# Hot Carrier Photocurrent through MOS Structure
## 1 Introduction  
Capacitors with optically-variable properties attract research interest due to their potential application in a variety of electronics devices. For example, the search for sustainable electric power supply is currently concentrated on photo-powered energy sources, and photocapacitors are an innovation in this field with promising applications in integrated devices such as medical sensors [1, 2] and next generation solar cells [3, 4, 5, 6]. Furthermore, the property of light-dependent capacitance of photovaractors is useful for optical control or modulation of microwave signals [7, 8]. It should be noted that all these optical devices operate on the basis of carrier generation, i.e., when light photon energy is not less than a semiconductor forbidden energy gap. If the photon energy is smaller than the band gap, free carrier absorption takes place in a semiconductor leading to the increase of their energy. Such free carriers having energy higher than the equilibrium one are called hot carriers. Carriers in a semiconductor can be heated not only by means of infrared radiation but also by a strong electric fields or microwaves. As a rule, no carrier generation is present during the carrier heating process. Two advantageous properties of the phenomenon can be highlighted. First, free carrier absorption is not a radiation wavelength-selective process. Second, the speed of the hot carrier effect is determined by the carrier energy relaxation time which is of picosecond order in most semiconductors. Thus, the hot carrier effect attracts interest not only from the fundamental point of view but it also finds a lot of applications. For example, it is employed in the detection of microwave [9, 10] and infrared radiation [11], and even within the whole region between both these ranges including the THz frequencies [12, 13]. In addition, the concept of hot carrier solar cell as remained an important focus of research topics during the last several decades [14, 15, 16]. When a potential barrier is present in a semiconductor, carrier heating causes their redistribution in the vicinity of it. This may result in the rise of so-called hot carrier thermo-electromotive force across the barrier. Early investigations of microwave impact on a semiconductor surface space charge region enabled the authors to conclude thatcarrier heating leads to the change of the surface potential [17]. The authors studied the rise of thermo-electromotive force across a metal-insulator (oxide)-semiconductor (MOS) structure, Al-SiO\({}_{2}\)-n-Si, illuminated with 10 GHz frequency microwaves. When silicon p-n diodes were exposed to microwave radiation, hot carrier thermo-emf was also induced [18], and analysis of its pulse's kinetics brought the authors to the conclusion that the thermo-emf is determined by the discharge of capacitance of the reverse biased p-n junction. CO\({}_{2}\) laser light also causes a rise of the hot carrier thermo-emf across wideband semiconductor l-h [19; 20] and p-n junctions [21; 22; 23]. It is worth noting that the hot carrier photocurrent flows in the opposite direction to the classical generation-induced photocurrent, i.e., the heated carriers diffuse the potential barrier. In the case of the reverse biased junction, when the carriers cannot overcome the potential barrier, the photocurrent pulse is still detected [22]. It was supposed that such photosignal rise was related to the p-n barrier capacitance. Later, it was assumed that carrier heating causes a change of the barrier capacitance, and this phenomenon was called a photocapacitive effect [24]. In this communication, we report photosignals induced by infrared CO\({}_{2}\) laser radiation across an Al-SiO\({}_{2}\)-p-Si structure. The work is remarkable in two ways. First, we declare a hot carrier photocapacitive effect in a MOS device--no carrier generation is present since the laser photon energy is much lower than the silicon energy bandgap, 0.12 eV \(<\) 1.12 eV.