Nonalloyed ohmic contacts for high-electron-mobility transistors based on AlGaN/GaN heterostructures

A microwave field-effect transistor with nonalloyed ohmic contacts is fabricated using the technique of regrowing a heavily doped region under the contact metallization by molecular beam epitaxy through a preliminarily formed dielectric mask. The fabricated field-effect transistor with a gate length of 0.18 µm and a total width of 100 µm has a current–amplification cutoff frequency of 66 GHz and ohmic contact resistivity of 0.15-0.18 Ω mm.     

The thermal stability of nonalloyed ohmic contacts to AlGaN/GaN heterostructures

Degradation of nonalloyed ohmic contacts with heavily doped GaN epitaxially grown to the heterostructures with two-dimensional electron gas has been investigated. The change in the relative contact resistivity at temperatures of up to 600°C for the Ti/Pd/Au, Cr/Au, and Cr/Pd/Au metallization compositions has been studied. It is demonstrated that the Cr/Pd/Au metallization composition, the resistivity of which decreases at working temperatures of 400°C, is the most resistant to the effect of temperature. It is shown for the first time that the largest contribution to the increase in the contact resistivity to two-dimensional electron gas upon heating above 400°C is made by the resistivity of the Cr/Pd/Au–n⁺-GaN metal–dielectric structure, while, at temperatures of 400°C and higher, the resistance between heavily doped GaN and two-dimensional electron gas decreases.

     

Alloyed Si/Al-based ohmic contacts to AlGaN/GaN nitride heterostructures

For the first time in Russia, the Si/Al/Ti/Au alloyed contact composition is investigated for the formation of ohmic contacts to AlGaN/GaN heterostructures using thermal annealing. The obtained results are compared with those for conventional Ti/Al/Ni/Au ohmic contacts. Use of the composition under investigation makes it possible to decrease the annealing temperature to 675–700°C, which results in improvement in the morphology of alloyed ohmic contacts in comparison with conventional contacts. The value of the contact resistance using the Si/Al-based composition to the AlGaN/GaN heterostructure is obtained in relation to the temperature and annealing duration. It is shown that no qualitative change in the resistance occurs at an annealing duration of several minutes in the temperature range of 700–750°C. In the temperature range of 675–700°C, there is an asymptotic decrease in the contact resistance with increasing annealing duration. The smallest value of the contact resistance amounts to 0.41 Ω mm.