3D Simulation of Heterostructure Devices

Introduction

The three-dimensional silicon and GaAs device simulator Device3D has now been extended to allow simulation of heterostructure devices within the ATLAS framework. The new product Blaze3D includes the modeling of graded and abrupt heterojunction barriers which is critical to the simulation of important classes of devices such as HBTs and HEMTs.Two typical examples of the application of Blaze3D are presented here. Firstly, the collector avalanche breakdown of a SiGe base HBT is simulated. Secondly, a conventional GaAs-AlGaAs HEMT with a resistive T-gate is simulated in transient mode. The progression of a negative gate voltage pulse along the length of the gate is accompanied by the progressive turn-off of the channel conduction through the 2DEG in the GaAs.

 

Creation of Virtual Device Structures

3-dimensional structures may be created using ATLAS command syntax or more easily by using DevEdit3D, the 3D extension to the powerful 'Devedit' device design and meshing package. DevEdit3D allows simulated or analytic doping profiles to be incorporated into device structures drawn on the screen and provides optimal 3D prismatic mesh generation based on user-defined constraints. The ability to automatically generate a conformal non-uniform mesh allows arbitrary device geometries to be studied. Concentration of mesh in active device regions, with relaxed mesh density elsewhere, helps minimize simulation times. A wide range of compound semiconductors are supported and layer composition fractions are easily defined.

Figure 1 shows a TonyPlot3D image of a 3D Si:SiGe HBT created using DevEdit3D and based on a device structure reported in[1]. The active device region is revealed by selectively removing a region of isolation oxide from the view. Concentration of the mesh in the confined SiGe base region may be noted.

 

 

Figure 1. 3D super self aligned SiGe HBT structure
created and meshed using DevEdit3D. Emitter and base
contacts are polysilicon. A section of oxide isolation is removed
from the view to reveal the confined SiGe base region with denser mesh.

 

A 3D HEMT structure (Figure 2) has the 10µm gate stripe extending along the z-axis with the channel conduction along the x-axis between source and drain. Because metals are treated as perfect conductors in ATLAS, the resistive gate is modeled by representing the gate metal as polysilicon with an appropriate resistivity and electron affinity.

 

Figure 2. 3D AlGaAs-GaAs HEMT structure with resistive
T-gate and end gate contact, from DevEdit3D.

 

 

Selection of Models

All the mobility models available in Blaze are incorporated in Blaze3D: concentration, transverse field and parallel field dependence (negative differential mobility or simple velocity saturation). Blaze3D also supports multiple recombination mechanisms (Auger, radiative and concentration dependent Schockley Read Hall) and band gap narrowing for simulation of bipolar transistors. Single event upset may be modeled as in Device3D and impact ionization may also be modeled to investigate breakdown limitations. Blaze3D carries the same comprehensive materials library as Blaze. This includes parameter defaults for more than 40 compound semiconductors. Arbitrary user-defined materials and properties are also supported. The SiGe HBT was simulated using the AUGER, CONSRH, FLDMOB, CONMOB, BGN and IMPACT models and biased into avalanche breakdown in the collector region under zero gate bias. For the simulation of the 3D HEMT, FLDMOB and CONMOB mobility models were invoked and single carrier mode was selected to reduce run time. The band offsets were set by defining the electron affinity of each layer. A -0.6V, 2pS gate voltage transient was applied to one end of the gate and the 2DEG channel observed to progressively turn off with time.

 

Simulation Results

Results of device simulation are generally in the form of terminal current-voltage characteristics and 3D structure files. The latter may be viewed and analyzed using TonyPlot3D which provides advanced imaging and dissection capabilities. Taking the example of the HBT breakdown, the region of avalanche multiplication may be imaged as isosurfaces of impact generation rate (Figure 3). A 2D cut plane exported to Tonyplot (Figure 4a) gives a different perspective and a 1D cut line allows a graph to be drawn (Figure 4c). Figure 4b shows the breakdown characteristic. It may be observed in these plots that the avalanche breakdown in this HBT occurs primarily in the n-SiGe collector 'extension' region. The purpose of this layer is to avoid a potential barrier at the SiGe base -Si collector interface [1].

 

Figure 3. Isosurfaces of impact ionization rate reveal that the
most intense avalanche multiplication is along the centre of the
emitter stripe and in the n-doped SiGe collector region.

 

Figure 4. (a) cut plane through 3D HBT structure at onset of avalanche
breakdown. Note the concentration of impact ionization in the centre of
the n-SiGe collector extension region. (b) HBT collector breakdown
characteristic. (c) cut line through 2D section showing graph of impact
ionization rate with depth under poly emitter stripe.

 

The response of the T-gated HEMT to a gate bias transient can be observed in Figure 5 which shows a 2D cut plane taken along the principal axis of the T-gate. The contours in the gate region represent the voltage which is clearly varying under the dynamic biasing as the gate is forced more negative (on the right side). The contours in the GaAs show the local electron channel current in the 2DEG and AlGaAs is consistent with the gate potential, being lower on the right hand side where the gate potential is more negative.

 

Figure 5: 2D cut plane taken from a Blaze3D solution for the 3D HEMT
during a negative gate bias transient. The section is along the major
axis of the resistive T-gate and shows the potential gradient along its length.
The channel conduction (particularly the parasitic conduction
in the AlGaAs) is consistent with the gate potential profile.

 

Conclusion

Blaze3D extends ATLAS simulation capability to include arbitrary three-dimensional heterostructure devices. Combined with DevEdit3D and TonyPlot3D, accurate structure definition and results analysis are possible for 3D device technology development. These features are demonstrated for typical examples of HBT and HEMT.

 

Reference

[1] A. Pruijboom et. al. IEDM 95-747 (1995).