In this work, the presence of anomalous low-frequency fluctuations during the initiation of higher frequency oscillations in InGaAs-based Gunn planar diodes has been evidenced and investigated. Accurate measurements showing the evolution of the power spectral density of the device with respect to the applied voltage have been carried out. Such spectra have been obtained in the wide frequency range between 10 MHz and 43.5 GHz, simultaneously covering both the low-frequency noise and the

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... al oscillation peak at some tens of GHz. This provides valuable information to better understand how these fluctuations appear and how these are distributed in frequency. For much higher frequency operation, such understanding can be utilized as a simple tool to predict the presence of Gunn oscillations without requiring a direct detection. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4896050] During the last decades, Gunn diodes have been widely used for the implementation of microwave oscillators. More recently, the improvement of the fabrication techniques, as well as the research on new materials and structures, have made possible the fabrication of Gunn diodes working up to several hundreds of GHz 1-3 and the design of structures prospectively reaching THz frequencies. 4 The oscillatory regime in such devices is usually considered as directly activated above a given threshold voltage. However, several studies have shown that the transition between a static state and the generation of a purely periodic signal is not so immediate, passing through a range of bias voltages for which unusual fluctuations appear in the device. 5-9 On the one hand, some noise is formed around the characteristic frequency of the oscillator that, for a higher voltage, usually converges into a single coherent peak. Besides the previous wellknown effect, Refs. 5-9 also demonstrated the presence of an anomalous low-frequency noise (ALFN) preceding the high-frequency oscillations, which vanishes when the fundamental peak is well established. Accordingly, Matsuno 5 reported strong current fluctuations at 100 kHz in a very limited voltage range near the onset of the Gunn instability in a GaAs sample. Similarly, Kabashima et al. 6 observed a current increase of several orders of magnitude at 50 kHz for voltages just below the generation of a pure oscillation in a X-band GaAs Gunn diode. In the latter study, such increase was attributed to the coexistence of both oscillatory and nonoscillatory states alternatively occurring with a random period, for certain range of voltages. The cathode contact characteristics can also be the origin of an increase of noise by leading to stationary field configurations in which local regions of negative differential mobility exist. 10 More recently, the study of this ALFN is gaining a lot of interest for the characterization of new mm-wave and THz Gunn sources. Following this line, Starikov et al., 7 Shiktorov et al., used Monte Carlo particle methods to simulate the spectral density evolution in InN, InGaAs, InP, and GaN devices working above 100 GHz, also predicting a noticeable enhancement of lowfrequency noise at the onset of the oscillations. This manuscript presents wide-frequency spectral power measurements from InGaAs-channel Gunn diodes oscillating at some tens of GHz. In this context, the aim of the present letter is double. On the one hand, first purpose is to provide a complete field of view of the simultaneous evolution of both the low-frequency noise and the main oscillation peak in a Gunn diode for different supply voltages. It extends the experimental results presented in the literature that were restricted to very limited frequency ranges. 5,6 Moreover, these results allow us to clearly identify the ALFN, analyze its dependence with frequency, and qualitatively correlate its appearance with the high-frequency oscillation. On the other hand, the second goal is more specific, and deals with the InGaAs-based planar diodes, and its demonstrated suitability to operate at mm-wave 1,2 and THz 3,4 frequencies. The power measurement at hundreds of GHz, typically made by means of sub-harmonic mixers or power meters, 2 may be strongly limited by the inherent high conversion losses or reduced sensitivity of the previous instruments at so high frequencies. For THz devices, the scenario becomes even more challenging, usually requiring free space setups and relatively high power levels to detect the presence of oscillations in the source. 11 Therefore, the knowledge on the ALFN mechanisms and its corresponding identification is proposed in this case as an alternative indirect method to determine the activation of higher frequency oscillations in devices based on similar InGaAs epitaxial structures properly adapted for (sub-)THz generation. The diodes characterized in this work are based on an epitaxial layer structure consisting of an un-doped In 0.23 Ga 0.77 As channel layer placed between two Al 0.23 Ga 0.77 As layers with a)