Design, fabrication and characterization of capacitively coupled gallium arsenide-based interdigitalgated plasma devices

Abstract

In the recent years, solid-state terahertz (THz) devices to fill the so-called “THz” gap have become a hot topic. Since the transit-time effect is so severe in this frequency range for conventional devices, one possibility is to utilize traveling wave interaction in semiconductor plasma. The theoretical results on the interactions of plasma wave with the EM space harmonic slow wave generated by interdigital slowwave circuits indicated the occurrence of negative conductance in two-terminal interdigital admittance when the carrier drift velocity slightly exceeds the phase velocity of the fundamental component of the EM waves. Experimental studies in microwave region using DC-connected interdigital gate structure are carried out and plasma wave interactions are successfully confirmed. However, the negative conductance is not obtained due to the non-uniformity of electric field distribution under such interdigital gate structure. This work presents an analysis including the newly proposed AlGaAs/GaAs HEMT plasma device with capacitively coupled interdigital gate structure. This structure is introduced in order to produce uniform field distribution and thus produce uniform drift velocity along the channel. The interdigital fingers are designed and fabricated on AlGaAs/GaAs HEMT substrate. The carrier mobility and the carrier sheet density of AlGaAs/GaAs HEMT structure obtained by Hall measurements at room temperature is 6040 cm2/V-s and 8.34 x 1011/cm2, respectively. Theoretical analysis of potential distribution in the interdigital-gated HEMT plasma wave device is carried out. The DC I–V characteristics of capacitively coupled interdigital structure showed that uniformity of electric field under the interdigital gates is improved compared to the DCconnected interdigital gate structure. Admittance measurements of capacitively coupled interdigital gate structure in the microwave region of 10–40 GHz showed the conductance modulation by drain–source voltage. This absolutely can be explained in terms of interactions between the input RF signals and 2DEG surface plasma waves. Absolute conductance values are smaller than the theoretical prediction, due to the small capacitance between interdigital fingers attenuating the propagation of RF signal at these frequencies. These results indicate the existence of plasma wave interactions. Further optimization of device structure and measurement method may lead to the occurrence of negative conductance.