New GaN and silicon juncionless field effect transistor THz detectors

We report the first experimental study of low-frequency noise in reverse biased P-InAsSbP/n-InAs infrared photodiodes at 300 K and 77 K. At room temperature, the current noise spectral density, SI, depends on frequency as 1/f over the entire current range and tends to the Nyquist noise when the frequency increases. At small reverse currents Irb ≤ 3.10-5 A, SI is proportional to Irb^2 ; at higher currents this dependence changes to SI~ Irb^4 . With temperature decrease down to 77 K, SI becomes proportional to Irb^0.5 , while the reverse current decreasesand the differential resistance grows by 4 orders of magnitude. The noise was also studied in the photovoltaic mode at 100 K, where SI is proportional to 2rb I . We conclude that at 100 K, the Nyquist noise is dominant and can be used for estimations of the specific detectivity of PInAsSbP/n-InAs diodes.

Nowadays high frequency electronics uses two distinct families of semiconductors-based transistors: field effect transistors and heterojunction bipolar transistors (HBTs). They compete reaching impressive cut off frequencies going up to THz range. Except their usual functions related to switching and amplifying of current or voltage both have been demonstrated as efficient direct THz radiation detectors. Indeed, both types of the transistors have shown that once equipped with antennas, they can capture THz radiation from the open space and deliver the voltage/current proportional to incoming THz radiation power (THz rectification).Most of the work was dedicated to the field effect transistors that rectify THz radiation by plasma related nonlinearities. After pioneering work of [1-2] only very small attention was devoted to HBTs. In this work, we present experimental studies of THz detection by different HBTs fabricated using InP double HBT (DHBT) technologies [3]. Different devices were investigated: single-finger devices and multi-finger devices formed using equally spaced parallel single-transistor fingers [4]. We have evaluated the room temperature detection performances of the devices in the sub-THz range from 250 GHz up to 650 GHz and analyse in details the physical mechanisms of THz detection. Finally, THz domain is an excellent non-contact probe of water content in biological tissues [5]. We also show that the sensitive HBTs detectors can be used for THz spectroscopy and 2D THz imaging to study the water dynamics of sorghum (a grass species cultivated for its grain) by monitoring the dehydration kinetics of its leaves.

Water movement and diffusion in wood is a well-documented research topic. It is strongly linked to the physical and mechanical properties of wood, and thus it has a high degree of importance and prominence for real-life application, particularly in wood drying. Here we employed the terahertz (THz) spectroscopy and imaging techniques to study the diffusion of water in wood. For our first experiments, we used balsa, a species of wood with a relatively homogenous structure (at the tissue level) and a low density that turns into fast diffusion rates and low absorption of the terahertz beam. This enables us to conduct several consecutive observations from various conditions (angles, diffusion directions, etc.) in a short time. These preliminary studies confirm the feasibility of terahertz spectroscopy and imaging techniques as a tool to observe the water diffusion in wood as a function of the direction (Fig. 1). We have also measured the evolution of the attenuation coefficient of the terahertz beam on wet and drying wood at different times. A model of the diffusion behavior based on these measurements, and according to the water content, will be developed and compared with other conventional water diffusion models based on gravimetry methods for the identification of the diffusion coefficients.

We present experimental studies of THz detection by different Heterojunction Bipolar Transistors (HBTs) fabricated using InP technology. We also show that the sensitive HBTs detectors can be used for THz spectroscopy and 2D THz imaging to study how the water content affects the polarization rate and biattenuation in fibrous sorghum leaf.