Realising wide bandgap P-SiC-emitter lateralheterojunction bipolar transistors with lowcollector–emitter offset voltage and high currentgain: a novel proposal using numerical simulation

M. Jagadesh Kumar and C.L. Reddy

Abstract: The authors report a novel method to reduce the collector–emitter offset voltageof the wide bandgap SiC-P-emitter lateral HBTs using a dual-bandgap emitter. In theirapproach, the collector–emitter offset voltage VCE(offset) is reduced drastically by eliminating thebuilt-in potential difference between the emitter–base (EB) and collector–base (CB) junctionsby using a SiC-on-Si P-emitter. It is demonstrated that the proposed dual-bandgap P-emitterHBT, together with the SiGe base and Schottky collector, not only has a very low VCE(offset)

but also exhibits high current gain, reduced Kirk effect, excellent transient response and highcutoff frequency. The performance of the proposed device is evaluated in detail usingtwo-dimensional device simulation, and a possible BiCMOS compatible fabrication procedure isalso suggested.

1 Introduction

Wide bandgap emitter bipolar transistors have severaladvantages over their homojunction counterparts, such as(i) large current gain independent of emitter doping, (ii)increased base doping against current gain trade-off and (iii)high speed operation [1–3]. A wide bandgap emitter BJTcan be realised either by reducing the base region bandgapwith respect to the emitter or by increasing the emitterregion bandgap with respect to the base. SiGe HBTs are anexcellent example of wide bandgap emitter BJTs, whichhave become very attractive in high speed applications dueto their superior performance [4, 5]. In the recent past, SiCas an emitter has also been shown to be a potentialcandidate for making wide bandgap emitter BJTs [6–8].While other wide bandgap HBT technologies based oncompound semiconductors such as InGaP/GaAs orAlGaAs/GaAs are available, HBTs based on SiC/Si areattractive because of their compatibility with the silicontechnology and the excellent properties of SiC. In spite of alarge lattice mismatch, wide bandgap SiC emitter hetero-bipolar transistors with large current gains have beensuccessfully reported [6–8]. However, wide bandgap emitterHBTs exhibit a finite collector–emitter offset voltage,VCE(offset) [9, 10] due to the large difference in the built-inpotential of emitter–base and collector–base junctions. Thisis detrimental in many digital applications and should beminimised while retaining the advantages of the widebandgap emitter.

A survey of literature reveals that only NPN widebandgap SiC emitter transistors have been reported. Often,in many applications, compatible PNP transistors with widebandgap SiC P-emitters are required. However, this is notpossible because, as we shall demonstrate in the followingsection, SiC emitter PNP transistors are more prone to thecollector–emitter offset-voltage problems than are the widebandgap SiC emitter NPN transistors. If the VCE(offset)

problem is eliminated in SiC wide bandgap emitter PNPtransistors, they will find wide usage in many applications.To the best of our knowledge, no solution has yet beenreported on how VCE(offset) can be minimised in widebandgap emitter PNP transistors.

The aim of this paper is therefore to propose for the firsttime a novel method of minimising the VCE(offset) in SiCwide bandgap emitter PNP transistors. We suggest thatmaking use of a dual-bandgap material consisting of SiCover Si in the emitter region will reduce the VCE(offset)

without significantly affecting the current gain. We usedtwo-dimensional numerical simulation[11] to verify theefficacy of our solution to minimise VCE(offset). In oursimulations, we have chosen a lateral experimental BJTstructure on SOI to implement our solution. In all oursimulations, we have used the mobility model of 4H SiCdue its availability compared to that of 3C SiC. We will alsodemonstrate that the use of SiGe base and a Schottkycollector in the proposed structure will further enhance thetransistor performance, making it a suitable candidate forhigh speed BiCMOS applications involving compatibleNPN and PNP transistors with wide bandgap SiC-emitters.We also have suggested in the end a possible fabricationprocedure for the device using well established fabricationsteps for lateral BJTs. It may be pointed out that the actualfabrication will not be without its problems. However, webelieve that the proposed structure may provide significantincentive for experimental exploration considering theimportance of wide bandgap emitter PNP transistors inmany circuit applications.

The authors are with the Department of Electrical Engineering, Indian Instituteof Technology, Delhi, Hauz Khas, New Delhi - 110 016, India

r IEE, 2004

IEE Proceedings online no. 20040295

doi:10.1049/ip-cds:20040295

Paper first received 17th April and in revised form 30th September 2003.Originally published online: 21st September 2004

IEE Proc.-Circuits Devices Syst., Vol. 151, No. 5, October 2004 399

2 Comparison of V CEðoffsetÞ problem in widebandgap emitter NPN/PNP HBTs

The collector–emitter offset voltage is defined as thedifference between the turn-on voltage of the emitter–base(EB) and base–collector (BC) junctions [12]:

VCEðoffsetÞ ¼ VEB � VBC at IC ¼ 0mA ð1Þ

To compare the collector–emitter offset problem in widebandgap PNP/NPN HBTs, we have chosen a lateral HBTstructure as shown in Fig. 1 in which the emitter region isSiC, and base and collector regions are silicon. The epitaxialfilm thickness of the HBT is chosen to be 0.2mm and theburied oxide thickness is 0.38mm. The emitter is doped at5� 1019 cm�3. The base width is 0.4mm and its doping ischosen to be 5� 1017 cm�3. The collector is doped at2� 1017 cm�3. All these parameters are chosen based onreported experimental results of lateral silicon NPN BJTs[13]. The input simulation parameters to the devicesimulator ATLAS are given in Table 1. The collector–emitter offset problem of SiC emitter PNP HBTs can bebest understood from Fig. 2, in which the emitter–base (EB)and base–collector (BC) junction diode characteristics areshown. Figure 2a shows that the turn-on voltage VEB forthe emitter–base junction of the PNP SiC emitter HBT is1.5V larger than that of the emitter–base junction of the

NPN SiC emitter HBT. On the other hand, the turn-onvoltage VBC of the collector–base junction for both the SiCemitter PNP/NPN HBTs is identical (B0.8V) as shown inFig. 2b since silicon is used for both the base and collectorregions. The output characteristics of the PNP and NPNwide bandgap SiC emitter HBTs are shown in Fig. 3. It canbe clearly observed from Fig. 3a that the SiC emitter PNPHBT exhibits a large collector–emitter offset voltage(B1.75V) compared to the SiC emitter NPN HBT due tothe large built-in voltage difference between the emitter–base (EB) and base–collector (BC) junctions [8, 9, 12] seenin Fig. 2. According to (1), the collector–emitter offsetvoltage for the SiC emitter NPN HBT is expected to beB0.25V and matches well with the offset voltage shown inFig. 3b. The collector–emitter offset voltage of the SiCemitter PNP HBT calculated using (1) comes out to beB1.75V and matches well with the offset voltagepredicted from the I/V characteristics shown in Fig. 3a.Owing to this large collector–emitter offset voltage exhibitedby the SiC emitter PNP HBTs, they cannot be used alongwith SiC emitter NPN HBTs. It can also be observed fromFig. 3 that the wide bandgap SiC emitter PNP HBT has alesser current gain than that of the wide bandgap NPNHBT, but often it is essential to have the performance ofPNP HBTs nearly identical to that of NPN HBTs inBiCMOS applications such as push–pull amplifier designand also in ECL and complementary (npn/pnp) logicdesign.

0.20 µm

0.38 µm

N(P)SiC

P(N)Si Si P+(N+)

3.8 µm 3.8 µm 0.4 µm

oxide

B C

P(N)

E

substrate

Fig. 1 Cross-sectional view of wide bandgap SiC emitter lateralPNP/NPN HBT

Table 1: ATLAS input simulation parameters

Parameter Value

SOI thickness tsi (initially) 0.20mm

Buried oxide thickness tbox 0.38mm

Field oxide thickness tox 0.18mm

Emitter length 3.80mm

Base length 0.40mm

Emitter region doping concentration 5� 1019 cm�3

Base region doping concentration 5� 1017 cm�3

Collector region doping concentration (only forNPN HBT)

2� 1017 cm�3

Barrier height lowering coefficient 2� 10�7 cm

SRH concentration parameter for electronsand holes NSRHN and NSRHP (both for Siand SiGe)

1� 1022 cm�3

−2 −1 0 4

0.004

0.008

0.012

0.016

PNP HBT NPN HBT

PNP HBTNPN HBT

0.020

curr

ent,

mA

0.004

0.008

0.012

0.016

0.020

curr

ent,

mA

voltage , V

21 3 5

−2 −1 0 4

voltage , V

21 3 5

a

b

Fig. 2 E–B and B–C diode characteristics of SiC emitter NPNHBT and SiC emitter PNP HBTa E–B junctionb B–C junction

400 IEE Proc.-Circuits Devices Syst., Vol. 151, No. 5, October 2004

3 Dual bandgap emitter approach to reduceVCE (offset ) in SiC emitter PNP HBTs

In this Section, we show that the collector–emitter offsetvoltage of wide bandgap SiC emitter PNP transistors can besignificantly reduced by replacing the SiC emitter in thelateral PNP HBT by a dual bandgap emitter consisting ofSiC on Si as shown in Fig. 4. The presence of a thin layer ofP-silicon in the emitter reduces the collector–emitter offsetvoltage drastically by eliminating built-in potential differ-ence between emitter–base (EB) and base–collector (BC)junctions. The reduction of collector–emitter offset voltagein a dual bandgap emitter PNP HBT can be bestunderstood from Fig. 5. As can be observed from Fig. 5,there is a large reduction in the built-in potential of anemitter–base (EB) junction after replacing the SiC emitterby the dual bandgap emitter. Now the theoreticallypredicted offset voltage for the dual bandgap emitter PNPHBT should be B0.05V according to (1). The outputcharacteristics of the dual bandgap PNP HBT (emitterthickness: 0.15mm SiC+0.05mm Si) are compared in Fig. 6with those of a SiC wide bandgap P-emitter PNP HBT. Weobserve that the VCE(offset) of the dual bandgap emitter PNPHBT matches with the value calculated from (1) and issignificantly smaller than the VCE(offset) of the SiC P-emitterHBT. However, while the introduction of a thin layer ofsilicon in the wide bandgap emitter reduces VCE(offset), it isalso accompanied by a reduction in the current gain. InFig. 7, the dependence of current gain is shown for different

relative values of SiC and silicon in the emitter region. Wenotice that, as the Si film thickness increases, the currentgain decreases. We demonstrate in the following Sectionthat the loss in current gain can be recovered by introducingthe SiGe base in the proposed structure shown in Fig. 4.

4 Application of SiGe base to dual bandgapemitter PNP HBT

To improve the current gain of the dual bandgap emitterPNP HBT, in our simulations we have replaced the silicon

−12

−10

−8

−6

−4

−2

0

0 −1 −2 −3 −4 −5collector voltage VC , V

collector voltage VC , V

30

10

20

30

40

50

0

colle

ctor

cur

rent

I C ,

µA

IB = -5 nA step

IB = 0 A

IB = 5 nA step

IB = 0 A

colle

ctor

cur

rent

I C, µ

A

1 2

a

b

Fig. 3 Common-emitter I/V characteristics of SiC emitter HBTsa P+NP HBTb N+PN HBT

0.20 µm

0.38 µm

PN

SiCP

Si SiP+

3.8 µm 3.8 µm0.4 µm

oxide

E C

Si Np-SiC p+ Sip-Si P

Si Si

N-substrate

B

Fig. 4 Cross-sectional view of dual bandgap emitter lateral PNPHBT

−2 −1 0 2

0.004

0.008

0.012

0.016

0.020

B-C junction E-B junction

voltage, V

curr

ent,

mA

1 3 4 5

Fig. 5 E–B and B–C diode characteristics of dual bandgap emitterPNP HBT

0 −1 −2 −5−3 −4

IB = −5 nA step

IB = 0 A

−12

−10

−8

−6

−4

−2

0

colle

ctor

cur

rent

, IC

, µA

collector voltage VC , V

P+NP HBT (dual bandgap emitter)

P+NP HBT (SiC emitter)

Fig. 6 Common-emitter I/V characteristics of SiC emitter PNPHBT compared with those of dual bandgap emitter PNP HBT

IEE Proc.-Circuits Devices Syst., Vol. 151, No. 5, October 2004 401

base by the SiGe base (20% Ge content) in the proposedstructure. The simulated current gain of the dual bandgapemitter with and without the SiGe base is shown in Fig. 8.The presence of SiGe in the base region improves theemitter injection efficiency, resulting in a higher currentgain. Although the application of SiGe base restores thecurrent gain, it is well known that PNP transistors sufferfrom large collector resistance due to low hole mobility.Further, SiGe base transistors suffer from the additionalproblem of excess stored base charge due to the accumula-tion of carriers at the collector–base junction [14]. Theapplication of a Schottky collector has been shown toimprove the performance of PNP bipolar transistors [8, 15,16]. Therefore, it would be of great interest to see how theusage of the Schottky collector to the dual bandgap emitterSiGe base PNP transistor will enhance its performance.

5 Application of Schottky collector to dualbandgap emitter SiGe base PNP HBT and its impacton device performance

To study the effect of the Schottky collector, we havereplaced the P-collector of the proposed structure shown inFig. 4 with a Schottky contact. Based on experimental

results, it has been reported [17] that platinum silicide givesthe highest barrier height (fBn¼ 0.82 eV) with an n-SiGebase. Therefore, making an appropriate Schottky contact tothe n-SiGe base is not a difficult task.

The Gummel plots of this dual bandgap SiC-on-Si P-emitter SiGe base HBT with and without the Schottkycollector transistor are compared in Fig. 9. We observe thatthe base current in the dual bandgap emitter SiGe baselateral Schottky collector PNMHBT is smaller than that ofthe dual bandgap emitter SiGe base PNP HBT. This ismainly because of the finite electron current Inm caused bythe electron flow from metal into the n-base [18]. As theelectron current from emitter to base is fixed by the emitter–base forward bias voltage, the electron current Inm frommetal to n-base flows into the base terminal [18], reducingthe total base current. As a result of this, the current gain ofthe dual bandgap emitter SiGe base lateral Schottkycollector PNM HBT is higher than that of the dualbandgap emitter SiGe base PNP HBT as shown in Fig. 10.An interesting point is that the base current of the dualbandgap emitter SiGe base lateral Schottky collector PNMHBT is less than that of the dual bandgap emitter SiGe basePNP HBT even at high–level injection of carriers as shownin Fig. 9, which clearly shows the suppression of the Kirk

0

100

200

300

400

500

600

curr

ent g

ain

�

10−14 10−12 10−10 10−8 10−6 10−4 10−2

collector current IC , A

(i)

(ii)(iii)(iv)(v)

Fig. 7 Gain against collector current characteristics of dualbandgap emitter PNP HBT for various combinations of emitterlayer for a constant thickness of 0.2mmVCE¼�1V(i) 0.2mm SiC; (ii) 0.15mm SiC+0.05mm Si; (iii) 0.1mm SiC+0.1mmSi; (iv) 0.05mm+0.15mm Si; (v) 0.2mm

0

100

200

300

400

500

curr

ent g

ain

�

10−14 10−12 10−10 10−8 10−6 10−4 10−2

collector current IC , A

P+NP HBT (dual bandgap emitter

with SiGe base)

P+NP HBT (dual bandgap emitter)

Fig. 8 Gain against collector current characteristics for dualbandgap (0.15mm SiC+0.05mm Si) emitter PNP HBT with andwithout SiGe baseVCE¼�1V

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

IC

10−4

10−8

10−12

10−16

colle

ctor

& b

ase

curr

ents

I C &

I B, A

IB

P+NM HBT

P+NP HBT

emitter−base voltage VEB ,V

Fig. 9 Gummel plot of dual bandgap emitter SiGe base lateralSchottky collector PNM HBT compared with that of dual bandgapemitter SiGe base PNP HBTVCE¼�1V

0

400

800

1200

10−12 10−10 10−8 10−6 10−4 10−2

collector current IC , A

curr

ent g

ain

�

P+NP HBTP+NM HBT

Fig. 10 Gain against collector current characteristics of dualbandgap emitter SiGe base lateral Schottky collector PNM HBTcompared with those of dual bandgap emitter SiGe base PNP HBTVCE¼�1V

402 IEE Proc.-Circuits Devices Syst., Vol. 151, No. 5, October 2004

effect [19] in the dual bandgap emitter SiGe base lateralSchottky collector PNM HBT. The simulated I/V char-acteristics of a dual bandgap emitter SiGe base lateralSchottky collector PNM HBT and a dual bandgap emitterSiGe base PNP HBT are shown in Fig. 11. As can be seen,the current–voltage characteristics of the dual bandgapemitter SiGe base lateral Schottky collector PNM HBT aresuperior to those of the dual bandgap emitter SiGe basePNP HBT in terms of reduced collector resistance.However, there is a finite offset voltage for the dualbandgap emitter SiGe base Schottky collector PNM HBTmainly due to the reduced built-in potential of the base–collector Schottky junction.

Figure 12 shows the transient behaviour of the dualbandgap emitter SiGe base lateral Schottky collector PNMHBT compared with the dual bandgap emitter SiGe basePNP HBT. It is clear that the dual bandgap emitter SiGebase lateral Schottky collector PNMHBT exhibits excellenttransient response with nearly zero base charge storage timedue to its metal collector and suppressed Kirk effect, and incomparison the dual bandgap emitter SiGe base PNP HBTshows a higher storage time due to the Kirk effect andalso the electron pile-up at the collector–base heterojunction[14, 19].

Figure 13 shows the unity gain cutoff frequency againstcollector current of the dual bandgap emitter SiGe base

lateral Schottky collector PNM HBT and is compared withthe dual bandgap emitter SiGe base PNP HBT. As can beobserved, the cutoff frequency of the dual bandgap emitterSiGe base lateral Schottky collector PNM HBT is higherthan that of dual bandgap emitter SiGe base PNPHBT dueto its metal collector and higher transconductance gm. Thedual bandgap emitter SiGe base lateral Schottky collectorPNMHBT exhibits an fT of 3.55GHz at a collector currentof 0.6mA, whereas for the comparable dual bandgapemitter SiGe base PNP HBT, fT falls to a negligible value atthe above current due to the Kirk effect and a decrease intransconductance.

6 Effect of doping and Ge % in the base

In all the above simulations we have assumed the base Geconcentration to be 20%, which is the practical upper limiton Ge in most practical applications. However, it will beinteresting to see how the current gain and the breakdownvoltage of the proposed structure change if the Geconcentration in the base region is varied. If ion-implanta-tion is used to create the SiGe base, it is quite possible thatthe Ge content may lie in the range 10 to 12 %. Therefore,we have next investigated the effect of base doping on peakcurrent gain and breakdown voltage BVCEO (for zero basecurrent) for various germanium concentrations in the SiGe-base of the dual bandgap emitter SiGe base lateral Schottkycollector PNM HBT.

Figure 14 shows the peak current gain against basedoping for various germanium concentrations in the SiGe-base of the dual bandgap emitter SiGe base lateral Schottkycollector PNM HBT. It can be observed from this Figurethat the peak current gain decreases as we decrease thegermanium concentration in the SiGe base for a given basedoping, and the gain also decreases as we increase the basedoping for a given Ge concentration because of low emitterinjection efficiency. Figure 15 shows the breakdown voltageBVCEO (for zero base current) against base doping forvarious germanium concentrations in the SiGe base of thedual bandgap emitter SiGe base lateral Schottky collectorPNM HBT. We note that for a given base doping, thebreakdown voltage BVCEO (for zero base current) increasesas we decrease the germanium concentration and, for agiven Ge concentration, the breakdown voltage increases aswe increase the base doping due to increasing critical electricfield. The above design curves provide useful indicators on

0

−5

−10

−15

−20

−25

0 −1 −3−2

colle

ctor

cur

rent

I C, µ

A

collector voltage VC ,V

IB = −5nA step

P+NM HBTP+NP HBT

IB = 0 A

Fig. 11 Common–emitter I/V characteristics of dual bandgapemitter SiGe base lateral Schottky collector PNM HBT comparedwith those of dual bandgap emitter SiGe base PNP HBT

0 8 10 12 14 1620

10

0

−10

−20

−30

−40

−50

base

cur

rent

IB

, µA

transient time T, ns2 4 6

P+NM HBT

P+NP HBT

Fig. 12 Transient behaviour of dual bandgap emitter SiGe baselateral Schottky collector PNM HBT compared with that of dualbandgap emitter SiGe base PNP HBT

0

1

2

3

4

5

10−8 10−7 10−6 10−5 10−4 10−3 10−2

cuto

ff fr

eque

ncy

f T, G

Hz

collector current IC , A

P+NM HBT

P+NP HBT

Fig. 13 Unity gain cutoff frequency against collector current ofdual bandgap emitter SiGe base lateral Schottky collector PNMHBT compared with that of dual bandgap emitter SiGe base PNPHBT

IEE Proc.-Circuits Devices Syst., Vol. 151, No. 5, October 2004 403

the required Ge concentration and base doping to realise agiven current gain and breakdown voltage.

7 Proposed fabrication procedure

Fabrication of the proposed structure can be realised byintroducing a few extra steps in the reported fabricationprocedure of the lateral BJTs on SOI [13]. We can start withan SOI wafer having an n-type epitaxial layer thickness of0.2mm and doping of 5� 1017 cm�3. In the first step, a thickCVD oxide is deposited and patterned as shown in Fig. 16a.The uncovered N-region is converted into a P-region byimplanting a p-type dopant at a calibrated tilt angle asdiscussed in [16] and as shown in Fig. 16b. The P-typeemitter region is then etched to a thickness of 0.05mm asshown in Fig. 16c. In the next step, we deposit the p+ SiCon the horizontal edge (at point X in Fig. 16d) of the siliconsurface, which acts as a seed, and the SiC grows [20, 21] asshown in Fig. 16d. Subsequent to this step, the CMPprocess is performed and then a thick CVD oxide isdeposited and patterned as shown in Fig. 16e. Followingthis step, a nitride film is deposited as shown in Fig. 16f. Inthe next step, an unmasked RIE etch is performed until theplanar silicon nitride is etched. This retains the nitridespacer at the vertical edge of thick CVD oxide as shown in

Fig. 16g. After a thick oxide is deposited as shown inFig. 16h, CMP process is carried out to planarise thesurface. Next, a nitride spacer is removed with selectiveetching, which will create a window in the oxide as shown inFig. 16i. Germanium can now be implanted [22–25] throughthis window to convert silicon in the base region to SiGe.Ge implantation can be performed at an energy of 130keVwith fluences of 1, 2 or 3� 1016 cm�2 according to thereported works in the literature [22]. To recrystallise theimplanted SiGe layer, a rapid thermal annealing (RTA)needs to be performed at 10001C for about 10 s. Thisprocess is to ensure complete recrystallisation of the SiGeamorphous layer [22].

After converting silicon in the base region to SiGe, wethen deposit n+-poly and then the wafer is once againplanarised using CMP, leaving n+-poly in the place wherethe nitride film was present earlier as shown in Fig. 16j.Following this step, a contact window is opened for a metalSchottky collector as shown in Fig. 16k and, subsequent tothis step, the p+ emitter contact window is opened asshown in Fig. 16l. Finally, platinum silicide is deposited toform the Schottky collector contact and ohmic contacts onthe emitter and n+-poly base region.

8 Conclusion

In this paper, we have first discussed the reasons for thesignificant collector–emitter offset voltage observed in widebandgap SiC P-emitter HBTs. Based on numerical simula-tions, we have demonstrated that using a dual bandgap SiC-on-Si emitter in the presence of the SiC P-emitter greatlyreduces the collector–emitter offset voltage of wide bandgapPNP HBTs. However, the presence of Si in the emitter

20 40 60 80 100

10

100

1000

20% Ge base

curr

ent g

ain

�

base doping ND , cm−3 (× 1017)

0% Ge base

10% Ge base

Fig. 14 Gain against base doping for various germanium con-centrations in base of dual bandgap emitter SiGe base lateralSchottky collector PNM HBT

20 40 60 80 1000

2

4

6

8

0% Ge base10% Ge base

20% Ge base

brea

kdow

n vo

ltage

, V

base doping ND , cm−3 (× 1017)

Fig. 15 Breakdown voltage BVCEO (for zero base current)against base doping for various germanium concentrations in baseof dual bandgap emitter SiGe base lateral Schottky collector PNMHBT

N

N-substrate

N-substrate

N-substrate N-substrate

N-substrateN-substrate

N-substrate N-substrate

N-substrate N-substrate

N-substrate

N-substrate

P N

NP

NP

P N

P N

P N

P

P

P

P

P

N

N

N

N

N

Si3N4

SiC X

LTO

N+ poly

a g

b h

c i

d j

e k

f l

Fig. 16 Proposed fabrication steps for dual bandgap emitter SiGebase lateral Schottky collector PNM HBT on SOI

404 IEE Proc.-Circuits Devices Syst., Vol. 151, No. 5, October 2004

results in a reduced current gain, and the low hole mobilityin the P-collector gives rise to a high collector resistance. Toovercome this problem, we have applied the SiGe base anda metal Schottky collector to the proposed structure anddemonstrated that the resulting device will not only havevery low collector–emitter offset voltage but will also exhibithigh current gain and negligible storage time. Based onreported experimental results for the lateral BJTs on SOI,we have also suggested a possible fabrication procedure forthe proposed structure. We conclude from our study of thedual bandgap SiC P-emitter HBT with the combination ofSiGe base and Schottky collector that the proposedstructure should be a good candidate for BiCMOSapplications requiring both NPN and PNP HBTs withcomparable performance.

9 Acknowledgment

Financial support from the Department of Science andTechnology (DST), Government of India, is gratefullyacknowledged. The authors also would like to thank theCouncil of Scientific and Industrial Research (CSIR),Government of India, for the Fellowship given to Mr.Linga Reddy.

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