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Fabrication and Characterization of Silicon Carbide Power Bipolar Junction Transistors
KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
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 [en]
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: urn:nbn:se:kth:diva-4623OAI: oai:DiVA.org:kth-4623DiVA: diva2:13123
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
List of papers
1. 1200-V 5.2-m Omega center dot cm(2) 4H-SiC BJTs with a high common-emitter current gain
Open this publication in new window or tab >>1200-V 5.2-m Omega center dot cm(2) 4H-SiC BJTs with a high common-emitter current gain
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2007 (English)In: IEEE Electron Device Letters, ISSN 0741-3106, E-ISSN 1558-0563, Vol. 28, no 11, 1007-1009 p.Article in journal (Refereed) Published
Abstract [en]

This letter presents fabrication of a power 4H-SiC bipolar junction transistor (BJT) with a high open-base breakdown voltage BVCEO approximate to 1200 V, a low specific ON-resistance R-SP_ON approximate to 5.2 m Omega . cm(2), and a high common-emitter current. gain beta approximate to 60. The high gain of the BJT is attributed to reduced surface recombination that has been obtained using passivation by thermal silicon dioxide grown in nitrous oxide (N2O) ambient. Reference BJTs with passivation by conventional dry thermal oxidation show a clearly lower current gain and a more pronounced emitter-size effect. BJTs with junction termination by a guard-ring-assisted junction-termination extension (JTE) show about 400 V higher breakdown voltage compared with BJTs with a conventional JTE.

Keyword
bipolar junction transistors (BJTs), current gain, emitter-size effect, high voltage, surface recombination, 4H-silicon carbide, inversion channel mobility, mosfets
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-17077 (URN)10.1109/led.2007.907418 (DOI)000250524200023 ()2-s2.0-36148960173 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. A comparative study of surface passivation on SiC BJTs with high current gain
Open this publication in new window or tab >>A comparative study of surface passivation on SiC BJTs with high current gain
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2007 (English)In: Materials Science Forum, ISSN 0255-5476, Vol. 556-557, 631-634 p.Article in journal (Refereed) Published
Abstract [en]

The effect of the different types of passivation layers on the current gain of SiC BJTs has been investigated. Measurements have been compared for BJTs passivated with thermal SiO2, plasma deposited (PECVD) SiO2 and BJTs without passivation. The maximum DC current gain of BJTs with thermal SiO2 was about 62 at I-c=20 mA and V-ce=40 V. On the other hand, the BJTs with a passivation by PECVD SiO2 had a DC current gain of only 25. The surface recombination current was extracted from measurements with BJTs of different emitter widths. The surface recombination current of BJTs with a thermally grown oxide was about 25% lower than unpassivated BJTs and 65% lower than that of PECVD passivated BJTs.

Keyword
bipolar junction transistor; surface passivation; surface recombination
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7932 (URN)10.4028/www.scientific.net/MSF.556-557.631 (DOI)000249653900149 ()2-s2.0-38449110894 (Scopus ID)
Note
QC 20100819Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2010-08-19Bibliographically approved
3. 4H-SiC power BJTs with high current gain and low on-resistance
Open this publication in new window or tab >>4H-SiC power BJTs with high current gain and low on-resistance
2007 (English)In: Materials Science Forum, ISSN 0255-5476, Vol. 556-557, 767-770 p.Article in journal (Refereed) Published
Abstract [en]

4H-SiC BJTs have been fabricated with varying geometrical designs. The maximum value of the current gain was about 30 at I-c=85 mA, V-CE=14 V and room temperature (RT) for a 20 mu m emitter width structure. A collector-emitter voltage drop V-CE of 2 V at a forward collector current 55 mA (J(C) = 128 A/cm(2)) was obtained and a specific on-resistance of 15.4 m Omega center dot cm(2) was extracted at RT. Optimum emitter finger widths and base-contact implant distances were derived from measurement. The temperature dependent DC IN characteristics of the BJTs have been studied resulting in 45 % reduction of the gain and 75 % increase of the on-resistance at 225 degrees C compared to RT. Forward-bias stress on SiC BJTs was investigated and about 20 % reduction of the initial current gain was found after 27.5 hours. Resistive switching measurements with packaged SiC BJTs were performed showing a resistive fast turn-on with a VCE fall-time of 90 ns. The results indicate that significantly faster switching can be obtained by actively controlling the base current.

Keyword
bipolar junction transistors; current gain; switching; degradation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7933 (URN)10.4028/www.scientific.net/MSF.556-557.767 (DOI)000249653900182 ()2-s2.0-38449112149 (Scopus ID)
Note
QC 20100819Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2010-08-19Bibliographically approved
4. 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, 743-745 p.Article 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.

Keyword
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
5. Influence of the base contact on the electrical characteristics of SiC BJTs
Open this publication in new window or tab >>Influence of the base contact on the electrical characteristics of SiC BJTs
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2007 (English)In: 19th International Symposium on Power Semiconductor Devices and ICs, ISPSD'07: Jeju Island; 27 May 2007 through 31 May 2007, 2007, 153-156 p.Conference paper, Published paper (Refereed)
Abstract [en]

In this paper, we have investigated how the specific on-resistance and common emitter current gain of SiC BJTs depend on the base contact resistance. The on-state characteristics of SiC BJTs were investigated before and after base contact annealing at different temperatures. The common emitter current gain and specific on-resistance was improved by 23 % and 300 % compared to the values of before base contact annealing, respectively. Large area SiC BJTs (active area 0.0324 cm(2)), have been measured up to 34 A collector current in pulsed mode showing a gain of 35, and a specific on-resistance of 2 8.79 m Omega center dot cm(2)

Series
International Symposium on Power Semiconductor Devices and ICs, ISSN 1063-6854
Keyword
silicon carbide; bipolar junction transistor; current gain; contact resistance; specific on-resistance
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-7935 (URN)10.1109/ISPSD.2007.4294955 (DOI)000249645200039 ()2-s2.0-36148982672 (Scopus ID)978-1-4244-1095-8 (ISBN)
Note
QC 20100819Available from: 2008-01-30 Created: 2008-01-30 Last updated: 2010-08-19Bibliographically approved
6. 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, 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.

Keyword
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: 2011-10-12Bibliographically approved
7. Surface passivation oxide effects on the current gain of 4H-SiC bipolar junction transistors
Open this publication in new window or tab >>Surface passivation oxide effects on the current gain of 4H-SiC bipolar junction transistors
Show others...
2008 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 92, no 8, 082113-1-082113-3 p.Article in journal (Refereed) Published
Abstract [en]

Effects of surface recombination on the common emitter current gain have been studied in 4H-silicon carbide (SiC) bipolar junction transistors (BJTs) with passivation formed by conventional dry oxidation and with passivation formed by dry oxidation in nitrous oxide (N2O) ambient. A gradual reduction of the current gain was found after removal of the passivation oxide followed by air exposure. Comparison of the measurement results for two different passivated BJTs indicates that the BJTs with passivation by dry oxidation in nitrous oxide (N2O) ambient show a half order of magnitude reduction of base current, resulting in a half order of magnitude increase of current gain at low currents. This improvement of current gain is attributed to reduced surface recombination caused by reduced interface trap densities at the base-emitter junction sidewall.

Keyword
inversion channel mobility, sic/sio2 interface, recombination, bjts, oxidation, mosfets
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-17400 (URN)10.1063/1.2888965 (DOI)000254297300051 ()2-s2.0-40049097542 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
8. Low-forward-voltage-drop 4H-SiC BJTs without base contact implantation
Open this publication in new window or tab >>Low-forward-voltage-drop 4H-SiC BJTs without base contact implantation
2008 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 55, no 8, 1907-1911 p.Article in journal (Refereed) Published
Abstract [en]

Bipolar junction transistors (BJTs) of 4H-SiC, with a low collector--emitter forward voltage drop YCE, have been fabricated without base contact implantation. A comparison of BJTs on the same wafer with and without base contact implantation shows less than 10% higher VcE for the BJTs without base contact implantation. Omitting the base contact implantation eliminates high concentrations of implantation-induced defects that act as recombination centers. This is advantageous because it allows a shorter distance Wp+ between the emitter edge and the base contact, without affecting the current gain when no base contact implantation is used. The BJTs without contact implantation show a constant current gain as Wp+ was reduced from 3 to I pm, whereas the gain decreased by 45% for the BJTs with base contact implantation for the same reduction of Wp+. A key to the successful fabrication of low-forward-voltage-drop SiC BJTs without base contact implantation is the formation of low-resistivity Ni/Ti/Al ohmic contacts to the base. The contact resistivity on the base region (N-A approximate to 4 x 10(17) cm(-3)) was measured with linear transmission line method structures to PC = 1.9 X 10(-3) Omega cm(2), whereas the contact resistivity with the base contact implantation was PC = 1.3 x 10-4 Omega cm(2), both after rapid thermal processing annealing at 800 degrees C.

Keyword
bipolar junction transistor (BJT), emitter injection efficiency, forward voltage drop, ohmic contact, 4H-silicon carbide, ohmic contacts, al/ti
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-17722 (URN)10.1109/ted.2008.926641 (DOI)000257950300020 ()2-s2.0-49249099302 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
9. High-Current-Gain SiC BJTs With Regrown Extrinsic Base and Etched JTE
Open this publication in new window or tab >>High-Current-Gain SiC BJTs With Regrown Extrinsic Base and Etched JTE
Show others...
2008 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 55, no 8, 1894-1898 p.Article in journal (Refereed) Published
Abstract [en]

This paper describes successful fabrication of 4H-SiC bipolar junction transistors (BJTs) with a regrown extrinsic base layer and an etched junction termination extension (JTE). Large-area 4H-SiC BJTs measuring 1.8 x 1.8 nun (with an active area of 3.24 mm') showed a common emitter current gain 0 of 42, specific on-resistance Rsp ON of 9 mQ - em', and open-base breakdown voltage BVcEO of-1.75 kV at room temperature. The key to successful fabrication of high-current-gain SiC BJTs with a regrown extrinsic base is efficient removal of the p+ regrown layer from the surface of the emitter-base junction. The BJT with p+ regrown layer has the advantage of lower base contact resistivity and current gain that is less sensitive to the distance between the emitter edge and the base contact, compared to a BJT with ion-implanted base. Fabrication of BJTs without ion implantation means less lifetime-reducing defects, and in addition, the surface morphology is improved since high-temperature annealing becomes unnecessary. BJTs with flat-surface junction termination that combine etched regrown layers show about 250 V higher breakdown voltage than BJTs; with only etched flat-surface JTE.

Keyword
base regrowth, bipolar junction transistors (BJTs), junction, termination extension (JTE), 4H-silicon carbide, 4h-sic bjts
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-17721 (URN)10.1109/ted.2008.926645 (DOI)000257950300018 ()2-s2.0-49249108274 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved

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