Enhancement of channel electric field in AlGaN/GaN multi-nanochannel high electron mobility transistors

We studied the drain current properties of an AlGaN/GaN multi-nano-channel (MNC) high electron mobility transistor (HEMT) fabricated on a sapphire substrate. We observed that the MNC HEMT exhibits the currents almost equal to those in the conventional planar device grown on the same chip. This result was unexpected since the actual gate width on the AlGaN surface in the case of MNC HEMT was only 20$%$ of that for the planar device. In order to explain our experimental results, we performed a three dimensional (3D) simulation of the planar and MNC HEMTs using the TCAD Sentaurus software. Especially, we calculated the transfer characteristics of the MNC HEMT with a different nanochannel width and compared them with experimental data. The simulation results exhibited a good agreement with experimental ones. On this basis, we showed that the unusual behavior of the current in the MNC HEMT results from the enhancement of the effective electron velocity (⁠$ve$⁠) under the gate. In particular, we found that $ve$ for the MNC HEMT was about 2.5 times higher than for the conventional HEMT, i.e., $2.44×107$ cm/s, which is close to the peak saturation velocity in GaN (⁠$2.5×107$ cm/s). Finally, we showed that such a strong enhancement of $ve$ in the MNC HEMT case is due to the formation of the high electric field in the nanochannel. The results obtained in this work are not limited only to MNC structures but they should also be useful in understanding the electric field and electron velocity distribution in other AlGaN/GaN HEMTs with 3D nanochannels such as AlGaN/GaN FinFETs.