chapter  7
24 Pages


The ultrahigh frequency operation is an important goal for the nitride semiconductor devices. For the microwave power GaN HEMTs to work at higher operation frequencies, especially in millimeter wave or even entering the THz band, the gate length is often required to be less than 150 nm, and in some cases 50 nm or even 10 nm, where the short channel effects of the devices become a major problem. The suppression of the short channel effect on the millimeter wave devices requires a thinning of the AlGaN barrier (reducing the distance between the gate and the 2DEG channel). The back barrier, high-resistance buffer layer, and so on are also employed to obtain better pinch-off characteristics and high output resistance. However, in order to maintain a high channel conductance of the heterostructure at a reduced barrier thickness, the Al content in the barrier needs to be increased to maintain a high 2DEG density. In terms of material properties, the strain induced by the increased lattice mismatch between high Al content AlGaN and GaN causes an increase in the AlGaN/GaN interface roughness as well as a degraded AlGaN barrier crystalline quality and even strain relaxation, hence decreasing the 2DEG sheet density and mobility and reducing device performance and reliability under long time, high bias, and high temperature. And in terms of device fabrication, the thin and highly defected AlGaN barrier makes the 2DEG highly sensitive to the barrier surface condition and strain, and thus the 2DEG depletion may arise from surface degradation caused by oxidization and etching damage in the fabrication; on the other hand, the greater insulativity of high Al content AlGaN inevitably leads to a higher ohmic contact resistance, which also imposes difficulty in the development of millimeter wave devices.