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The capacitance-voltage (C-V) characteristics of Al0.25Ga0.75N/GaN heterostructure barrier varactors (HBV) grown by metal-organic chemical vapor deposition on a c-oriented sapphire substrate were studied. The favorable material properties of wide-band-gap III-N's are important in realizing the HBVs for frequency triples and distributed line pulse sharpens for practical use. The HBV structure consists of a 10-nm-thick undoped Al0.25Ga0.75N barrier sandwiched between two undoped GaN layers with a thickness of 5 nm. The barrier structure is further sandwiched between two n-GaN (n = 5 × 1017 cm–3) layers with respective thicknesses of 500 nm and 1 µm to form the top and the bottom ohmic contacts. At room temperature, the C-V characteristics show a maximum capacitance Cmax at approximately –1.8 V because of the induced piezoelectric field within the heterostructure, and the I-V characteristics show an increase in leakage current below –1.5 V and above 0.5 V. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ohmic contacts are formed on the epitaxial n-GaN film by the electron beam evaporation of Ti/Al and their rapid thermal alloying with device separation by a fairly fast Electron Cyclotron Resonance (ECR) plasma etching. The etched surface has a poor ohmic contact with a typical contact resistance of 8.1 × 10 - 3 Ω.cm 2 , while the as-received and rinsed surface has a better contact with a lower contact resistance of 7.5 × 10 - 6 Ω.cm 2 . The resistivity of the film is determined as a function of the temperature, in good agreement with the numerical analyses. The ohmic characteristics are evaluated by the 1/f noise measurements. Low ohmic contacts give the typical 1/f noise characteristics which increase with the square of the sample current 1, while higher ohmic contacts give a linear noise power dependence on the sample current 1. We are able to evaluate, therefore, the ohmicity of the electrodes most sensitively from the 1/f noise measurements. Assuming the relevant total electron number between the electrodes, the Hooge parameter is estimated as α H = 5.3 x 10 - 6 at room temperature, in qualitative agreement with the cross correlational 1/f noise model.

Low cost highly sensitive preamplifier was made by using the commercially available operational amplifier AD797 which has the input noise equivalent power of −190 dBV2/Hz with 80 dB amplification.

The Hooge parameters of compound semiconductors are found to be in inverse proportional to the mean free paths of materials. The newly developed model of 1/f phonon energy partition fluctuation in thermal equilibrium predicts the value of Hooge parameter as αH = a/λ, the ratio of the lattice constant a and the mean free path λ. Several reported experimental results on αH for very pure semiconductors are found on the a/λ line. Experimental verification is given by measuring noise in InGaAs/InAlAs heterostructure, where optical phonon effects can be observed due to negligible impurity scattering. The Hooge parameter of about 1 in p-InGaAs and 10-3 to 10-5 in n-InGaAs reflects the two order difference in the mobility and corresponding λ values.

The Si surface of n-SiC is etched by an Ar electron cyclotron resonance plasma in order to smooth the as-received substrates. Low-frequency current noise characteristics of Ohmic contacts of the wide-band-gap semiconductor n-SiC are investigated. Ohmic contacts with a diameter of 200 µm have a minimum Ohmic contact resistance of the order of 10-4 Ω·cm2 at an alloying temperature of 1000°C for 30 min. These low resistance Ohmic contacts show typical 1/f current noise characteristics that increase with the square of the sample current I 2. A sample with a higher contact resistance exhibits, however, current noise power increase with increasing sample current I. The relevant total electron number between the electrodes of the bulk substrate is estimated from the resistance and by finite element method simulation, from which the Hooge parameter αH is estimated to be approximately 40.