3D Simulation of Power Devices Using
Giga3D and MixedMode3D

Introduction

Recent additions to the ATLAS device simulation framework have added the ability to simulate 3D electrothermal effects in Giga3D and mixed circuit simulation with 3D device simulation in MixedMode3D. The new modules add to the existing 3D device simulation within ATLAS as shown in Figure 1. They show the migration of existing 2D device simulation models and techniques to a full 3D approach. ATLAS currently supports 3D simulation of most common device technologies:

Device3D - MOS, Bipolar, EEPROMs

Blaze3D - MESFETs, HEMTs, HBTs

Giga3D - power devices, SOI, ESD effects

MixedMode3D - power devices embedded in circuitry , ESD effects, latchup

TFT3D - thin film transistors

Thermal3D -thermal effects in packaging

Quantum3D - quantum moments solver

The new modules Giga3D and MixedMode3D can be used with both Device3D and Blaze3D to model silicon and non-silicon technologies respectively.

Figure 1. 3D device simulation within ATLAS.

2D Versus 3D Simulation

For many devices 2D ATLAS simulation has been sufficient to describe the device behavior to the limit of simulation accuracy. Using 3D simulations will always be at least an order of magnitude slower than 2D and have more limitations on the accuracy of the initial structure definition.

However in many technologies the device physics requires a 3D approach to simulation. Examples include substrate contacts in SOI, current crowding in power device structures, width effects in FETs. In addition for Giga3D, current filimentation is essentially a cylindrically symmetric event for which full 3D electrothermal device simulation is required.

Defining Devices for Giga3D
Accuracy in definition the initial structure for 3D device simulations can often be the limiting factor in overall simulation reliability. ATLAS accepts 3D structures defined either using the ATLAS syntax or from DevEdit3D. The familiar ATLAS syntax has been extended to support objects in the z-direction. This includes the THERMCONTACT definition required in Giga3D.

DevEdit3D can be used as an interactive or batch mode tool to pre-process a 3D device structure. It accepts input from ATHENA, SSuprem3 or ASCII doping profiles. It also allows arbitrary device structures including structures with circular masks.

Defining a Netlist for MixedMode3D
The SPICE-like netlist used in MixedMode3D is defined between .BEGIN and .END statements in an ATLAS input file. Circuit simulation primitives are defined in the standard SPICE manner: D for diodes, Q for bipolars, L for inductors. Circuit elements to be simulated by device simulation are given the identifier 'A'. A typical netlist might include:

AIGBT 1=gate 2=emitter 4=collector inf=myigbt.str

Node numbers from the circuit are paired with electrode names from the ATLAS device on the 'A' line.

Power Device Simulation Example

Currently power device simulation engineers have been applying Silvaco's MixedMode simulator to good effect. By coupling together a spice circuit with 2D device simulations of the power device, much information about device performance has been obtained. However, this has neglected three important aspects of the physical device operation

• current filamentation occurs in a localized 3 region
• heat generation is 3D
• 3D boundary conditions

Figure 2 illustrates a GTO thyristor device which may exhibit all three of these problems. The GTO device has been designed with a forward blocking voltage of 3500V. To model the operation of this device in practice we have embedded the GTO thyristor into a circuit shown in Figure 4. MixedMode3D simulation of this circuit produces the result shown in Figure 5. This circuit response can be explained as follows. The GTO thyristor is initially driven into the ON state by increasing the gate current to 2.1A and the supply voltage to 3000V. The closure of a switch is then modeled by forcing the resistor R3 to change from 1Mohm to 1mohm in only 100ns. This results in the gate turning off the device and the anode current falling to zero. However, the high negative dI/dt in the load circuit results in a large positive voltage in the load inductor L1. This produces an over-voltage on the anode contact of the GTO and causes the voltage to exceed 3000V. The over-voltage may cause the device to exceed its forward blocking voltage, causing impact ionization to occur, and as a result the turn-off of the GTO will be affected. Accurate Mixedmode3D simulation is vital in order to properly characterize these effects.

Figure 2. GTO thyristor geometry. This device can only be simulated correctly in 3D.

Figure 3. Isosurfaces of temperature in a power diode with current crowding into the anode.

Figure 4. Circuit schematic for a GTO thyristor. The GTO element is simulated using 3D device simulation.

Figure 5. Currents in the GTO thyristor during turn-off through external circuit.

Conclusion

The addition of Giga3D and MixedMode3D to ATLAS allows users to perform full 3D simulations of electrothermal device behavior. For power device applications the combination with MixedMode3D is particularly powerful to model the switching of devices within a circuit environment.