GaN is an excellent canditate for high power and high temperature applications due to its wide band gap. It has high breakdown field strength and high saturation electron velocity, which also allows GaN based devices to be used for high frequency applications such as modern 5G communications. GaN-based High Electron Mobility Transistors (HEMTs) are therefore used extensively for high frequency and high power applications. Al0.83In0.17N/GaN hetero-structure has many advantages over conventional AlGaN/GaN system, like higher spontaneous polarization charge leading to higher 2-DEG and stress-free barrier layer, as Al0.83In0.17N is lattice matched with GaN. Lower barrier thickness is sufficient in Al0.83In0.17N/GaN hetero-structure compared to AlGaN/GaN system to get the same 2DEG density. Thus, better gate control over the channel is obtained in Al0.83In0.17N/GaN HEMT/MISHEMT devices leading to higher transconductance and cut-off frequency (fT). However, although Al0.83In0.17N/GaN HEMTs/MISHEMTs have many advantages, there are relibility issues. These relibility isssues occur due to the trapping of charge carriers in the surface states, traps in buffer layers, and at hetero-interfaces. When HEMT is subjected to pulsed measurement i.e. turned to on-state after being biased in off-state, the drain saturation current falls to lower value and the on-state resistance is higher than that in DC measurements. This is known as current collapse. This is ascribed mainly due to the trapping of charge carriers in surface states in the drain-access region. Therefore surface passivation is needed to suppress the current collapse. Thin GaN cap layer on the top of AlInN barrier layer passivates the surface states. It also helps to suppress the gate leakage current.
We have fabricated and characterized HEMTs on two different wafers. The wafers are identical in all respects except that one is with a GaN cap layer and other is without. From characterization results we observed that GaN cap layer helps to reduce the gate leakage current in HEMT devices. It also helps to improve the off-state breakdown voltage, and reduces the current collapse effect. In this talk, I will discuss fabrication steps and mechanisms by which the gate leakage current reduces in HEMT devices with GaN cap layer. I will also discuss how the GaN cap layer helps to reduce current collapse. Finally I will discuss how the short chanel effects in the small-geometry HEMTs and MISHEMTs can be suppressed, which is my next plan of work.