behave as a bulk device and the electric field

build-up begins at the collector-base junction. On the other hand, a shallow Xj makes

the electric field to build-up at the NN+-junction and the device acts as a conventional

LBT structure on SOI.


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Fig. 16 shows the electric field profile at different collector-emitter voltages

and at different drift dopings (ND). As seen from the figure, both CTLBT and LBT

breakdown at the NN+-junction side at low dopings. However, the nature of

breakdown is different in both the structures. The CTLBT structure breaks down at

low drift dopings as the drift region is entirely depleted and because the electric field

at the NN+-junction is high. For LBT, breakdown occurs because the electric field at

the silicon-box interface is high. In this case, the electric field distribution is two-

dimensional in nature. The component of the electric field at the NN+

junction side

depletes the lowly doped thin silicon film. When the depletion front reaches the

silicon-BOX interface, the electric field increases substantially causing breakdown.

The breakdown characteristics at high drift dopings are, however, same for both the

structures, which is due to high electric field at the collector-base junction end.

Fig. 17 shows the effect of drift region doping on the breakdown voltage of

CTLBT and LBT. Clearly, both the structures show maximum breakdown voltage for

an optimum doping. However, the optimum drift doping for maximum breakdown

voltage in CTLBT is smaller than that of the LBT structure. This can be explained as

follows. In CTLBT, due to the presence of the collector-tub, the critical electric field

at the collector-base junction determines the maximum breakdown voltage, which is

limited by the drift doping. In the case of LBT, a high drift doping makes the vertical

component of the electric field large, and, therefore, the lateral electrical field

component becomes responsible to cause depletion in the drift region beginning from

the collector-base junction end. However, once the drift region is entirely depleted,

the vertical component becomes dominant and the electric field at the silicon-BOX

interface determines the maximum breakdown voltage. The critical electric field for

breakdown at the silicon-BOX interface is increased with the increase in drift region


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doping and hence the LBT shows optimum breakdown voltage at higher doping when

compared with the CTLBT structure.

Fig. 18 shows the influence of the drift region length (LD) on the breakdown

voltage both for the LBT and CTLBT. For LBT, the breakdown voltage saturates at

lower values of LD, however, for CTLBT, the saturation is observed at higher values

of LD. This is because, in CTLBT, the collector-tub makes the lateral electric field

dominant, and therefore, the drift region length can accommodate the spread of lateral

electric field and the breakdown voltage increases with increasing LD. The breakdown

voltage, however, saturates when both the vertical and lateral components of the

electric field become high as evidenced by the two peaks in the electric field

distribution curve (Fig. 7). For LBT structure, both the vertical and lateral component

contributes to the breakdown process and a longer collector drift region length is of no

consequence for increasing the breakdown voltage.

V. Conclusions

Two-dimensional numerical simulation studies of a collector-tub lateral bipolar

transistor (CTLBT) structure on silicon-on-insulator (SOI) are presented. The CTLBT

has an N-diffusion region at the collector high-low (NN+) junction. The collector-

emitter breakdown voltage (BVCEO) of CTLBT is about 2.7 times higher than that of

the conventional LBT on SOI with identical doping profile. The increased breakdown

voltage in CTLBT is explained as due to the shifting of the electric field from the

collector high-low junction side to the base-collector junction side and also due to the

distribution of the applied reverse potential in the substrate and drift regions. This

potential redistribution, for a given BOX thickness and SOI film thickness, is found to


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be dependent on substrate doping (NS), drift doping (ND) and collector-tub junction

depth (Xj). To realize the CTLBT structure in a standard CMOS process, a process

flow is proposed which needs only additional two masks.


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Figure captions

Fig. 1 Process flow for CTLBT and LBT structure.

Fig. 2(a) Lateral bipolar transistor (LBT) and (b) Collector-tub lateral bipolartransistor (CTLBT) on SOI.

Fig. 3 Doping profile for CTLBT and LBT as obtained from ATHENA.

Fig. 4 Gummel plots of CTLBT compared with that of the LBT.

Fig. 5 Collector current versus Current gain of CTLBT compared with that of the

LBT.

Fig. 6 Common-emitter I-V characteristics of CTLBT and LBT

Fig. 7 Electric field at the silicon film/BOX interface of CTLBT compared with that

of the LBT.

Fig. 8 Potential contours at the substrate of CTLBT at breakdown voltage (255V),

potential contours=10V/step, LD=40m, NS=5.0x1013

cm-3

, ND=2.5x1015

cm-3

,

tOX=1.2m, and Xj=3.0m.

Fig. 9 Electric field vector diagram for CTLBT showing electric field spreading at the

Silicon film/BOX interface and breakdown at the collector-base interface or at the

collector junction/substrate interface.

Fig. 10 Potential contours at the Silicon film of LBT at breakdown voltage (93V),

potential contours=10V/step, LD=40m, NS=5.0x1013

cm-3

, ND=2.5x1015

cm-3

,

tOX=1.2m, and Xj=3.0m.

Fig. 11 Electric field vector diagram for LBT showing field crowding at the BOX and

breakdown at the silicon-BOX interface.

Fig. 12 Effect of substrate doping on the breakdown voltage of CTLBT.

Fig.13 Electric field profile of CTLBT at different substrate dopings.

Fig. 14 Breakdown voltage of CTLBT at different collector-tub junction depths.

Fig. 15 Electric field profile at the silicon film/field oxide interface of CTLBT at

various junction depths.

Fig. 16 Electric field profile along the silicon/BOX and silicon-field oxide interface of

CTLBT for different drift region dopings compared with that of the LBT.

Fig. 17 Effect of drift doping on the breakdown characteristics of CTLBT compared

with that of the LBT.


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Fig. 18 Effect of drift length on the breakdown voltage of CTLBT compared with that

of the LBT.


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Fig. 1

Phosphorus

P- substrate

N

(a)

N+

P- substrate

N

N(b)

N+

P- substrate

N

N(c)

Phosphorus Phosphorus

PN+ N+

P- substrate

N

N (d)

Boron

PN+ N+

P- substrate

N

N (e)

P+ polysilicon contact

(f)

B CE

PN+ N+

P- substrate

N

N


16/24

Fig. 2

(a)

N+

P- substrate

P N N+

CBE

BOX

(b)

N+

P- substrate

Buried oxide

P N N+

N-collectortub

BOX

CBE


17/24

Fig. 4

0.2 0.4 0.6 0.8 1.0

IC

IB

CTLBT, Xj=3.0m

LBT- - - -

ND=2.5x10

15cm

-3

NS=5.0x10

13cm

-3

LD=40m

tOX

=1.2m

VCB=0 V

10-15

10-13

10-11

10-9

10-7

10-5

10-3

10-17

IC,IB[A]

Base-emitter voltage, VBE

[ V ]

0 10 20 30 40 50 60

1018

1014

10

15

1016

1017

1019

1020

CollectorEmitter

Drift regionP

N

N+

N+

Base

Netdoping[cm-3]

Lateral distance [ m ]


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0 50 100 150 200 250 300

0

1

23

4

5

6

7

- - - -CTLBT, X

j=3.0m

LBTIB=0 to 0.25A @ 0.05A

IB=0

Collector-emitter voltage, VCE

[ V ]

ND=2.5x10

15cm

-3

NS=5.0x10

13cm

-3

LD=40m

tOX

=1.2m

Collector

current[A

]

Fig. 6

0

5

10

15

20

25

30

35

- - - -

VCB

=0 V

CTLBT, Xj=3.0m

LBT

ND=2.5x10

15cm

-3

NS=5.0x10

13cm

-3

LD=40mtOX

=1.2m

10-7

10-9

10-11

10-5

10-13

Curre

ntgain,

Collector current, IC

[ A ]


19/24

Fig. 8

Fig. 7

E B C

BOX

N+ N N+P

Depletion layer

N

VCEO=0 V VCEO=255 V

60

60


Verticaldistance[m]

5040300 10 20

70

5040

0102030

-3.2

-2.2

-1.2

P-substrate

E B C

BOX

N+ N N+P

Depletion layer

N

VCEO=0 V VCEO=255 V

60

60


Verticaldistance[m]

5040300 10 20

70

5040

0102030

-3.2

-2.2

-1.2

P-substrate

10 20 30 40 50 600.0

0.5

1.0

1.5

2.0

2.5

BVCEO

=255V

CTLBT, Xj=3.0m

LBT- - - - BVCEO=93V

BVCEO=25V

VCEO

increment @ 25 V

ND=2.5x10

15cm

-3

NS=5.0x10

13cm

-3

LD=40m

tOX

=1.2m

E

lectricfield[x105Vcm-1]



20/24

Fig. 10

E B C

BOX

N+ N N+P

Depletion layer

N

60

60


Ver

ticaldistance[m]

5040300 10 20

70

5040

0102030

-3.2-2.2

-1.2

P-substrate

Breakdown

E B CE B C

BOX

N+ N N+P

Depletion layer

N

60

60


Ver

ticaldistance[m]

5040300 10 20

70

5040

0102030

-3.2-2.2

-1.2

P-substrate

Breakdown

Fig. 9

E B C


BOX

N+ N N+P

60

60

Verticaldist

ance[m]

5040300 10 20

70

5040

01020

30

-3.2

-2.2

-1.2

VCEO=93 V

Depletion layer

P-substrate

VCEO=0 V

E B C


BOX

N+ N N+P

60

60

Verticaldist

ance[m]

5040300 10 20

70

5040

01020

30

-3.2

-2.2

-1.2

VCEO=93 V

Depletion layer

P-substrate

VCEO=0 V


21/24

Fig. 11

1013

1014

1015

80

100

120

140

160

180

200N

D=1.0x10

15cm

-3

LD=40m

Xj=3.0m

tOX

=1.2m

B

reakdownvoltage[V]

Substrate doping [ cm-3

]

Fig. 12

E B C

Depletion layer

60

60


Verticaldistance[m]

5040300 10 20

70

5040

0102030

-3.2

-2.2

-1.2

P-substrate

Breakdown

BOX

N+ N N+P

E B CE B C

Depletion layer

60

60


Verticaldistance[m]

5040300 10 20

70

5040

0102030

-3.2

-2.2

-1.2

P-substrate

Breakdown

BOX

N+ N N+P


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Fig. 13

3 6 9 12 15 18 21180

200

220

240

260

280

ND=1.0x10

15cm

-3

NS=5.0x10

13cm

-3

LD=40m

tOX

=1.2m

BreakdownVoltage[V]

Collector-tub junction depth [ m ]

Fig. 14

10 20 30 40 50 60

0.0

0.5

1.0

1.5

2.0

ND=1.0x10

15cm

-3

LD=40m

Xj=3.0m

tOX

=1.2m

Elec

tricfield[x105Vcm-1]


NS=1.0x10

13cm

-3, BV

CEO=110V

NS=5.0x10

13cm

-3, BV

CEO=195V

NS=1.0x10

15cm

-3, BV

CEO=95V


23/24

Fig. 16

10 20 30 40 50 600.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

NS=5.0x1013cm-3

LD=40m

Xj=3.0m

tOX

=1.2m

Si-BOX interface

Si-BOX interfaceSi-field oxide interface

Si-field oxide interfaceLBT

CTLBT

ND=1.6x10

16cm

-3

ND=1.0x10

14cm

-3

Electricfield[

x105Vcm-1]


10 20 30 40 50 600.0

0.5

1.0

1.5

2.0

ND=1.0x10

15cm

-3

NS

=5.0x1013

cm-3

LD=40m

tOX

=1.2m

Xj=2.6m, BV

CEO=192V

Xj=13.2m, BV

CEO=270V

Xj=19.5m, BV

CEO=226V

Electricfield[x105Vcm-1]


Fig. 15


24/24

Fig. 18

100

150

200

250N

S=5.0x10

13cm

-3

LD=10m

tOX

=1.2m

1016

1015

1014

B

reakdownvoltage[V]

Drift doping [ cm-3

]

CTLBT, Xj=3.0m

LBT

Fig. 17

10 20 30 40 5050

100

150

200

250

300

ND=2.5x10

15cm

-3

NS=5.0x10

13cm

-3

tOX

=1.2m

Breakdownvoltage[V]

Drift region Length, LD

[ m ]

CLBT, Xj=3.0m

LBT

Date post:	30-May-2018
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Realizing High-voltage Thin Film Lateral Bipolar Transistors on SOI with a Collector-tub

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