(Digital Presentation) Analysis of MIS-HEMT Kink Effect in Saturation Region
Authors: Perina Welder , Martino Joao , Agopian Paula 
Published: September 2023

The High Electron Mobility Transistor (HEMT) has been widely used in the field of power electronics and high frequency operation since 1983 (1), while presenting a simple circuit design for analog applications (2, 3), and for extreme harsh environments as well (3, 4). However, HEMTs presents a problem with gate leakage and current collapse which can be solved by introducing an insulator between the gate and semiconductor while maintaining the high performance, hence the Metal Insulator Semiconductor High Electron Mobility Transistor (MIS-HEMT) appears as a solution. The AlGaN/GaN heterostructures forms, at the interface, a sheet with high electron density (2DEG – 2 Dimension Electron Gas), which also presents a high electron mobility. The MIS structure presents a well know conduction between source and drain. As a result, the MIS-HEMT device presents multiple conductions. The focus of this work is to study how the multiple conductions of the MIS-HEMT impact on the output characteristics.

The studied devices are, a MIS-HEMT with the gate insulator composed of a 2 nm Si3N4, and a GaN MOSFET of 25 nm of Al2O3 as the gate insulator, both devices were fabricated at imec Leuven – Belgium, and have the same dimensions: gate width of 5 µm and gate length of 600 nm. Their respective schematics are presented at figure 1.

The drain current (IDS) as a function of drain voltage (VDS) for a MIS-HEMT (left) and GaN MOSFET (right) with multiple overdrive voltage (VGT) is presented in figure 2. The VDS ranges for these devices are different because they have different saturation points, although it is possible to notice that the MIS-HEMT shows greater current drive when compared to MOSFET considering the same VDS, which is likely due to the formation of the 2DEG as the main conducting mechanism in the MIS-HEMT.

Previous works (5 – 8) showed that the MIS-HEMT have multiple conduction mechanisms dependent on VGT, and figure 3 shows the effect of those multiple conductions on the output current of the device where it is very likely that one of those conductions have different VDS sat. It is possible to notice higher values of IDS with increasing VGT, as expected. However, for high enough VGT, there is an anomalous IDS increase (kink) in the saturation region, that occurs due to the different conduction mechanism (MOS and HEMT conductions). This effect will be called as MIS-HEMT kink effect (MH kink effect). This MH Kink effect occurs when the HEMT conduction mechanism overcomes the MOS conduction.

The output conductance (gD) as a function of VDS for the MOSFET is presented in figure 4. Figure 5 shows gD as a function of VDS for multiple VGT for the MIS-HEMT, where it is possible to see a peak in the gD (in saturation region) shifting to the right for increasing VGT, caused by the MH kink effect.

In order to better visualize, the VDS value for which the MH kink takes place, it is plotted as a function of VGT in figure 6. It is possible to see that as VGT increases, the MK kink effect shifts for higher VDS values, thanks to the higher electron density in 2DEG making the HEMT conduction less dependent on VDS.

Figure 1