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High power bipolar junction transistors in silicon carbide
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
2005 (English)Licentiate thesis, comprehensive summary (Other scientific)
Abstract [en]

As a power device material, SiC has gained remarkable attention to its high thermal conductivity and high breakdown electric field. SiC bipolar junction transistors (BJTs) are interesting for applications as power switch for 600 V-1200 V applications. The SiC BJT has potential for very low specific on-resistances and this together with high temperature operation makes it very suitable for applications with high power densities. One disadvantage of the BJT compared with MOSFETs and Insulated Gate Bipolar Transistors (IGBTs) is that the BJT requires a more complex drive circuit with higher power capability. For the SiC BJT to become competitive with field effect transistors, it is important to achieve high current gains to reduce the power required by the drive circuit. Although much progress in SiC BJTs has been made, SiC BJTs still have low common emitter current gain typically in the range 10-50. In this work, a record high current gain exceeding 60 has been demonstrated for a SiC BJT with a breakdown voltage of 1100 V. This result is attributed to an optimized device design, a stable device process and state-of-the-art epitaxial base and emitter layers.

A new technique to fabricate the extrinsic base using epitaxial regrowth of the extrinsic base layer was proposed. This technique allows fabrication of the highly doped region of the extrinsic base a few hundred nanometers from the intrinsic region. An important factor that made removal of the regrowth difficult was that epitaxial growth of very highly doped layers has a faster lateral than vertical growth rate and the thickness of the p+ layer therefore has a maximum close to the base-emitter sidewall. A remaining p+ regrowth spacer at the edge of the base-emitter junction is proposed to explain the low current gain.

Under high power operation, the SiC BJTs were strongly influenced by self-heating, which significantly limits the performance of device. The DC I-V characteristics of 4H-SiC BJTs have also been studied in the temperature range 25 °C to 300 °C. The DC current gain at 300 °C decreased 56 % compared to its value at 25 °C. Selfheating effects were quantified by extracting the junction temperature from DC measurements.

To form good ohmic contacts to both n-type and p-type SiC using the same metal is one important challenge for simplifying SiC Bipolar Junction Transistor (BJT) fabrication. Ohmic contact formation in the SiC BJT process was investigated using sputter deposition of titanium tungsten to both n-type and p-type followed by annealing at 950 oC. The contacts were characterized with linear transmission line method (LTLM) structures. The n+ emitter structure and the p+ base structure contact resistivity after 30 min annealing was 1.4 x 10-4 Ωcm2 and 3.7 x 10-4 Ωcm2, respectively. Results from high-resolution transmission electron microscopy (HRTEM), suggest that diffusion of Si and C atoms into the TiW layer and a reaction at the interface forming (Ti,W)C1-x are key factors for formation of ohmic contacts.

Place, publisher, year, edition, pages
Stockholm: KTH , 2005. , p. viii, 44
Series
Trita-EKT, ISSN 1650-8599 ; 2005:6
Keywords [en]
Silicon Carbide (SiC), power device, biplar junction transistor, TiW, ohmic contact, current gain
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-3854OAI: oai:DiVA.org:kth-3854DiVA, id: diva2:9720
Supervisors
Note
QC 20101208Available from: 2006-02-14 Created: 2006-02-14 Last updated: 2010-12-08Bibliographically approved
List of papers
1. Electrical characteristics of 4H-SiC BJTs at elevated temperatures
Open this publication in new window or tab >>Electrical characteristics of 4H-SiC BJTs at elevated temperatures
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2005 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 483-485, p. 897-900Article in journal (Refereed) Published
Abstract [en]

The DCI-V characteristics of 4H-SiC BJTs have been studied in the temperature range 25 ° C to 300 ° C. The DC current gain at 300 ° C decreased 56% compared to its value at 25 ° C. Under high power operation, the SiC BJTs were strongly influenced by self-heating, which significantly limits the performance of device. Pulsed measurements were performed and compared to DC measurements to distinguish the effects of self-heating. From DC IN measurements, the junction temperature and thermal resistance were extracted to 102 ° C and 19 ° C/W respectively for a power level of 7.3 W at ambient temperature 25 ° C.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2005
Keywords
4H-SiC, Bipolar junction transistor, Junction temperature, Self-heating
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-5368 (URN)10.4028/www.scientific.net/MSF.483-485.897 (DOI)000228549600213 ()2-s2.0-35148894549 (Scopus ID)978-087849963-2 (ISBN)
Conference
5th European Conference on Silicon Carbide and Related Materials, ECRSCRM2004; Bologna; 31 August 2004 through 4 September 2004
Note

QC 20101208

Available from: 2006-02-14 Created: 2006-02-14 Last updated: 2017-11-21Bibliographically approved
2. Geometrical effects in high current gain 1100-V 4H-SiC BJTs
Open this publication in new window or tab >>Geometrical effects in high current gain 1100-V 4H-SiC BJTs
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2005 (English)In: IEEE Electron Device Letters, ISSN 0741-3106, E-ISSN 1558-0563, Vol. 26, no 10, p. 743-745Article in journal (Refereed) Published
Abstract [en]

This paper reports the fabrication of epitaxial 4H-SiC bipolar junction transistors (BJTs) with a maximum current gain beta = 64 and a breakdown voltage of 1100 V. The high beta value is attributed to high material quality obtained after a continuous epitaxial growth of the base-emitter junction. The BJTs show a clear emitter-size effect indicating that surface recombination has a significant influence on beta. A minimum distance of 2-3 mu m between the emitter edge and base contact implant was found adequate to avoid a substantial beta reduction.

Keywords
4H-SiC, bipolar junction transistor (BJT), breakdown voltage, current gain, emitter-size effect, bipolar junction transistors, 1800 v
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-15068 (URN)10.1109/led.2005.856010 (DOI)000232208700015 ()2-s2.0-27144457411 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. Investigation of TiW contacts to 4H-SiC bipolar junction devices
Open this publication in new window or tab >>Investigation of TiW contacts to 4H-SiC bipolar junction devices
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2006 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 527-529, p. 887-890Article in journal (Refereed) Published
Abstract [en]

One important challenge in SiC Bipolar Junction Transistor (BJT) fabrication is to form good ohmic contacts to both n-type and p-type SiC. In this paper, we have examined contact study in a SiC BJT process with sputter deposition of titanium tungsten contacts to both n-type and p-type regions followed by annealing at different temperatures between 750 T and 950 T. The contacts were characterized using linear transmission line method (LTLM) structures. To see the formation of compound phases, X-ray Diffraction (XRD) theta-2 theta scans were performed before and after annealing. The results indicate that 5 minutes annealing at 950 T of the n(+) contact is sufficient whereas the p(+) contacts remain non-ohmic after 30 minutes annealing. The n(+) emitter structure contact resistivity after 5 min annealing with 750 degrees C and 950 degrees C was 1.08 x 10(-3) Omega cm(2) and 4.08 x 10(-4) Omega cm(2), respectively. Small amorphous regions of silicon and carbon as well as titanium tungsten carbide regions were observed by high-resolution transmission electron microscopy (HRTEM), whereas less carbide formation and no amorphous regions were found in a sample with unsuccessful formation of TiW ohmic contacts.

Keywords
bipolar junction transistor, titanium, tungsten, ohmic contacts
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-5370 (URN)000244227200210 ()2-s2.0-37849035711 (Scopus ID)
Conference
International Conference on Silicon Carbide and Related Materials (ICSCRM 2005) Pittsburgh, PA, SEP 18-23, 2005
Note
QC 20101208Available from: 2006-02-14 Created: 2006-02-14 Last updated: 2017-11-21Bibliographically approved
4. A 4H-SiC BJT with an Epitaxially Regrown Extrinsic Base Layer
Open this publication in new window or tab >>A 4H-SiC BJT with an Epitaxially Regrown Extrinsic Base Layer
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2005 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 483-485, p. 905-908Article in journal (Refereed) Published
Abstract [en]

4H-SiC BJTs were fabricated using epitaxial regrowth instead of ion implantation to form a highly doped extrinsic base layer necessary for a good base ohmic contact. A remaining p(+) regrowth spacer at the edge of the base-emitter junction is proposed to explain a low current gain of 6 for the BJTs. A breakdown voltage of 1000 V was obtained for devices with Al implanted JTE.

Keywords
bipolar junction transistor; extrinsic base; epitaxial regrowth
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-7936 (URN)10.4028/www.scientific.net/MSF.483-485.905 (DOI)000228549600215 ()2-s2.0-35148858873 (Scopus ID)
Note
QC 20100819. 5th European Conference on Silicon Carbide and Related Materials. Bologna, ITALY. AUG 31-SEP 04, 2004 Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2017-12-14Bibliographically approved

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