Recently the high electron mobility transistors (HEMT) have drawn the attention of the scientific community for power electronic applications thanks to their wide bandgap and consequently high breakdown voltage (1-3). A new metal oxide/insulator semiconductor high electron mobility transistor (MOSHEMT) appears in order to minimize the leakage current as reported in (4). In this work, the basic parameters of MOSHEMT are evaluated experimentally for different gate insulators (10nm of Al2O3 and 5nm of SiNx).
From the transfer curves presented in figure 1, one can notice a shift in the threshold voltage and different hysteresis behavior when comparing both insulators. The hysteresis analysis shows a better behavior for SiNx devices, which present negligible hysteresis values. Figure 2 presents the hysteresis width extracted from devices with Al2O3 as a function of gate length for different drain bias. Although usually the Vth variation is associated with positive charge trapping, which causes a negative Vth shift, in this case the opposite Vth variation was observed. This positive Vth shift may also be related with a degradation of the on-resistance, i.e., a higher voltage is required to obtain the same current level, as reported in (3). As the drain voltage and consequently the drain current increase, the Vth shift becomes smaller reducing the hysteresis values, as can be seen in figure 2.
Focusing on the threshold voltage variation between devices with different gate dielectrics, although both the gate insulator thicknesses and the permittivity impact on the threshold voltage (Vth) of the transistor in opposite direction (5), the dominant factor of the Vth shift, in this case, is a negative linear relation between Vth and the dielectric thicknesses. The MISHEMT with 5 nm of SiNx presents a Vth almost 2V higher than MOSHEMT with 10nm of Al2O3.
In addition to the better Vth value (closest to the normally-off device), SiNx dielectric transistors are less affected by the short channel effect (SCE) when compared to devices with Al2O3, as can be observed in figure 3. From Fig. 3 it is possible to notice that reducing the channel length (Lg) from 600nm to 200nm, the threshold voltage for transistors with SiNx dielectric (figure 3B) drops 175mV while for devices with Al2O3 layer (figure 3A) this reduction reaches 450mV. Considering DIBL (|Vthlinear – Vthsat|/ VDSsat – VDSLinear) with VDSsat=2V, it is also observed that for both splits the DIBL starts to degrade for Lg = 400 nm, however the values and the degradation of DIBL are much higher for devices with Al2O3 than for devices with SiNx dielectric. This behavior can be related to the thinner dielectric and better coupling between the gate and the GaN layer.
When the MOSHEMTs were evaluated at high temperatures (from 25ºC to 150ºC), two different behaviors were obtained. The transfer characteristic of MOSHEMT with Al2O3 layer (figure 4A) presents an expected behavior, showing the zero temperature coefficient point (VZTC) due to the competition between the threshold voltage reduction and mobility degradation, while for the MISHEMTs with SiNx layer the VZTC was not found. In order to explain this unexpected behavior the threshold voltage and transconductance were evaluated (figures 5 and 6).
MOSHEMTs with Al2O3 gate present a reduction of the threshold voltage as the temperature increases (figure 5A) due to the reduction of the fermi potential, for all analyzed channel lengths. However, for SiNx MISHEMTs Vth is reduced when the temperature increases from 25ºC to 50ºC, but for higher temperatures Vth starts to increase again creating a rebound effect. The threshold voltage anomalous behavior can be explained by the double conduction of the device which leads us to extract an effective Vth.
This double conduction can only be observed when we evaluate the transconductance curve of the MISHEMTs (figure 6). It is possible to observe that for temperatures higher than 50ºC the transconductance curve presents two different slopes before the maximum value, representing two different conduction channels. Figure 6B shows the transconductance and drain current as a function of gate voltage at 100ºC and it is easy to see the two different slopes representing the two different threshold voltages, due to the turn on/off of the SiNx/AlGaN inversion layer. Looking for the transition point between both conductions, we can also observe a small peak in the drain current.
Figure 1
