RELIABILITY AND MICRO-STRUCTURAL PROPERTIES OF GAAS SCHOTTKY DIODES FOR SUBMILLIMETER-WAVE APPLICATIONS

Whisker contacted GaAs Schottky barrier diodes are the standard device s for mixing and multiplier applications in the THz frequency range. T his is mainly due to their minimum parasitics and mature technology. B ut with the decreasing size of the anode contact, which is required fo r operation at high frequencies (up to approx. 3 THz), the reliability and the micro-structural understanding of the Schottky barrier become s increasingly important. This contribution presents new results conce rning the reliability of Schottky diodes and the physical properties o f small-area Schottky junctions, especially at low current densities. For these purposes a number of different Schottky diodes have been fab ricated with different epilayer doping concentrations and anode diamet ers. Measured I/V characteristics show that the diode current deviates considerably from the ideal thermionic current behavior with decreasi ng diode diameter. This deviation shows an exponential dependence on t he diode voltage and is a function of the doping concentration of the active layer. For a given doping concentration in the epi-layer and de creasing anode diameter, this phenomenon shifts the minimum of the ide ality factor towards higher current densities. An explanation is given in terms of a difference of the crystallinity of the polycrystalline platinum films on the GaAs for decreasing SiO2 aperture size in connec tion with a reduced Pt mobility in the electrolyte. The reliability of Schottky barrier diodes under thermal and electrical stress has been investigated on different THz Schottky diode structures. The results s how that the barrier height and the ideality factor of the fabricated structures are not affected by thermal stress. Electrical stress induc ed by large forward currents up to a current density of 10 kA/mm(2) ev en leads to a slight increase of the barrier height and a reduction of the series resistance.