Normally-off AlGaN/GaN HFET with p-type GaN Gate and AlGaN Buffer


1. Introduction

To break through the material limits of Silicon and to realize the drastic performance improvement needed to meet the severe requirements in the future, wide bandgap semiconductors such as SiC and GaN have attracted much attention. AlGaN/GaN HEMTs are generally promising candidates for switching power transistors due to their high breakdown strength and the high current density in the transistor channel giving a low on-state resistance. Further, there exists a high requirement for simulation tools to accurately predict device performance prior to fabrication because of the high inherent cost of the cut-and-try method. Additionally, there are strong polarization fields in the AlGaN/GaN material system (spontaneous and piezoelectric polarization). Failure to include this strong polarization field will introduce distortion to the calculated band diagrams and thus compromise simulation results. Those simulations helped in the understanding of basic physics in GaN HEMTs. Thus we have decided here to simulate and compare to experimental data a normally-off AlGaN/GaN HEMT with a p-type gate based on [1]. Indeed The inherent normally-on behavior of AlGaN/GaN HEMTs would exclude them from most power-electronic applications. The p-GaN gate transistors presented here combine the high-mobility 2DEG transistor channel with secure normally-off operation, as is required for applications in power electronics. However, the required Vth > +1 V is often achieved by a low Al-concentration in the AlGaN barrier, giving a reduced electron density in the 2DEG of the transistor channel and compromising RON. Here, a low Al-concentration AlGaN buffer beneath the GaN channel is introduced to gain both a high electron concentration in the 2DEG and a high Vth.


2. Models

GaN based devices exhibit piezoelectric as well as spontaneous polarization. Gradients of polarization charges lead to charge accumulation at hetero-interfaces and strongly induced localized two-dimensional electron gas (2DEG). Polarization modeling is thus critical for GaN based devices. ATLAS provides three different polarization models for GaN and the related nitrides. These three models represent a historical progression more than anything else and as you would expect the later models supercede the prior models with more functionality. The latest model TEN.POLAR calculates the piezoelectric and spontaneous polarization but also includes contributions by external mechanical strain and axial strain due to lattice mismatch.

ATLAS uses specific physical models and material parameters to take into account the mole fraction of the AlgaN/GaN system. We choose to model low field mobility using the ALBRCT model allowing the control of electrons and holes separately thus taking into account the fact that the gate is p-type. This mobility model is also a function of lattice temperature. We have selected a nitride specific high field dependent mobility model. This model is based on a fit to Monte Carlo data for bulk nitride and is set by adding GANSET.N (for electrons) in the model statement. In some cases, lattice heating may be important. This typically occurs at high current operation, just like the case of power devices. The lattice heating model should be used to simulate the heat-flow in the device and reproduce negative differential resistance. To enable heat flow simulation, the LAT.TEMP parameter is set on the MODEL statement.

Semiconductor materials exhibit some defects. The presence of these defects, or traps, in semiconductor substrates may significantly influence the electrical characteristics of the device. Trap centers, whose associated energy lies in a forbidden gap, exchange charge with the conduction and valence bands through the emission and capture of electrons. The trap centers influence the density of space charge in semiconductor bulk and the recombination statistics. Device physics has established the existence of three different mechanisms which add to the space charge term in Poissons’s equation in addition to the ionized donor and acceptor impurities. These are interface fixed charge, interface trap states, and bulk trap states. We will use bulk trap states during our simulation.


3. Simulation

The structure was created using ATLAS syntax. Mesh was optimized in order to get an accurate and quick simulation paying special attention at the interfaces where induced charge from polarization are present.

The structure as well as the polarization charges automatically calculated from the polarization model are shown in Figure 1.

Figure 1. GanFET structure and corresponding polarization charges.

While a Schottky-type metal on the AlGaN barrier acts as gate for normally-on HEMTs, a p-type doped semiconductor as gate is able to deplete the transistor channel when unbiased, thus yielding a normally-off device, as can be seen in Figure 2 where the electron quasi fermi level lies below the conduction band.

Figure 2. IdVg and IgVg Characteristics.

The simulation results of the IdVg and IgVg characteristics are shown in Figure 3. The Threshold Voltage is around 1.25V. The sub-threshold leakage current drops significantly immediately below the threshold voltage, however the drop slows down to around 4µA/mm at VGS=0V. The leakage current is determined by the traps. The gate current in the on-state (defined as VGS=5V) is around 3µA/mm and thus around 5 orders of magnitude below the drain current.

Figure 3. Band diagram under the gate.

The output characteristic shown in Figure 4 exhibts a negative differential resistance due to lattice heating and simulated by solving the lattice heating equation set by the LAT.TEMP parameter of the MODELS statement.

Figure 4. Output characteristis.

A breakdown voltage of 870 V for a 18 μm gate-drain spacing is shown in Figure 5. The relatively high leakage current is induced by the traps. As a consequence and for this particular case, a 64bit version of ATLAS was used whereas in other circumstances higher precision versions are needed.


Figure 5. Breakdown volatge characteristic.


4. Conclusion

A normally-off GaN transistor for power applications with a low on-state resistance and high breakdown strength was simulated. The combination of a p-type GaN gate with an AlGaN back-barrier yields in a sufficiently high threshold voltage for power electronic applications. Very good agreement between simulations and experiment was obtained.


5. References

  1. “Normally-off AlGaN/GaN HEFT with p-type GaN Gate and AlGaN Buffer” O. Hilt, A. Knauer, F. Brunner, E. Bahat-Treidel and J. Würfl . Ferdinand-Braun-Institut, Leibniz Institut fuer Hoechstfrequenztechnik Proceedings of The 22nd International Symposium on Power Semiconductor Devices & ICs, Hiroshima.


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