<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> In this paper, we propose a novel concept for a semiconductor planar detector. At room temperature, the diode exhibits markedly high responsivity and strong spectral dependence of it: the responsivity value around 2000 V/W is inherent in <formula formulatype="inline"><tex Notation="TeX">${K}_{ a}$</tex> </formula> microwave frequency range, and while going up to frequency range <emphasis emphasistype="italic">D</emphasis>, it decreases by about two orders. Rectification of microwave currents and intervalley electromotive force are discussed to be mainly responsible for high responsivity and its rapid decrease within microwave range. Nevertheless, experiments at higher radiation frequencies performed under the action of pulsed CO<formula formulatype="inline"><tex Notation="TeX">$_{2}$</tex></formula> laser radiation reveal the planar diode as a fast IR detector and a promising candidate for terahertz radiation sensing. </para>