Designing a High-Voltage IGBT Structure with TCAD

 

Introduction

The Insulated Gate Bipolar Transistor (IGBT), is a power device that combines the high-power characteristics of bipolar transistors with the fast-switching and voltage-drive characteristics of MOSFETs. It is used in a wide variety of applications, ranging from electric vehicles, industrial applications such as general-purpose inverters and switching power supplies, to consumer applications such as air conditioners and washing machines, making it a key device of the modern power electronics. From the viewpoints of lowering the loss and enhancing the efficiency of the application devices to which it is incorporated, the structure of the IGBT is being continuously improved. To illustrate the effectiveness of TCAD simulations in designing a power device structure, high-voltage structures of planar type and trench type, which are typical IGBT cell structure types, were designed using ATLAS 2D device simulator and then compared to each other.

 

Simulations

The following characteristics are needed for designing an IGBT structure.

  1. Breakdown characteristics
  2. IC-VG characteristics
  3. IC-VC characteristics
  4. Switching characteristics

To compare different types of cell structures like planar and trench, the structures should have a similar breakdown voltage and a similar threshold voltage. In other words, it is necessary to adjust the structures so that they have approximately the same breakdown voltage and threshold voltage. This corresponds to the simulations of above 1. and 2. From the simulation results of 3. and 4., the collector-to-emitter saturation voltage VCE(sat) and the fall time Tf are obtained. The tradeoff between VCE(sat) and Tf reveals a superior IGBT structure.

 

Cell Structures

Figure 1 shows the planar (left) and trench (right) cell structures. A typical set of major design rules were assumed for both structures: a cell pitch of 34μm and a poly-silicon gate width of 28μm for the planar structure, and a cell pitch of 4μm and a trench gate depth of 3μm for the trench structure. The substrate structure other than the surface neighborhood, that is, density and thickness of drift layer, buffer layer, and P-substrate, was assumed to be common so that the both structures have an active area of 1mm2, a total thickness of 400μm, a base depth of 2.5μm, a gate oxide thickness of 1000Å, and an electron and hole life time of 1e-6s.

Figure 1. Planar and trench cell structures.

 

Simulation Results

1. Breakdown Characteristics
The target breakdown voltage in this design was assumed to be 900V or more. The required resistivity of the drift region, or epitaxial layer, to achieve the target was assumed to be 45Ω · cm. The breakdown characteristics were simulated using the thickness as a parameter. Figure 2 shows the result. It shows that both the planar and trench structures have 900V or more breakdown voltage, which is also aproximately the same value. The substrate structure of this simulation (of the trench type) is illustrated in Figure 3. It can be said that it is reasonable that both cell structures have aproximately the same breakdown voltage since they share a common substrate structure except for the surface neighborhood.

Figure 2. Breakdown voltage simulation.

 

Figure 3. Substrate structure (trench type).

 

 

2. IC-VG Characteristics
Both the planar and trench structures were adjusted to have a target threshold voltage (Vth) of about 3.5V at IC =1mA, and IC-VG characteristics were simulated. It was assumed that VCE is 10V, and a base surface concentration was used as a parameter for adjusting the threshold voltage. Figure 4 shows the result. The waveforms show that, for both structures, the threshold voltage is adjusted to the targeted value, which is also aproximately the same value. The structures are determined only after the conditions for the threshold voltage and breakdown voltage are both met. Otherwise, the simulations of 1. and 2. will be repeated until the conditions are satisfied.

Figure 4. IC-VC simulation.

 

3. IC-VC Characteristics
To calculate the saturation voltage of the planar and trench structures, the IC-VC characteristics were simulated at VGE=15V. Figure 5 shows the result. The waveforms show that the trench structure has a lower saturation voltage. For example, the saturation voltage at IC=10A is 3.15V for the planar structure and 2.35V for the trench structure.

Figure 5. IC-VC simulation.

 

4. Switching Characteristics
To calculate the fall time, a mixed-mode simulation of the switching characteristics was performed. The measurement circuit is shown in Figure 6. The fall time at peak current ICP=2.5A was measured under the major measurement conditions of VCE = 300V, L = 1.5mH, and the device temperature of 125°C. A compact model was used for Di. Figure 7 shows the switching waveforms of the collector current obtained by the simulation. The fall time, defined as the time difference (the time at ICP × 0.1 minus the time at ICP × 0.9), was 510ns for the planar structure and 470ns for the trench structure, which are relatively close. Figure 8 shows the hole density distribution inside the device corresponding to points indicated by [A], [B], and [C] in Figure 7 for the trench cell structure. It shows that the number of holes decreases with time and as the collector current decreases, indicating a very close relationship between the hole density and tail current in the buffer layer.

Figure 6. Switching measurement circuit.

 

Figure 7. Switching waveforms.

 

Figure 8. Hole density distribution (trench type).

 

5. Performance Comparison
Figure 9 shows the correlation of the tradeoff between the fall time and saturation voltage obtained by simulations 3. and 4., where the life times are 6e-7s, 1e-6s, and 2e-6s. The relationships similar to those of the actual devices were reproduced; the shorter the fall time the higher the saturation voltage, and the longer the fall time the lower the saturation voltage.

Figure 9. Tradeoff correlation (performance index).

 

Conclusion

From the performance comparison about, it can be concluded that the trench type is the superior cell structure for the IGBT. The above simulation results from ATLAS proved that TCAD is highly effective for designing an IGBT structure. In this article, cell structures with different gate structures, planar and trench, were compared in terms of the design and performance, but any cell structures with different substrate structures, such as punch-through and non-punch-through types, can be compared in a similar manner.

 

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