Ti/4H-SiC Schottky diode with breakdown voltage up to 3 kV

In this study the breakdown voltage for Ti/4H-SiC type Schottky diode with six guard rings and JTE layer has been calculated by mean of numerical simulations. It is established that increase of the n-type 4H-SiC epitaxial layer from 14 up to 20 μm and addition of JTE layer lead to increase of breakdown voltage value on ∼900 V in contrast to the same diode without JTE. The above-mentioned diode’s structure gives the possibility for designing and production of diode with higher breakdown voltage value up to 3 kV.


Introduction
It is known that semiconductor material silicon carbide (SiC) is promising for the development of power electronics, microelectronics and optoelectronics devices. This is due to the large band gap of the semiconductor SiC (>3 eV), high thermal conductivity, high breakdown fields and the rate of electron saturation, as well as significant radiation and thermal stability [1]. One of the simplest devices based on SiC, but at the same time important for microelectronics, is Schottky diode. For example, Schottky diodes for power electronics based on 4H-SiC have already been manufactured by the domestic industry, in particular, by the "GRUPPA KREMNY EL" domestic company (Bryansk). It is obvious that for the further development of the domestic component base based on SiC it is necessary to study in detail the influence of the diode structure parameters on its current-voltage characteristics to optimize the Schottky diode operation in power electronics, which can be done using physical simulation [2]. Earlier, in our previous paper [3][4][5] have been studied of 4H-SiC type Schottky diodes with Ni and Ti Schottky anode contacts without guard rings. Therefore, the main goal of this study was to design a perspective SiC Schottky diode with guard rings using physical simulation methods in the ATLAS software program and to study its electrical characteristics.

Materials and methods
The following Schottky diode with six guard rings and donor concentration in the epitaxial layer 4H-SiC N D = 3×10 15 сm −3 was taken as the object of simulation in program.
As can be seen earlier, to calculate the breakdown voltage (BV m ) of the Schottky diode in a planeparallel p-n junction, we first determined the critical breakdown field strength (E c ) from the condition of equality to the unit of the ionization integral [6]: where α n and α p -the ionization coefficients of electrons and holes, d -the epitaxial layer thickness.
In 4H-SiC, the ionization coefficients of the exponential dependence on the reverse field:  [7]. By substituting expressions (2) and (3) where e s -4H-SiC dielectric permittivity, N D -the donor concentration in the 4H-SiC epitaxial layer, q -the elementary charge.
Then, after the numerical solution of equation (4) and determination of the critical breakdown field strength, the breakdown voltage can be estimated by the following formula [6,7] 2 EW c BV m = where E c -the critical breakdown field strength, determined from the integral equation (4), W -the thickness of the space charge area (SCA). Further, in the approximation, it can be assumed that the breakdown occurs at a time when the thickness of the SCA is equal to the thickness of the epitaxial layer d [7]. A calculated value of the critical breakdown field strength E c and the breakdown voltage of BV m with donor concentration N D = 3×10 15 сm −3 and different values of the thickness d of the 4H-SiC epitaxial layer are shown in Table 1. As it can be seen from Table 1 breakdown voltage of more than 2 kV is provided at a layer thickness of 18 µm. With this in mind, the breakdown voltage was simulated for the 4H-SiC epitaxial layer thickness of 18, 20 and 22 µm. The Schottky diode with six guard rings was taken as the object of modeling. In Fig. 1a shown the simulated schematic silicone carbide Schottky diode structure for calculation. For numerical simulation were chosen the following the Schottky diode parameters: the  been carried out in the ATLAS program, with taking into account close to reality situation the incomplete impact ionization and anisotropy in the direction (0001) by the Hummel-Newton method. Fig. 1b shows the reverse I-V characteristics of Schottky diode with thickness of the epitaxial layer (h) and distance between rings (l) calculated in ATLAS for diode with JTE (h=20 mm, l=2,5 mm). As follows from Fig. 1b the diode breakdown starts at 2,87 kV i.e. breakdown voltage value equals ~3 kV. For comparison in Fig. 1b also presented I-V characteristics of Ti/4H-SiC Schottky diodes with reduced thickness of the epitaxial layer (h=14 mm) and increased distance between guard rings (l=5 mm) and without JTE layer. In this case the value of breakdown voltage value equals 1,94 kV.