Physics/2. Solid state physics.

V.V.Fedina, postgraduate FE chair; I.V. Fedin, postgraduate FE chair

Tomsk State University of Control Systems and Radio-electronics, 634050, Russia, Tomsk, Lenina ave., 40

Simulation of GaN HEMT in Silvaco TCAD

Introduction

Wide band gap semiconductors such as SiC and GaN have attracted much attention of design engineers because of high material quality.

For now, AlGaN/GaN High Electron Mobility Transistors (HEMTs) can replace outdated silicon devices due to their high breakdown strength and the high current density in the transistor channel giving a low on-resistance [1-2]. The normally-off GaN high-mobility 2DEG transistor is required for applications in power electronics with threshold voltage >1 V. At the present time a process for preparing normally-off GaN transistors is use gate area based on p-GaN layer. This method is the most promising of all. The aim of this study is simulation enhancement mode GaN HEMT in Silvaco TCAD. The simulation is consist of depending’s computation. This study present depending’s of threshold voltage (U_th) and saturation current (I_D-S) from thickness and aluminum mole fraction of barrier layer AlxGa1-xN.

Experimental part

Fig. 1 shows a cross-section of the proposed normally-off pGaN/AlGaN/GaN HEMT. It consists of 2 µm Fe doped GaN buffer layer, 35 nn undoped GaN channel, Al_0.25Ga_0.75N barrier layer and 60 nm Mg high doped p-GaN cap layer. 140 nm thick nickel was used for gate contacts. The final structure was passivated by silicon nitride (SiN). The gate to drain space (L_D-G) is 1 μm, drain to source (L_D-S) is 8.4 μm, gate length is 1 μm and gate width is 100 μm.

Fig. 1. Schematic image of p-GaN/AlGaN/GaN transistor

Fig. 2 shows band diagram of gate area.

Fig. 2. Band diagram of modelling transistor

Fig. 2 shows that double heterojunction is forming in p-Gate region between pGaN/AlGaN and AlGaN/GaN. This is proved true by work [1].

Fig.3 shows simulated transfer characteristic and real transfer characteristic of p-GaN/AlGaN/GaN transistors. Real HEMT was prepared with the same periphery and aluminum mole fraction as the model of transistors.

The threshold voltage (U_th) of modeled normally-off transistor is 2.6 V. The threshold voltage (U_th) of real normally-off transistor is 1.9 V. It can be, that cause of discrepancy is in simplification of the model which consists in absence of real Schottky barrier between p-GaN and gate metal in this model.

Fig. 3. Simulated transfer characteristic (a) and real transfer characteristic (b) of p-GaN/AlGaN/GaN transistors

Also in this study modeling dependence of threshold voltage (U_th) and saturation current (I_D-S) from aluminum mole fraction of barrier layer Al_xGa_1-xN was performed. The thickness of p-GaN layer has been fixed on 60 nm.

Fig.4. The dependence of the threshold voltage (U_th) and saturation current (I_D-S) from aluminum mole fraction of barrier layer AlxGa1-xN

Fig.5 (a) shows that increasing of aluminum mole fraction from 0.1 to 0.3 leads to decreasing of threshold voltage from 3.3 to 2 V. Fig.5 (b) shows that increasing of aluminum mole fraction 0.1 to 0.3 leads to increasing saturation current (ID-S) from 3 to 60 mA. Results are conform with work [2].

Also in this study modeling dependence of threshold voltage (U_th) and saturation current (I_D-S) from barrier layer thickness (d) was performed. Mole fraction of aluminum in AlGaN was fixed on x(Al)=0.25. Fig 5 (a) shows that increasing of barrier layer thickness from 5 to 25 µm leads to decreasing of threshold voltage from 3,2 to 1 V. Fig 5 (b) shows, that increasing of barrier layer thickness from 5 to 15 µm leads to increasing saturation current (ID-S) from 26 to 46 mA.

Fig. 5. The dependence of threshold voltage (U_th) and saturation current (I_D-S) from AlGaN barrier layer thickness

Further increasing of AlGaN leads to decreasing saturation current. This data agreement with work [1].

Conclusion

This work present results of normally-off pGaN/AlGaN/GaN-based HEMT Silvaco TCAD simulation.

From the results it is concluded that:

1) Double heterojunction is formed in p-Gate region between pGaN/AlGaN and AlGaN/GaN;

2) Increasing of mole fraction of aluminum in AlGaN from 0.1 to 0.3 leads to decreasing of threshold voltage from 3.3 to 2 V and increasing of saturation current from 3 to 60 mA.

3) Increasing of barrier layer thickness from 5 to 25 µm leads to decreasing of threshold voltage from 3.2 to 1 V and increasing drain-source saturation current up to 46 mA;

4) Modeling data agreed with different literary source.

REFERENCES

1. Taube A. Modelowanie normalnie wyłączonych tranzystorów HEMT AlGaN/GaN z bramką p-GaN // XII Krajova Konferencja Elektroniki. 2013.

2. Madhurima V. Characteristics of AlGaN/GaN HEMT with PType GaN Gate and AlGaN Buffer // International Journal of Innovative Research in Computer and Communication Engineering. Vol. 1, Issue 10, December 2013. P. 2358-2362.