III-V HETEROJUNCTION BIPOLAR TRANSISTORS

Yan Yan

Department of Electrical EngineeringUniversity of Notre Dame, Notre Dame, IN 46556-5637

April 27, 2004

III-IV BIPOLAR TRANSISTOR TECHNOLOGY

Characteristic Parameters

Current gain Cutoff frequency / Speed Parasitic capacitances

Methods to improve performances

Device design Choice of material systems

GaInAs/InP Buried Metal HBT- REDUCTION OF BASE-COLLECTOR CAPACITANCE

• Buried Tungsten wires of the same area as the emitter metal was used to reduce CBCext

• SBCT of BM-HBT was estimated to be 22% that of conventional HBT, CBC 30% of conventional HBT

• fT = 86 GHz, fMAX > 135 GHz of device with an emitter area of 0.5 x 2.5 μm2

• fT = 82 GHz, fMAX > 200 GHz of device with an emitter area of 0.3 x 1.5 μm2

Schematic view of fabricated BM-HBT

Layer structure for the buried metal - HBT

I – V Characteristics

Common-emitter collector I-V characteristicsof BM-HBT with an emitter area of 2x10 μm2

• Current gain about 70 at the collector voltage of 4V• S-parameters were measured from 50 MHz to 30 GHz using an HP8722 network analyzer• Extrapolations of fT and fMAX were carried out from the -20dB/decade regions of current gain (|h21|2) and Mason’s unilateral gain (U), respectively• the values of fT and fMAX reached peak points (fT = 33.5 GHz, fMAX = 47.3 GHz) at IC = 4mA and VC = 6V

SEM image of BM-HBT

SEM image of the fabricated BM-HBT with anEmitter width of 0.3 μm after formation of the Dummy mesa. Good alignment between theWires and the emitter is observed.

A SEM view of a cross section cut by a focuse Ion beam. Measured collector thickness was 290 nm.

AlGaAs-GaInP Composite Emitter in GaInP/GaAs HBT

- Improved Emitter Transit Time

Emitter Transit Time

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Composite Graded Emitter vs. Conventional Emitter

• Self-aligned HBTs are grown by CBE (chemical beam epitaxy)• Increase in fT from 44 GHz to 62 GHz• CBE 3 times lower for composite emitter HBT without significant IC variation• Common limitation in high speed performance of HBTs: large CBE (limited mobile carrier transport thus charges accumulation in the emitter)• Composite graded AlGaAs layer forms an electron launcher at the interface with the GaInP layer, which injects the electrons at a higher kinetic energy toward the remaining part of the emitter, It leads to lower free carrier concentration (Qe) and smaller CBE

Energy band diagram of GaInP/GaAs (a)Composite emitter (b)conventional design HBT

Comparison of eletric field and electron density

Comparison of (a)electric field and (b)electron density Profilesfor GaInP conventional and AlGaAs0GaInP composite emitterdesign HBTs.

• Compositonally graded AlGaAs emitter HBTs have much stronger electric fields present in the emitter• The electron density is dramatically decreased due to the presence of a drift velocity component in this region of the emitter• GaInP conventional emitter HBTs do not have a built-in electric field within the emitter region, and the electron density in this case is increased due to slow transport of carriers and thus carrier accumulation

Comparison of electron velocity

Comparison of electron velocity profiles for GaInPconventional and AlGaAs-GaInP emitter designHBTs in the composite emitter region

• The figure focuses on the velocity characteristics responsible for the improved frequency characteristics• In the case of the composite emitter design, the electron velocity is high due to the drift velocity component in the special emitter region• On the other hand, the electron velocity of the conventional emitter design is slower since diffusion carrier transport is dominant in the emitter region, which consists only of GaInP• An estimate of זE using simulation

to evaluate ∆Qe/∆Jc showed values of 0.13ps and 0.57ps, respectively.

Comparison of CBE and fT

Comparison of CBE and fT characteristics for GaInP/ GaAsHBT (a)composite emitter and (b)conventional emitter

• CBE for a composite emitter HBT was found to be at least 3 times lower than the conventional emitter HBT under high IC operation

• CBE for a composite emitter HBT presents a weak Jc dependence

• fT was improved from 44 GHz to 62 GHz by using the composite emitter in the HBT

InGaP/GaAs HBT with WSi/Ti Electrode and Buried SiO2 in the Extrinsic Collector

- decrease of emitter size and CBC

Schematic cross-section of device structure:(a)Conventional HBT and (b)small-scale HBTwith a WSi/Ti base electrode and buried SiO2

• The width of the base contact is reduced by using a self-aligning process• The buried SiO2 reduces the parasitic capacitance because the dielectric constant of SiO2 is about 1/3 of that of GaAs• WSi/Ti is used as the base electrode instead of conventional gold-based electrode. Both WSi and Ti can be deposited by sputtering with good step coverage and selectively patternede on GaAs and SiO2 by RIE. A thin Ti film inserted between WSi and GaAs reduces the contact resistance, and made it possible to reduce the width of the base contact without the large increase in the base resistance• The emitter size effect on current gain was suppressed by using InGaP as the emitter

Device Performance

• The DC current gain of 20 is obtained for an HBT with SE of 0.3 x 1.6 μm2 due to the suppression of emitter size effect by using InGaP as the emitter material;

• An HBT with SE of 0.6 x 4.6 μm2 exhibited fT of 138 GHz and fmax of 275 GHz at IC of 4 mA;

• An HBT with SE of 0.3 x 1.6 μm2 exhibited fT of 96 GHz and fmax of 197 GHz at IC of 1 mA

Motivation for work in InAs bipolar transistors

Historic trend – Increased the amount of Indium in the base of a HBT

• Higher electron mobility & saturation velocity → shorter base transit time

• Improved base resistance/base contact resistance• Faster device

Advantages compared to the traditional III-Vs• Lower electron effective mass (0.022 m0)• Higher electron mobility (up to 33000 cm2 V-1 sec-1 at room

temperature)• Higher peak velocity

Cracking study of AlxIn1-xAs on InAsAlxIn1-xAs grown on an InAs substrate is tensile stained, and there exists acritical thickness for the epilayer to form cracks to relieve strain

AlxIn1-xAs epilayers on InAs substrates as a function of Al composition

• Two spicific regions of interest were studied, one with low Al concentration (7-9%) for the HBT devices, and one with higher Al concentration (40- 50%) for other devices.• For x=9%, maximum thickness is around 450 Å• Samples were examined by Normarski contrast interference Microscopy to determine whether they were cracked or crack free

InAs HBT device structure and I-V

βmax for BJT is 50, βmax for HBT is 100

Device structure of InAs BJT and HBT devicesRoom temperature common emitter J-V characteristics of an InAs HBT

SUMMARY

Base collector capacitance reduced by as high as 70% in BM-HBT

Base emitter capacitance reduced to one-third by composite InGaP/GaAs emitter

Parasitic capacitance reduced by 50% in InGaP/GaAs HBT using WSi base electrode and buried SiO2 layer

First results for InAs bipolar transistors (β ~ 100)

Reference1. T. Arai, Y. Harada, S. Yamagami, Y. Miyamoto, K. Furuya, “First fabrication of

GaInAs/InP buried metal heterojunction bipolar transistor and reduction of base-collector capacitance”, Jpn. J. Appl. Phys., Vol. 39 (2000), pp. L503-505

2. T. Arai, S. Yamagami, Y. Miyamoto, K. Furuya, “Reduction of base-collector capacitance in submicron InP/GaInAs heterojunction bipolar transistors with buried tungsten wires”, Jpn. J. Appl. Phys., Vol. 40 (2001), pp. L735-L737

3. T. Arai, S. Yamagami, Y. Miyamoto, and K. Furuya, “Fabrication of submicron buried metal heterojunction bipolar transistor by EB-lithography”,

www.pe.titech.ac.jp/qee_root/symposium/ PDF/araimiyamoto.pdf 4. J. Park, D. Pavlidis, S. Mohammadi, J. Guyaux and J. Garcia, “Improved

emitter transit times using AlGaAs-GaInP composite emitter in GaInP/GaAs heterojunction bipolar transistors”, IEEE transactions on Electron Device. Vol. 48, No. 7, July 2001, pp1297-1303

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Reference

6. T. Oka, K. Hirata, K. Ouchi, H. Uchiyama, K. Mochizuki and T. Nakamura, “

Small-scale InGaP/GaAs HBT’s with WSi/Ti base electrode and buried SiO2”, IEEE transactions on electron devices, vol. 45, No. 11, Nov 1998, pp.2276-2282

7. K. Averett, S. Maimon, X. Wu, M. Koch and G. Wicks, “InAs-based bipolar transistors grown by molecular beam epitaxy”, Journal of vacuum science technology B, 20 (3), May/Jun 2002, PP. 1213-1216

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