| Gallium Nitride (GaN) and related materials (especially Aluminum Gallium Nitride, AlGaN) have recently attracted a lot of interest for applications in high power electronics capable of operation at elevated temperatures and high frequencies. The AlGInN system offers numerous advantages. These include wider bandgaps, good transport properties, the availability of heterostructures (particularly AlGaN/GaN), the experience base gained by the commercialization of GaN-based laser and light emitting diodes and the existence of a high growth rate epitaxial method (hydride vapor phase epitaxy, HVPE) for producing very thick layers or even quasi-substrates. These attributes have led to rapid progress in the realization of a broad range of GaN electronic devices.; AlXGa1−XN (X = 0∼0.25) Schottky rectifiers were fabricated in a lateral geometry using p+-implanted guard rings and rectifying contact overlap onto an SiO2 passivation layer. The reverse breakdown voltage (VB) increased with the spacing between Schottky and ohmic metal contacts, reaching 9700 V for Al0.25Ga0.75 N and 6350 V for GaN, respectively, for 100 μm gap spacing. Assuming lateral depletion, these values correspond to breakdown field strengths of ≤9.67 × 105 V-cm−1, which is roughly a factor of 5 lower than the theoretical maximum in bulk GaN. The figure of merit (VB)2/RON, where RON is the on-state resistance, was in the range 94–268 MW-cm−2 for all the devices. Edge-terminated Schottky rectifiers were also fabricated on quasi-bulk GaN substrates grown by HYPE. For small diameter (75 μm) Schottky contacts, VB measured in the vertical geometry was ∼700 V, with an on-state resistance (RON) of 3 mΩ·cm 2, producing a figure-of-merit VB2/RON of 162.8 MW-cm−2.; GaN p-i-n diodes were also fabricated. A direct comparison of GaN p-i-n and Schottky rectifiers fabricated on the same GaN wafer showed higher reverse breakdown voltage for the former (490 V versus 347 V for the Schottky diodes), but lower forward turn-on voltages for the latter (∼3.5 V versus ∼5 V for the p-i-n diodes). The forward I-V characteristics of the p-i-n rectifiers show behavior consistent with a multiple recombination center model. The reverse current in both types of rectifiers was dominated by surface perimeter leakage at moderate bias. Finally, all of the devices we fabricated showed negative temperature coefficients for reverse breakdown voltage due to high defect level, which is a clear disadvantage for elevated temperature operation.; Bipolar devices are particularly interesting for high current applications such as microwave power amplifiers for radar, satellite and communication in the 1∼5 GHz range, powers >100 W and operating temperatures >425°C. pnp Bipolar Junction Transistors and pnp Heterojunction Bipolar Transistors were demonstrated for the first time. For power microwave applications, small area self-aligned npn GaN/AlGaN HBTs were attempted. The devices showed very promising direct current characteristics. |
