Device simulation using Sentaurus Device Sentaurus Device is used to simulate the diode IV and the reverse junction-breakdown characteristics of the SiCSchottky diode. The top contact (anode) is declared as a Schottky contact with a workfunction value of 6.35 eV in the Electrode section: Electrode {... { Name='top_schottky' Schottky Workfunction=6.35 Voltage=0.0 } This value corresponds to the photoelectric workfunction of platinum. An important injection mechanism for Schottky contacts in SiC is tunneling through the steep and thin Schottky barriers. This tunneling mechanism is activated for the electrons at the top Schottky contact with: Physics (Electrode='top_schottky') { Recombination (eBarrierTunneling) } The magnitude of the tunneling current depends on the band-edge profile along the entire path between the points connected by tunneling. This makes tunneling a nonlocal process, that is, computation of the tunneling current at a certain point depends on quantities at other points in the structure. To evaluate the tunneling model, Sentaurus Device constructs internally an auxiliary, so-called nonlocal, mesh. The construction of the nonlocal mesh is controlled with the NonLocal command in the contactspecific or interface-specific Math section, with the option Length. The value of Length gives the maximum tunneling distance in centimeters. Select Length to be at least as large as the thickness of the tunneling barrier. For example, with: Math (Electrode='top_schottky') { NonLocal(Length=1e-5) Digits(NonLocal) = 3 EnergyResolution(NonLocal)=0.001 } nonlocal mesh lines are constructed for vertices up to a distance of 100 nm from the top_schottky electrode. The parameter Digits(Nonlocal) determines the relative accuracy (the number of valid decimal digits) to which the tunneling currents are computed. The EnergyResolution(NonLocal) parameter (given in eV) is a lower limit for the energy step that is used to perform integrations over band-edge energies. Here, the energy resolution is set to 1 meV and the relative accuracy of the tunneling current computation is set to three digits. In large bandgap semiconductors, the intrinsic carrier density is extremely small. For this reason, it is necessary to adjust some of the default settings for the numeric accuracy in the global Math section: Math {... Digits=7 ErrEff(electron)=1e-12 ErrEff(hole)=1e-12 RHSmin=1e-30 RHSmax = 1e30 CdensityMin=1e-30 } The Digits parameter defines the relative error convergence criterion; it approximates the number of digits of accuracy to which an equation must be solved before it is considered to have converged. Due to the small intrinsic carrier density in SiC, it is recommended to increase this accuracy setting from the default of 5 to 7. The parameter ErrEff(electron|hole) defines to which carrier concentration level the accuracy of the solution variable effectively is controlled. The default value of 10 10 cm –3 is inappropriate for SiC. A much tighter accuracy setting of 10 –12 cm –3 is recommended. This value is of the order of the SiC intrinsic carrier concentration. During the Newton iterations, Sentaurus Device will accept a solution as having converged if either the weighted error estimated (‘L 2 norm’) becomes less than 1 or the value of the right-hand side (RHS) becomes smaller than a certain value, given by the parameter RHSmin. By default, this value is 10 –5 . It is recommended to tighten this setting for SiC to 10 –30 . For transient simulations (not used here), the iterations are stopped if the RHS exceeds a certain value, defined by the parameter RHSmax. For SiC, initially large RHS values may occur, so it is recommended to increase this setting from the default value of 10 15 to 10 30 . The parameter CdensityMin controls the minimum current density for which impact ionization is considered. The default value of 3 x 10 –8 A/cm –2 is too large for SiC. The recommended value is 10 –30 A/cm –2 . Further, the very low natural thermal leakage current in (clean) SiC may lead to slow convergence. To improve speed and robustness, it is recommended to increase the leakage current level. Even an increase by 2–3 orders of magnitude does not alter the relevant electrical behavior of SiC devices. 4 Copyright © 2007 Synopsys, Inc. All rights reserved.
Optimum n-drift region of a 4H-SiC Junction Barrier Schottky Diode (JBS) was analyzed by simulation with consideration of the anisotropic impact ionization. According to the detailed simulations using SRIM and Sentaurus, model parameters of empirical equations were obtained through fitting, which showed that the anisotropic avalanche model (2D-ANISO) differs significantly from the 1. Advantages of the 1200 V SiC Schottky Diode with MPS Design Single- and three-phase inverters in solar, UPS or energy storage applications today demand for high efficiency, compact designs and extended reliability. Inverter implementation in these applications is limited by silicon devices’ high dynamic.
For example, the breakdown voltage is not affected by increasing the thermal leakage current. A simple way to increase the leakage current is to apply a small optical generation rate to the whole device with the OptBeam statement: Physics {... OptBeam( WaveInt = 1e12 # [1/cm2/s] SemAbs = 1e0 # [1/cm] SemSurf = -0.4 # [cm] SemWind = ( 0.0 90e-4 ) # [cm] ) } Here, a peak optical generation rate of 10 12 electron–hole pairs per cm 2 per second is defined. The absorption depth is defined as 1 cm. The parameter SemSurf defines at which position the peak optical generation is applied. If the value lies outside of the device, such as in the example given here, the nearest surface is used. The lateral extent of the light beam is controlled by specifying a window. Here, the window extends over the whole structure. To improve the accuracy of the integration procedure used to evaluate the optical pair generation rate for coarse meshes, it is recommended to activate the box integration method when using OptBeam with: Math {... RecBoxIntegr } The Okuto–Crowell model for the avalanche generation is used here. This model is activated with: Physics {... Recombination (... Avalanche (OkutoCrowell Eparallel) ) } The parameter file sdevice.par of Sentaurus Device provides calibrated Okuto–Crowell parameters for SiC devices with high breakdown voltages for near roomtemperature operations. The robust numerics of Sentaurus Device allow users to perform breakdown simulations by attaching a large resistor to one of the electrodes and, then, sweeping the bias of this electrode to very large values. After the onset of impact ionization, most of the voltage drop occurs across this resistor if the value chosen is sufficiently large. Therefore, this technique achieves an automatic switching from a voltage-controlled prebreakdown regime to a currentcontrolled postbreakdown regime. The breakdown IV characteristics can be seen by plotting the terminal current versus the inner contact voltage instead of the outer. The appropriate value for the attached resistor is of the order of R = V/I at the onset of impact ionization. This technique is used here for the simulations of reverse-bias diode breakdown. The values of the external resistor are defined in the Electrode section: Electrode { {Name='top_schottky' Schottky Workfunction=6.35 Voltage=0.0 Resist = 1e10 } } Here, a value of 10 10 Ωμm is used. Figure 2 shows the forward diode characteristics and Figure 3 shows the reverse breakdown characteristics of the Schottky diodes with and without guard rings. Current [A/μm] Figure 2 Anode Current [A/μm] Figure 3 without guard rings with guard rings 1.5x10 -03 1.0x10 -03 5.0x10 -04 0.0x10 +00 0 2 4 6 8 10 Voltage [V] Forward diode characteristics: anode current as function of anode voltage for SiCSchottky diode with (red) and without (blue) guard rings simulated with Sentaurus Device 10 -10 10 -11 without guard rings with guard rings 10 -12 10 -13 10 -14 10 -15 10 -16 10 -17 -1200 -1000 -800 -600 -400 -200 0 Anode Voltage [V] Reverse breakdown characteristics: anode current as function of anode voltage for SiCSchottky diode with (red) and without (blue) guard rings simulated with Sentaurus Device Copyright © 2007 Synopsys, Inc. All rights reserved. 5
- Page 1 and 2: Sentaurus Technology Template: SiC
- Page 3: Introduction The purpose of this te