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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.

An instrument to improve power spectrum density (PSD) measurements, a highly sensitive preamplifier and a data acquisition unit are designed as well as a programmable power battery source. The preamplifier has 8 junction field effect transistor (JFET) inputs in parallel resulting in an input noise equivalent power of —186 dBV2/Hz. The data acquisition system consists of a low-pass filter, an A/D converter and a RAM, which transfer the data to the connected PC. The FFT or other signal processing is carried out within the PC. The battery power source is controlled by the PC, enabling us to obtain the device-under-test (DUT) bias DC required by the program. This measuring system works well at room temperature where the DUT and the power source could be put into a magnetic shielding box to reject spurious line noises. In the case where the DUT is placed in a cryostat to measure the temperature dependence of noise properties, however, the shielding of the DUT from the power line becomes a serious problem. Particularly in order to use a compressor cooling system, spurious noises come into the measuring system through the power lines of the compressor. In such a case the preamplifier must be placed as close as possible to the DUT, and the power line to the compressor is sometimes disconnected from the system.

In Athena Olympics in 2004 a Japanese girl got the gold medal in Women Marathon games. Just before the beginning, she was listening to some domestic melodies in order to concentrate on the race. The rhythm or power of that music is found to have the typical 1/f noise characteristics. The 1/f music is found effective to concentrate as well as to relax themselves for a fairly long time range, while some short time trial runner uses a kind of white noise like music.

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.