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Simulation and Characterization of Silicon Carbide Power Bipolar Junction Transistors
KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The superior characteristics of silicon carbide, compared with silicon, have suggested considering this material for the next generation of power semiconductor devices. Among the different power switches, the bipolar junction transistor (BJT) can provide a very low forward voltage drop, a high current capability and a fast switching speed. However, in order to compete on the market, it is crucial to a have high current gain and a breakdown voltage close to ideal. Moreover, the absence of conductivity modulation and long-term stability has to be solved.

In this thesis, these topics are investigated comparing simulations and measurements. Initially, an efficient etched JTE has been simulated and fabricated. In agreement with the simulations, the fabricated diodes exhibit the highest BV of around 4.3 kV when a two-zone JTE is implemented. Furthermore, the simulations and measurements demonstrate a good agreement between the electric field distribution inside the device and the optical luminescence measured at breakdown.

Additionally, an accurate model to simulate the forward characteristics of 4H-SiC BJTs is presented. In order to validate the model, the simulated current gains are compared with measurements at different temperatures and different base-emitter geometries. Moreover, the simulations and measurements of the on-resistance are compared at different base currents and different temperatures. This comparison, coupled with a detailed analysis of the carrier concentration inside the BJT, indicates that internal forward biasing of the base-collector junction limits the BJT to operate at high current density and low forward voltage drop simultaneously. In agreement with the measurements, a design with a highly-doped extrinsic base is proposed to alleviate this problem.

In addition to the static characteristics, the comparison of measured and simulated switching waveforms demonstrates that the SiC BJT can provide fast switching speed when it acts as a unipolar device. This is crucial to have low power losses during transient.

Finally, the long-term stability is investigated. It is observed that the electrical stress of the base-emitter diode produces current gain degradation; however, the degradation mechanisms are still unclear. In fact, the analysis of the measured Gummel plot suggests that the reduction of the carrier lifetime in the base-emitter region might be only one of the causes of this degradation. In addition, the current gain degradation due to ionizing radiation is investigated comparing the simulations and measurements. The simulations suggest that the creation of positive charge in the passivation layer can increase the base current; this increase is also observed in the electrical measurements.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xiv, 82 p.
Series
Trita-ICT/MAP AVH, ISSN 1653-7610 ; 2012:08
Keyword [en]
silicon carbide, power device, BJT, diode, simulation, characterization, current gain, on-resistance, breakdown voltage, forward voltage drop, degradation
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
SRA - ICT
Identifiers
URN: urn:nbn:se:kth:diva-95320ISBN: 978-91-7501-365-7 (print)OAI: oai:DiVA.org:kth-95320DiVA: diva2:527741
Public defence
2012-06-08, C1, Electrum, KTH-ICT, Isafjordsgatan 26, Kista, 10:00 (English)
Opponent
Supervisors
Note
QC 20120522Available from: 2012-05-22 Created: 2012-05-22 Last updated: 2012-05-22Bibliographically approved
List of papers
1. High-Voltage 4H-SiC PiN Diodes With Etched Junction Termination Extension
Open this publication in new window or tab >>High-Voltage 4H-SiC PiN Diodes With Etched Junction Termination Extension
Show others...
2009 (English)In: IEEE Electron Device Letters, ISSN 0741-3106, E-ISSN 1558-0563, Vol. 30, no 11, 1170-1172 p.Article in journal (Refereed) Published
Abstract [en]

Implantation-free mesa-etched 4H-SiC PiN diodes with a near-ideal breakdown voltage of 4.3 kV (about 80% of the theoretical value) were fabricated, measured, and analyzed by device simulation and optical imaging measurements at breakdown. The key step in achieving a high breakdown voltage is a controlled etching into the epitaxially grown p-doped anode layer to reach an optimum dopant dose of similar to 1.2 x 10(13) cm(-2) in the junction termination extension (JTE). Electroluminescence revealed a localized avalanche breakdown that is in good agreement with device simulation. A comparison of diodes with single-and double-zone etched JTEs shows a higher breakdown voltage and a less sensitivity to varying processing conditions for diodes with a two-zone JTE.

Keyword
Junction termination extension, PiN diode, 4H-SiC
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-18894 (URN)10.1109/led.2009.2030374 (DOI)000271151500018 ()2-s2.0-70350612871 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
2. Modeling and Characterization of Current Gain Versus Temperature in 4H-SiC Power BJTs
Open this publication in new window or tab >>Modeling and Characterization of Current Gain Versus Temperature in 4H-SiC Power BJTs
Show others...
2010 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 57, no 3, 704-711 p.Article in journal (Refereed) Published
Abstract [en]

Accurate physical modeling has been developed to describe the current gain of silicon carbide (SiC) power bipolar junction transistors (BJTs), and the results have been compared with measurements. Interface traps between SiC and SiO2 have been used to model the surface recombination by changing the trap profile, capture cross section, and concentration. The best agreement with measurement is obtained using one single energy level at 1 eV above the valence band, a capture cross section of 1 x 10(-15) cm(2), and a trap concentration of 2 x 10(12) cm(-2). Simulations have been performed at different temperatures to validate the model and characterize the temperature behavior of SiC BJTs. An analysis of the carrier concentration at different collector currents has been performed in order to describe the mechanisms of the current gain fall-off at a high collector current both at room temperature and high temperatures. At room temperature, high injection in the base ( which has a doping concentration of 3 x 10(17) cm(-3)) and forward biasing of the base-collector junction occur simultaneously, causing an abrupt drop of the current gain. At higher temperatures, high injection in the base is alleviated by the higher ionization degree of the aluminum dopants, and then forward biasing of the base-collector junction is the acting mechanism for the current gain fall-off. Forward biasing of the base-collector junction can also explain the reduction of the knee current with increasing temperature by means of the negative temperature dependence of the mobility.

Keyword
Bipolar junction transistor (BJT), current gain, interface traps, silicon carbide (SiC), simulations, temperature modeling, 1800 v, interface, oxide, 4h
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-19258 (URN)10.1109/ted.2009.2039099 (DOI)000274993100022 ()2-s2.0-77649179200 (Scopus ID)
Note
QC 20100525Available from: 2010-08-05 Created: 2010-08-05 Last updated: 2017-12-12Bibliographically approved
3. Influence of Emitter Width and Emitter-Base Distance on the Current Gain in 4H-SiC Power BJTs
Open this publication in new window or tab >>Influence of Emitter Width and Emitter-Base Distance on the Current Gain in 4H-SiC Power BJTs
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2010 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 57, no 10, 2664-2670 p.Article in journal (Refereed) Published
Abstract [en]

The influence of the emitter-base geometry on the current gain has been investigated by means of measurements and simulations. Particular attention has been placed on the emitter width and on the distance between the emitter edge and the base contact. When the emitter width is decreased from 40 to 8 mu m, the current gain is reduced by 20%, whereas when the distance between the base contact and the emitter edge is decreased from 5 to 2 mu m, the current gain is reduced by 10%. Simulations have been used to investigate the reasons for the current gain reduction. The reduction of the emitter width induces two mechanisms of current gain reduction: earlier forward biasing of the base-collector junction and higher recombination in the emitter region. Both mechanisms result from the higher current density flowing under the emitter region. Placing the base contact very close to the emitter edge increases the base current by increasing the gradient of the electron concentration toward the base contact. The effect of increasing the base doping in the extrinsic region has been simulated, and the results demonstrate that the current gain can be improved if a high doping concentration in the range of 5 x 10(18) cm(-3) is used.

Keyword
Bipolar junction transistor (BJT), current gain, emitter-base distance, emitter width, silicon carbide (SiC), simulations
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-29823 (URN)10.1109/TED.2010.2061854 (DOI)000283346500040 ()2-s2.0-77956993929 (Scopus ID)
Note
QC 20110217Available from: 2011-02-17 Created: 2011-02-17 Last updated: 2017-12-11Bibliographically approved
4. Modeling and Characterization of the ON-Resistance in 4H-SiC Power BJTs
Open this publication in new window or tab >>Modeling and Characterization of the ON-Resistance in 4H-SiC Power BJTs
Show others...
2011 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 58, no 7, 2081-2087 p.Article in journal (Refereed) Published
Abstract [en]

The ON-resistance of silicon carbide bipolar transistors is characterized and simulated. Output characteristics are compared at different base currents and different temperatures in order to validate the physical model parameters. A good agreement is obtained, and the key factors, which limit the improvement of R-ON, are identified. Surface recombination and material quality play an important role in improving device performances, but the device design is also crucial. Based on simulation results, a design that can enhance the conductivity modulation in the lowly doped drift region is proposed. By increasing the base doping in the extrinsic region, it is possible to meet the requirements of having low voltage drop, high current density, and satisfactory forced current gain. According to simulation results, if the doping is 5 x 10(18) cm(-3), it is possible to conduct 200 A/cm(2) at V-CE = 1 V by having a forced current gain of about 8, which represents a large improvement, compared with the simulated value of only one in the standard design.

Keyword
Bipolar junction transistor (BJT), extrinsic base, forced current gain, ON-resistance, 4H-silicon carbide (SiC)
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-36242 (URN)10.1109/TED.2011.2141141 (DOI)000291952900034 ()2-s2.0-79959514175 (Scopus ID)
Note
QC 20110711Available from: 2011-07-11 Created: 2011-07-11 Last updated: 2017-12-11Bibliographically approved
5. Investigation of Current Gain Degradation in 4H-SiC Power BJTs
Open this publication in new window or tab >>Investigation of Current Gain Degradation in 4H-SiC Power BJTs
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2012 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 717-720, 1131-1134 p.Article in journal (Refereed) Published
Abstract [en]

The current gain degradation of 4H-SiC BJTs with no significant drift of the on-resistance is investigated. Electrical stress on devices with different emitter widths suggests that the device design can influence the degradation behavior. Analysis of the base current extrapolated from the Gummel plot indicates that the reduction of the carrier lifetime in the base region could be the cause for the degradation of the gain. However, analysis of the base current of the base-emitter diode shows that the degradation of the passivation layer could also influence the reduction of the current gain.

Keyword
4H-SiC, BJT, degradation, current gain, temperature
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-95318 (URN)10.4028/www.scientific.net/MSF.717-720.1131 (DOI)000309431000270 ()2-s2.0-84861349620 (Scopus ID)
Conference
14th International Conference on Silicon Carbide and Related Materials 2011, ICSCRM 2011; Cleveland, OH;11 September 2011 through 16 September 2011
Funder
StandUp
Note

QC 20120522

Available from: 2012-05-22 Created: 2012-05-22 Last updated: 2017-12-07Bibliographically approved
6. Impact of Ionizing Radiation on the SiO2/SiC Interface in 4H-SiC BJTs
Open this publication in new window or tab >>Impact of Ionizing Radiation on the SiO2/SiC Interface in 4H-SiC BJTs
2012 (English)In: IEEE Transactions on Electron Devices, ISSN 0018-9383, E-ISSN 1557-9646, Vol. 59, no 12, 3371-3376 p.Article in journal (Refereed) Published
Abstract [en]

Degradation of SiO2 surface passivation for 4H-SiC power bipolar junction transistors (BJTs) as a result of ion irradiation has been studied to assess the radiation hardness of these devices. Fully functional BJTs with 2700 V breakdown voltage are implanted with 600 keV helium ions at fluences ranging from 1 x 10(12) to 1 x 10(16) cm(-2) at room temperature. These ions are estimated to reach the SiO2/SiC interface. The current-voltage characteristics before and after irradiation show that the current gain of the devices starts degrading after a helium fluence of 1 x 10(14) cm(-2) and decreases up to 20% for the highest fluence of ions. Simulations show that the helium ions induce ionization inside the SiO2, which increases the interface charge and leads to a degradation of the BJT performance. Thermal annealing of the irradiated devices at 300 degrees C, 420 degrees C, and 500 degrees C further increases the amount of charge at the interface, resulting in increased base current in the low-voltage range.

Keyword
Bipolar junction transistor (BJT), device passivation, ion radiation effects, 4H-SiC
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-61424 (URN)10.1109/TED.2012.2222414 (DOI)000311680400032 ()2-s2.0-84870260028 (Scopus ID)
Note

QC 20130109. Updated from submitted to published.

Available from: 2012-01-17 Created: 2012-01-17 Last updated: 2017-12-08Bibliographically approved

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