The Forward and Reverse Characteristics of Silicon Carbide Diodes at High Temperatures

P. Rabkin (Silvaco), R. Cottle (Silvaco), Professor M. Shur (University of Virginia), and P. Blakey (Silvaco)

 

The thermal and electronic properties of silicon carbide lead to very high semiconductor figures of merit for high-power, high-speed, high-temperature, high-frequency and radiation hard applications. This has caused growing interest in semiconductor devices based on SiC material systems. A recent review of the reasons for the interest in SiC technologies, and the increasing range of research and development activities in the area, was given in [1].

Simulations of SiC structures were performed previously using the 2D numerical simulator Blaze [2]. The subject of the investigation was a hetero-JFET with a n-type beta-SiC channel and p-type alpha-SiC substrate. The DC characteristics at room temperature were simulated, and shown to be in good agreement with experimental data. Simulation was then used to estimate the high frequency characteristics of the device.

Blaze has subsequently been extended in ways that improve its ability to simulate general devices based on wide band gap semiconductors. An example of the capabilities of the most recent version of Blaze is presented here.

The Present Work
Blaze was used to calculate the forward, reverse, and reverse breakdown characteristics of a SiC diode at elevated temperatures. The study was motivated by a recent paper [3] that described the simulation of an alpha-SiC diode that has been well characterized experimentally [4]. The results that were obtained using Blaze, and which are presented here, show that Blaze produces results that are in good agreement with experiment. Blaze can thus provide state-of-the-art SiC device simulation capabilities to the entire SiC device community.

The methodology used in the present work was to first determine mobility and recombination parameters by fitting calculated results to experimental [4] and other calculated forward characteristics [3]; and to then simulate the reverse characteristics and breakdown behavior of the diode, using the same mobility and recombination parameters. Simulated reverse breakdown characteristics were compared with experimental data from [4] and simulated results from [3].

Reverse breakdown calculations were performed for a range of different 'critical fields' Ec, where the ionization rate is assumed to have the form A.exp(-Ec/E), and E is the electric field. The experimental breakdown voltage is around 700 V [4]. This allows bracketing of the critical field values associated with impact ionization in the 6H(alpha)-SiC polytype, and gives insight into the impact ionization parameters to be used when simulating other SiC devices of the same polytype.

 

Calculated Results
The p-n-n- diode structure reported in [4] and simulated in [3] has a 1µm thick p-emitter doped at 1018cm, a 6µm thick n-base region doped at 2.3 10cm, and and a 330µm thick n+- emitter doped at 4.5 10cm. The material parameters used in the present work follow those used in [3] except for small differences in mobilities and their temperature dependencies, doping dependencies of carrier lifetimes, and the effective mass ratio for electrons and holes. The electron and hole capture times were assumed to be 10 ns. Because of the high energy band gap of about 3eV, and its comparatively strong temperature dependence, energy band gap narrowing due to the temperature and doping becomes very important, and must be accounted for properly. The simulations performed here used the band gap narrowing parameters suggested in [3].

Simulated forward characteristics for room temperature operation (300 K) and at an elevated temperature (623 K) are presented in Figure 1. These results are in a good agreement with measured and calculated data reported in [3] for a wide range of current densities.

 

Figure 1. SiC diode forward characteristics for
room (300K) and elevated (632 K) temperatures.

 

Simulation of the reverse characteristics and breakdown behavior of SiC devices is complicated by a very low intrinsic concentration ( ni ~ 1.6 10 cm at 300 K). The reverse current of a p-n junction is made up of two components: conventional diffusion current; and generation current in the depletion region. The first component is proportional to ni , and the second component is proportional to ni. In turn, ni ~ exp(-Eg/2kT), where Eg is energy band gap, and k is Boltzmann's constant. (There are other temperature dependencies involved, such as mobilities and lifetimes, but the exponential dependence of ni is the most significant.) The thermal generation current dominates up to temperatures of about 1000 K, when the components become comparable with a magnitude of about 10 A/cm [3]. In 6H-SiC at 300 K the generation current density is about 10A/cm. This makes simulation of the reverse characteristics and breakdown difficult at temperatures around 300 K and below, since the precision of the calculation of carrier concentrations and currents has to be significantly increased.

Figure 2 shows the calculated reverse characteristics and breakdown behavior for the diode at an elevated temperature of 623 K. The calculations were performed for a range of values of critical fields. The experimental breakdown voltage of about 700V is also indicated. Suggested values of critical fields vary from publication to publication with a factor of around two between minimum and maximum suggested values [1]. In this work reverse characteristics were calculated with values of Ec between ~1.2 10V/cm (the value suggested in [3]) and 2 10V/cm. The values which gave the best agreement with measured data were 1.8 10V/cm for electrons and holes. The lower fitting value used in [3] may be due to an inaccurate discretization of impact ionization (see the article 'Improved Simulation of Breakdown' elsewhere in this issue).

 

Figure 2. SiC diode reverse characteristics and
breakdown for different critical field
values for electrons and holes.

 

Additional Capabilities
Blaze is part of the ATLAS system for 2D device simulation. The other components of the framework can be used to provide added functionality. Two other ATLAS products of interest to the SiC device community are Giga and MixedMode. Giga works in conjunction with Blaze to account for heat flow, lattice heating, and realistic heat sinks. MixedMode is a SPICE-like circuit simulator that supports the use of numerical physically-based devices simulated using ATLAS as well as compact device models. The combination of Blaze, Giga and MixedMode can predict the performance of circuits that include SiC devices, while taking into account heat flow within the device.

Conclusions
Blaze
provides robust and reliable simulation capabilities for semiconductor devices based on wide bandgap semiconductors. Calculated results demonstrate the ability of Blaze to simulate the forward, reverse, and breakdown characteristics of a SiC diode at elevated temperatures. The combination of Blaze, Giga, and MixedMode provides exceptionally versatile simulation capabilities to the SiC device research and development community.

References

[1]. R. Davis, G. Kelner, M. Shur, J. Palmour, and J. Edmond.
"Thin Film Deposition and Microelectronic and Optoelectronic Device Fabrication and Characterization in Monocrystalline Alpha and Beta Silicon Carbide,"
Proceedings of the IEEE, vol. 79, No.5, pp. 677-701, 1991.

[2]. P. Rabkin, M.Shur, and P. Blakey.
"DC and AC Analysis of a Silicon Carbide HJFET,"
Simulation Standard, vol. 3, No. 5, pp. 2-3, 1992.

[3]. R. Helbig.
"Progress in New Materials for Power Electronics: SiC",
Proceedings of 5th International Symposium on Power Semiconductor Devices and ICs,
Monterey, CA, 1993.

[4]. J.W. Palmour, J.A. Edmond, H.S Kong, and C.H. Carter,
Proceedings of the 4th International Conference on Amorphous and Crystalline SiC,
Santa Clara, CA, 1991.