Optimal drift region for diamond power devices

Abstract

In power devices such as Schottky Barrier Diodes or Field Effect Transistors, the breakdown voltage is linked to the design of the drift layer but also to the physical properties of the material used. Diamond, with its high critical electric field due to its large band gap, opens the way to power components able to withstand very high voltage with outstanding figures of merit. Nevertheless, a particular attention has to be paid to the design of the drift layer to take benefit of these outstanding properties. Indeed, the drift region thickness, doping level and consequently the punch through or non-punch through designs must be well designed to reach the desired breakdown value and to minimize the ON state resistance at the same time. Here, a focus on the optimization of the specific ON state resistance as function of the breakdown voltage figure of merit has been carried out, while optimizing the drift layer and calculating the specific ON state resistance of unipolar high voltage diamond power devices. Based on the ionization integral calculation with impact ionization coefficients adapted to diamond, we performed an accurate analysis to find the best punch through design of the drift layer offering the lowest ON state resistance at a given breakdown voltage value. This theoretical study has been first applied in a one dimensional discrete approach of the breakdown voltage. An additional 2D cylindrical coordinate analysis was performed to quantify the radius effect on the breakdown voltage value, and to compare the 2D breakdown voltage with the 1D breakdown voltage, for different drift region designs. These results offer preliminary design rules to fabricate more efficient unipolar diamond power devices. At the material level, this analysis also points out that thicknesses and doping levels required to achieve such structures are quite challenging for crystal growth in the context of high voltage power devices.

Design of Diamond Power Devices: Application to Schottky Barrier Diodes

“…The performances in the ON-and OFF-states of the aforementioned devices are strongly dependent on the drift region design. There is a trade-off between the ON-state losses represented by the specific ON-state resistance R on S and the BV, i.e., there is only one drift region design (doping and thickness) which minimizes the ON-state losses for a given BV and operating temperature [21] as discussed in the next section. The effect of the Schottky barrier in this trade-off will be also further discussed.…”

“…In Silicon, 95% of the ON-state voltage drop in vertical 600V MOSFETs is due to the sole drift region [22]. In bulk diamond, Figure 2 shows the specific ON-state resistance R on .S of a vertical drift region as a function of breakdown voltage predicted by 1D avalanche model [21,23] and for a Non Punch Through condition (NPT). This specific ON-state resistance is the lowest possible with unipolar vertical diamond power devices having a breakdown voltage between 1kV and 10kV.…”

“…In the case of n-type diamond drift regions, the typical dopants would be Phosphorus or Nitrogen, but both the extremely deep donor levels (0.57 eV and 1.7 eV, respectively) make such options almost unsuitable for power electronics applications. For the prediction of the specific ON-state resistance, up to date models have been used [25]: incomplete Boron ionization (with doping and temperature dependence: Equations (1)-(3) with p the hole concentration), hole mobility in CVD diamond (with doping and temperature dependence: Equations (4)-(6)) and the drift region thickness set as the thinnest possible after reaching NPT condition in OFF-state as predicted by 1D impact ionization integral [21]. As in [25], the physical parameters were set to g h = 0.25, Nv = 2.17 × 10 19 cm −3 , β min = 0, β max = 3.11, Nβ = 4.1 × 10 18 cm −3 , γβ = 0.617, µ min = 0, µ max = 2016 cm 2 /(V•s), Nµ = 3.25 × 10 17 cm −3 , γµ = 0.73.…”

“…As it will be described hereinafter, the breakdown voltage in diamond devices and in diamond SBDs in particular is currently mostly governed by excessive leakage currents and premature breakdown due to poor electric field management. Moreover, the NPT condition does not lead to the minimum specific ON state resistance for each breakdown, since there is an optimal punch through coefficient under PT condition [21]. For all these reasons, four different designs of the drift region will be considered-see Figure 3a,b and Table 3-for a 1D breakdown voltage of 1 kV (as predicted by impact ionization integral based on temperature independent impact ionization coefficients in CVD diamond from [23]).…”

Mobility Models Based on Forward Current-Voltage Characteristics of P-type Pseudo-Vertical Diamond Schottky Barrier Diodes

“…Diamond is a promising semiconductor for power switches due to its wide band gap, high critical field, and high thermal conductivity [1][2][3]. Diamond has a wide band gap of 5.5 eV, which corresponds to the boundary between semiconductors and insulators.…”