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A 4H-SiC BJT with an Epitaxially Regrown Extrinsic Base Layer
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.ORCID iD: 0000-0001-8108-2631
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2005 (English)In: Materials Science Forum, ISSN 0255-5476, Vol. 483-485, 905-908 p.Article 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.

Place, publisher, year, edition, pages
2005. Vol. 483-485, 905-908 p.
Keyword [en]
bipolar junction transistor; extrinsic base; epitaxial regrowth
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
URN: urn:nbn:se:kth:diva-7936DOI: 10.4028/www.scientific.net/MSF.483-485.905ISI: 000228549600215Scopus ID: 2-s2.0-35148858873OAI: oai:DiVA.org:kth-7936DiVA: diva2:13119
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: 2011-10-12Bibliographically approved
In thesis
1. Fabrication and Characterization of Silicon Carbide Power Bipolar Junction Transistors
Open this publication in new window or tab >>Fabrication and Characterization of Silicon Carbide Power Bipolar Junction Transistors
2008 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Silicon carbide bipolar junction transistors (BJTs) are attractive power switching devices because of the unique material properties of SiC with high breakdown electric field, high thermal conductivity and high saturated drift velocity of electrons. 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. For SiC BJTs the common emitter current gain (β), the specific on-resistance (RSP_ON), and the breakdown voltage are important to optimize for competition with silicon based power devices. In this thesis, power SiC BJTs with high current gain β ≈ 60 , low on-resistance RSP_ON ≈ 5 mΩcm2, and high breakdown voltage BVCEO ≈ 1200 V have been demonstrated. The 1200 V SiC BJT that has been demonstrated has about 80 % lower on-state power losses compared to a typical 1200 V Si IGBT chip.

A continuous epitaxial growth of the base-emitter layers has been used to reduce interface defects and thus improve the current gain. A significant influence of surface recombination on the current gain was identified by comparing the experiments with device simulations. In order to reduce the surface recombination, different passivation layers were investigated in SiC BJTs, and thermal oxidation in N2O ambient was identified as an efficient passivation method to increase the current gain.

To obtain a low contact resistance, especially to the p-type base contact, is one critical issue to fabricate SiC power BJTs with low on-resistance. Low temperature anneal (~ 800 oC) of a p-type Ni/Ti/Al contact on 4H-SiC has been demonstrated. The contact resistivity on the ion implanted base region of the BJT was 1.3 × 10-4 Ωcm2 after annealing. The Ni/Ti/Al p-type ohmic contact was adapted to 4H-SiC BJTs fabrication indicating that the base contact plays a role for achieving a low on-resistance of SiC BJTs.

To achieve a high breakdown voltage, optimized junction termination is important in a power device. A guard ring assisted Junction Termination Extension (JTE) structure was used to improve the breakdown voltage of the SiC BJTs. The highest breakdown voltage of the fabricated SiC BJTs was obtained for devices with guard ring assisted JTE using the base contact implant step for a simultaneous formation of guard rings.

As a new approach to fabricate SiC BJTs, epitaxial regrowth of an extrinsic base layer was demonstrated. SiC BJTs without any ion implantation were successfully demonstrated using epitaxial regrowth of a highly doped p-type region and an etched JTE using the epitaxial base. A maximum current gain of 42 was measured for a 1.8 mm × 1.8 mm BJT with a stable and reproducible open base breakdown voltage of 1800 V.

Place, publisher, year, edition, pages
Stockholm: KTH, 2008. xvi, 74 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2008:01
Keyword
silicon carbide, power device, BJT, current gain, specific on resistance (RSP_ON), breakdown voltage, forward voltage drop, surface recombination, ohmic contact.
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-4623 (URN)
Public defence
2008-02-15, Sal E, Forum, Isafjordsgatan 39, Kista, Stockholm, 10:15
Opponent
Supervisors
Note
QC 20100819Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2010-08-19Bibliographically approved
2. High power bipolar junction transistors in silicon carbide
Open this publication in new window or tab >>High power bipolar junction transistors in silicon carbide
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. viii, 44 p.
Series
Trita-EKT, ISSN 1650-8599 ; 2005:6
Keyword
Silicon Carbide (SiC), power device, biplar junction transistor, TiW, ohmic contact, current gain
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-3854 (URN)
Supervisors
Note
QC 20101208Available from: 2006-02-14 Created: 2006-02-14 Last updated: 2010-12-08Bibliographically approved

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Zetterling, Carl-Mikael

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