NOVEL 4H-SIC BIPOLAR JUNCTION TRANSISTOR (BJT) WITH IMPROVED CURRENT GAIN

NOVEL 4H-SIC BIPOLAR JUNCTION TRANSISTOR (BJT) WITH IMPROVED CURRENT GAIN Thilini Daranagama 1, Vasantha Pathirana 2, Florin Udrea 3, Richard McMahon 4 1,2,3,4 The University of Cambridge, Cambridge, United Kingdom 2,3 Cambridge Microelectronics Ltd, Cambridge, United Kingdom e-mail: td325@cam.ac.uk, vp222@cam.ac.uk, fu10000@cam.ac.uk, ram1@cam.ac.uk Abstract Silicon Carbide (SiC) is becoming increasingly of interest to the power electronics industry due to its superior characteristics such as high critical electric field, larger bandgap and higher thermal conductivity in comparison to Silicon (Si). Taking advantage of these material properties, high voltage devices could be fabricated with lower power losses and high temperature and frequency operability, which could surpass the limits enforced by Si power devices. SiC BJTs in particular are advantageous for the medium to highvoltage application range (e.g. HVDC multi-level converters) as they exhibit lower on-state losses while maintaining superior switching performance. The only drawback is being a current driven device in contrast to voltage-controlled SiC MOSFETs. Therefore, devices with a large current gain (β) are highly desirable for simplifying the base drive. In this paper, a novel circular structure with improved β has been demonstrated. Although the material quality of epilayers and surface passivation layer are the most significant factors limiting β, the geometrical design also plays a vital role in achieving a higher β. Therefore, the influence of the emitter geometry on β and the Rittner effect have been discussed by comparing rectangular and circular cell structures, with the aid of the software Sentaurus TCAD. Keywords Bipolar Junction Transistor (BJT), Current Gain, Rittner Effect, Silicon Carbide (SiC), TCAD Simulations. BV CBO V CE V BE I C /J C I B /J B W EM β J Bulk J Scr J Inj K Bsurf P E A E NOMENCLATURE Open emitter breakdown voltage. Collector-emitter voltage. Base-emitter voltage. Collector current/ collector current density. Base current/ base current density. Width of the emitter of the half-cell. Common-emitter current gain. Bulk recombination current density. Space-charge recombination current density. Base-emitter back-injected current density. Normalised periphery recombination base current. Emitter perimeter. Emitter area. I. INTRODUCTION SiC bipolar devices are attractive for high voltage applications such as HVDC voltage-source converters (VSC). VSCs are increasingly favoured over traditional linecommutated converters using thyristors as they are faster to switch, compact and have better controllability. However, the lower power capacity and higher losses imposed by the use of Si IGBTs have limited the use of VSCs. These limitations could be overcome by replacing Si IGBTs with SiC devices. At the high voltages required in such applications (>10 kv), SiC BJTs are preferred over SiC MOSFETs due to the absence of the large drift resistance arising from the unipolar nature of the MOSFETs. The positive temperature coefficient of the on-state resistance of SiC BJTs means that they can easily be paralleled to achieve the high current levels required in HVDC applications [1]. Moreover, previous work has demonstrated that SiC BJTs have better switching performance than SiC unipolar devices at higher power levels [2]. SiC BJTs are therefore leading candidates for HVDC VSCs. Despite their potential benefits, industry is reluctant to use SiC BJTs due to their requirement for a continuous base current (I B ) during on-state. This results in higher losses and complexity in the drive circuit, in comparison to the drive circuits required for SiC unipolar devices. Although SiC BJTs have been demonstrated with substantially higher current gains than Si BJTs [3][4], further improving the β will lead to smaller base current requirements, overcoming the barrier imposed by the base drive. Therefore, in this work, a novel circular cell 4H-SiC BJT with improved current gain has been reported. While the wafer quality and the fabrication conditions are key factors for achieving a higher β, geometrical design also plays a vital role in designing SiC BJTs with high current gains. Therefore, analysing the device at a structural level to understand the factors influencing β is essential for designing high voltage SiC BJTs with reasonable gain values. Therefore, in this paper, the effects of geometrical parameters of the emitter region on β have been investigated by comparing simulation results of structures having rectangular and circular cell geometries. For the devices investigated, an open emitter breakdown voltage (BV CBO ) of 2 kv has been chosen. Furthermore, in BJTs, β increases with collector current density up to a certain level after which it begins to fall. This is referred to as the Rittner effect and could be controlled by careful design [5]. Therefore, the influence of the geometrical parameters of the emitter region on the Rittner current density has also been discussed. 978-1-4799-8779-5/15/$31.00 2015 IEEE

II. DESIGN AND PHYSICAL MODELS The simplified cross-section of the simulated 4H-SiC BJTs is shown in Figure 1. The top view of the conventional rectangular geometry SiC BJT is shown in Figure 2 as structure A. Two novel circular structures of the same crosssection have been analysed; structure B is with the base contact at the centre and structure C is with the emitter contact at the centre of the device, as shown in Figure 2. The n-type collector-drift epilayer has been chosen to be 10 μm thick with a doping level of 7x10 15 cm -3 to achieve an open emitter breakdown voltage (BV CBO ) of 2 kv. The following 4H-SiC physical models have been used in the Sentaurus Device simulator: incomplete ionization model, carrier mobility model taking in to account the dependencies with doping, incomplete ionization and saturation of carrier drift velocity at high electric fields, Shockley Read Hall (SRH) recombination accounting the doping dependency, Auger recombination, bandgap narrowing and Fermi Dirac statistics. Due to the hexagonal crystal structure, carrier transport in 4H-SiC exhibits anisotropy. Therefore, anisotropic properties have been taken in to account in mobility and impact ionization models. Surface recombination at the SiC/SiO 2 interface at the etched sidewalls of the emitter-base perimeter has been modelled with acceptor type traps using the specification detailed in [6], i.e., with a single energy level at 1 ev from the valence band, a capture cross section of 1 10 15 cm 2 and a trap concentration of 1 10 12 cm 2. All structures have been simulated to an active area of 17 mm 2 and at 300 K. III. SIMULATION OF SIC BJTS A. Influence on Current Gain In Figure 3, the simulated forward I-V curves for the three structures have been plotted. The current gains (β), calculated as I C /I B, have been plotted against I C in Figure 4. I B = 1 A I B = 0.6 A I B = 0.2 A Fig. 1. Cross-section of the half-cell of the simulated structures. Fig. 3. Simulated I C vs. V CE curves of the structures A, B and C for 0.2 A, 0.6 A and 1 A base currents. Fig. 2. The view from the top (two dimensional) of the simulated structures. Fig. 4. Current gains in the active region of operation, plotted against I C for structures A, B and C. V CE = 5 V. Points when β begins to fall have been marked with crosses.

Ritter current density, which is defined in this paper as the current density when β is maximum (in order to make the explanations clearer), is marked with an x in all three curves. It is clear from the figures that the circular structure B with the base contact at the centre of the device exhibits the highest current gain and structure C with the emitter contact at the centre exhibits the lowest current gain across all the current levels. Table I summarises the geometrical parameters for the three structures. According to Table I, structure B has the largest emitter area. This is because the emitter is at the perimeter of the cell. The higher emitter area in structure B will increase the emitter injection and spread the current more uniformly across the device, improving the current gain. However, due to the poor quality of the interface between SiC and SiO 2, the interface trap density at the etched sidewalls of the emitter-base perimeter of SiC BJTs is substantially higher compared to Si BJTs. Therefore, some electrons injected in to the base from the emitter will recombine at the SiC/SiO 2 interface, within the acceptor traps in this case. Hence, it is important to investigate the surface recombination at the SiC/SiO 2 interface in these structures. The base current in a BJT can be expressed as in Equation 1 [7], Emitter edge/ SiO 2 interface W GAP Fig. 5. Trapped electron densities in the base region near the SiC/SiO 2 interface along the cross section of A-A (see Figure 1). Simulated I C = 10 A for all devices. V CE = 5 V. trapped at the SiC/SiO 2 interface. B. Influence of Emitter Width on the Maximum Current Gain The current gain β vs. I C curves for devices having emitter widths W EM = 1 and 8μm have been plotted in Figure 6. Figure 7 plots the maximum current gains as a function of emitter width W EM. From Figure 7, maximum β increases J B = J C β = J Bulk+J Scr + J Inj + K Bsurf P E A E (1) where J Bulk (A.cm -2 ), J Scr (A.cm -2 ) and J Inj (A.cm -2 ) are the bulk recombination current density, the space-charge recombination current density, and the base-emitter backinjected current density, respectively. In this equation, K Bsurf.P E /A E is the emitter periphery surface recombination current density and K Bsurf is a constant for a given collector current density. From Equation 1, for a given collector current density, β is inversely proportional to the ratio of emitter perimeter over emitter area (P E /A E ). From Table I, structure B has the lowest value of P E /A E and therefore, even when SiC/SiO 2 interface recombination is considered, structure B will give the highest current gain. To further illustrate the influence of interface traps on the current gain, the trapped electron densities along the horizontal cutline A-A (as seen from Figure 1) in the base region near the SiC/SiO 2 interface have been plotted in Figure 5. The simulation data for the plots are taken at I C = 10 A, which is below the Rittner current densities of all the devices. From the plots, it is clear that the structure B exhibits the lowest density of trapped electrons especially near the emitter edge of the SiC/SiO 2 interface and structure C, the highest. Hence, for the same collector current, the structure B will operate with a lower base current compared to structure C, as less emitter injected electrons will get TABLE I Calculated Values of the Geometrical Parameters Parameter Structure Structure Structure A B C Emitter Area, A E (μm 2 ) 6.80E+06 10.9E+06 2.72E+06 Emitter Perimeter, P E (μm) 1.36E+06 1.63E+06 1.09E+06 P E / A E (μm -1 ) 0.20 0.15 0.4 Fig. 6. Plots of current gains simulated at V CE = 5 V against I C for the devices with W EM = 1 and 8 μm. Fig. 7. Simulated maximum current gains for different emitter widths. V CE = 5 V.

rapidly with increasing emitter width at lower values of W EM in all three structures. However after about 11 μm, the increase in β is not as significant. This can be attributed to the emitter current crowding effect [6]. When the BJT is biased in to its on-state, the base current has to flow laterally under the emitter region, causing a lateral voltage drop due to the base sheet resistance. This causes a non-uniform forward biasing of the base-emitter junction. If the emitter is very wide, forward-biasing of the emitter-base junction closer to the base contact will be higher, as plotted in Figure 8 for structure B. This will cause more emitter current to flow in this area. Therefore, the current carrying capability of a BJT does not scale-up with W EM and instead, the gain tends to saturate when the emitter gets wider. Furthermore, from Figures 6 and 7, it is clear that the structure B exhibits a higher maximum β than structures A and C across the entire range of W EM. In order to explain this phenomenon, emitter areas and perimeters of the three structures have been plotted against W EM in Figures 9 and 10, respectively. A E continues to increase with W EM for all the structures and is highest for structure B at all values of W EM, implying a higher emitter injection. However, at lower values of W EM, P E is substantially higher in structure B, implying a higher surface recombination current negatively affecting the overall current gain. Therefore, these effects balance out so that at smaller values of W EM, the difference in the current gains between structure B and the other structures is smaller. This can be further explained by considering the P E /A E plots in Figure 11. From Equation 1, β is inversely proportional to P E /A E. The ratio P E /A E is highest for structure C and lowest for structure B at all values of W EM and the difference is higher at higher emitter widths. This explains the maximum gain trends between the structures observed in Figure 7. C. Influence of Emitter Width on the Rittner Current Density In Figure 12, the Rittner current densities of the three structures have been plotted against the corresponding emitter widths. At lower values of W EM, significantly higher Rittner current densities can be observed for structure B. This is a significant observation as it implies that the current gain W GAP Edge of the base contact W P+ Fig. 9. Emitter area plotted against W EM for the three structures. Fig. 10. Emitter perimeter plotted against W EM for the three structures. of structure B continues to increase with collector current for a broader range of collector currents. A higher Rittner current density is advantageous in terms of base drive design, particularly in applications where the current demand could vary. Therefore, it is important to investigate the factors affecting the Rittner current density. Comparing the emitter perimeter plots of Figure 10 to the Rittner current density plots of Figure 12, similar patterns for the three structures can be observed. Therefore, the trends of the Rittner current density can be attributed to the trends of the emitter perimeter. The dependency of the Rittner current density on P E can be explained as follows. As seen in Figure 10, in structures A and B, P E decreases across the whole spectrum of W EM whereas in structure C, P E increases up to about 8 μm and begins to fall. Total perimeter of these structures, when modelling a 17 mm 2 active area, is calculated as, Number of cells = 17 mm2 active area of a cell (2) Fig. 8. Lateral potential along the base region taken at the cross section A-A (see Figure 1) of structure B. The simulated bias conditions are V BE = 3.0 V and V CE = 5 V. Total P E = Number of cells * P E of a single cell (3)

Edge of the base contact Beginning of the emitter contact Fig. 11. Ratio of emitter perimeter to emitter area, plotted against W EM for the three structures. Fig. 13. Lateral potential in the base along the A-A cross section (see Figure 1) of the three structures with W EM = 1 μm. Simulation conditions are I C = 25 A and V CE = 5 V. Emitter/base junction Base/collector junction Fig. 12. Rittner current densities for different emitter widths. V CE = 5 V. Structure A is a rectangular structure and increasing W EM will reduce the number of cells that can fit in 17 mm 2. Hence, the total P E will continue to reduce with increasing W EM. Structure B has the base at the centre of the device and the emitter goes all around the perimeter of the circle. Therefore, emitter perimeter of a single cell does not change when emitter is made wider. However, the number of cells that can fit in 17 mm 2 will reduce. Hence the total emitter perimeter will reduce in structure B when W EM is increased. In structure C, however, both P E and active area of a single cell increase with increasing W EM. Therefore, the total P E increases up to 8μm and then begins to decrease at higher values of W EM, as the increase in cell area dominates. When the emitter perimeter is smaller, the device needs to be driven harder by the base voltage to obtain the same collector current. In order to explain this phenomenon, solutions at I C of 25 A of the structures with W EM = 1 μm have been considered. In structure C with W EM = 1 μm, 25 A is well above the Rittner current level (9 A) whereas in structures A and B, Rittner current levels are at 41 A and 71 A, respectively. From Figure 13, which contains plots of the lateral potential of the solutions along the A-A cross section Fig. 14. Effective doping (with incomplete ionization) and electron concentrations in the base along the vertical cross section B-B (see Figure 1) for the three structures with W EM = 1 μm. I C = 25 A and V CE = 5 V. in Figure 1, it is clear that the base of structure C has to be driven much harder than the other structures. The higher V BE required to bias the structure C causes more electrons to enter the base region from the emitter, resulting in the electron concentration in the base exceeding beyond the base doping level. This is illustrated in Figure 14, where carrier concentrations in the base along the vertical cross section of B-B (see Figure 1) have been plotted. This phenomenon is referred to as the high level injection in base. When high level injection occurs in the base, the majority carrier concentration (holes) increases in order to satisfy the charge neutrality. The enhancement of the hole concentration in the base causes more holes to enter the emitter, resulting in a reduction in the emitter injection efficiency and hence, the current gain of the BJT with structure C. In the other two devices, when conducting a 25 A collector current, the carrier densities are lower than the effective doping

concentration. Therefore, Rittner effect will occur at much higher collector currents. IV. CONCLUSIONS In this paper, a novel circular cell SiC BJT with improved current gain has been demonstrated. By comparing a conventional rectangular geometry device with two novel circular cell structures, the influence of the emitter region on the current gain and the Rittner current density of SiC BJTs have been discussed with the aid of Sentaurus TCAD simulation results. The simulation study shows that the circular structure with the base contact at the centre of the device (structure B) exhibits the highest current gain at all values of emitter widths. This is attributed to the larger emitter area and the lower surface recombination current density arising from the smaller P E /A E ratio of structure B. At higher values of W EM, the difference between the maximum current gains is even higher. Furthermore, structure B has the highest Rittner current densities at lower values of W EM. This is because of the lower V BE biasing required for this device, ultimately delaying the high level injection in the base to much higher collector current densities. The reason for the lower V BE is the higher emitter perimeter of structure B at lower values of W EM. Overall, it can be concluded that by implementing the structure B instead of conventional rectangular structures, SiC BJTs with improved current gains and Rittner current densities can be designed. W EM must be carefully chosen depending on the target application to provide the best tradeoff between a higher current gain and a higher Rittner current density. REFERENCES [1] M. Chinthavali, P. Ning, Y. Cui, and L. M. Tolbert, Investigation on the Parallel Operation of Discrete SiC BJTs and JFETs, in Twenty-Sixth Annual IEEEApplied Power Electronics Conference and Exposition (APEC), pp. 1076 1083, 2011. [2] T. Daranagama, N. Udugampola, R. McMahon, and F. Udrea, Comparative analysis of static and switching performance of 1.2 kv commercial SiC transistors for high power density applications, in The 1st IEEE Workshop on Wide Bandgap Power Devices and Applications, pp. 48 51, 2013. [3] J. Zhang, X. Li, P. Alexandrov, L. Fursin, X. Wang, and J. H. Zhao, Fabrication and Characterization of High-Current-Gain 4H-SiC Bipolar Junction Transistors, IEEE Trans. Electron Devices, vol. 55, no. 8, pp. 1899 1906, 2008. [4] M. Domeij, H. Lee, E. Danielsson, C. Zetterling, M. Östling, and A. Schöner, Geometrical effects in high current gain 1100-V 4H-SiC BJTs, IEEE Electron Device Lett., vol. 26, no. 10, pp. 743 745, 2005. [5] B. J. Baliga, Bipolar Transistors, in Power Semiconductor Devices, ch. 5, pp. 198 257, PWS, Boston, MA, 1996. [6] B. Buono, R. Ghandi, M. Domeij, B. G. Malm, C.-M. Zetterling, and M. Östling, Influence of Emitter Width and Emitter Base Distance on the Current Gain in 4H- SiC Power BJTs, IEEE Trans. Electron Devices, vol. 57, no. 10, pp. 2664 2670, 2010. [7] N. G. M. Tao, H. Liu, and C. R. Bolognesi, Surface recombination currents in Type-II NpN InP-GaAsSb- InP self-aligned DHBTs, IEEE Trans. Electron Devices, vol. 52, no. 6, pp. 1061 1066, June 2005.