Current-voltage characteristics (CVC) of industrial diodes (GA401, GA402, DK-s7M, KD514A, D18, tunnel GI401) are typical for all such diodes. CVC of MIM diodes correspond to those described in the literature [14] - [19] [21] [22] [23] .

Figure 3 shows the room-temperature V_oc(P) for these diodes. In all cases one can see the maximum of V_oc(P), but polarity reversal was not achieved in some diodes (Figure 3(b) and Figures 3(d)-(f)) because of they are destroyed at high P. For p-n-junction and Schottky diodes (Figures 3(a)-(c)) takes place abrupt change and hysteresis in V_oc(P).

V_oc(P) of all diodes investigated is linear for small P. Figure 4 shows V_oc(P) for parametric diode GA402 and detector diode D604 for small signal as an example. It is seen that diode GA402 has a higher voltage sensitivity and linearity than the D604.

We have mentioned above that diodes as detectors of microwave and optical signals work in short-circuit mode (resistance of a load R_L << R_i – intrinsic resistance of the diode as signal source). Dependence of short-circuit current I_sc(P) on incident power P was measured on diodes GA402 (Figure 5) as the power P₀ for polarity reversal was smaller and these diodes were more stable for these investigations. It was found that the diodes can be divided into two groups according to the form of I_sc(P) (curves 1 in Figure 6(a) and Figure 6(b), which are similar to that of MIM-diodes in ref. [19] ).

Figure 5 shows the effect of external direct bias on shape of the I_sc(P) of diodes. Initial current and voltage drop in this experiment were compensated by circuit showed in Figure 3. The forward bias (curves 2 - 5 in Figure 6(a) and Figure 6(b)), reducing the potential barrier, ultimately removes the maximum I_sc(P). The reverse bias (curves 6 - 10 in Figure 5(a) and Figure 5(b)) increases the height of the potential barrier and ultimately leads to rectification in the opposite direction. In this case, the maximum also disappears and detected signal is too small.

Previously, similar dependences I_sc(P) were observed for point-contact MIM diodes by Kwok et al. [18] , Pyee et al. [19] (Figure 6) at 10 GHz. Sillivan et al. [21] and Green et al. [22] [23] noted a change in the sign of the detected signal when mixing or detecting laser radiation. In the case of MIM-diodes, the shape of the I_sc(P) depended on the metals forming the diode, i.e. on the height and shape of the potential barrier formed by the oxide layer. In particular, Pyee et al. [19] made 40 types of diodes from wire W, Cu and C, TaC filaments on a metal base from Al, Hg, Mg, Mo, Nb, Ni, Pb, Sn, Ta, W, the I_sc(P) of which it turned out to be possible to break in 5 groups (Figure 5). However, we are not aware of such studies on diodes with p-n-junction.

Let us especially note the presence of hysteresis in the V_oc(P) of some examples of diodes GA402 (Figure 3(a) and Figure 7(a)), GA401 (Figure 3(b)) and

KD514A (Figure 3(c)), the origin of which has not been clarified. Based on what has been said above about the role of the shape and height of the potential barrier in the detector properties of diodes, it can be assumed that this hysteresis is due precisely to the features of electronic processes in the space charge region of specific specimens of diodes.

Figure 7 shows V_oc(P) of diodes GA402 and MIM (W-Mo) serviceable (1) and after deliberate breakdown (2). In this experiment, the opposite of the polarity of the detected signal and thermoEMF in diodes is manifested.

The opposite of the polarity of the rectified voltage and thermoEMF is also confirmed by the oscilloscope trace of the microwave pulse of 1 μs duration, detected by the diode DK-s7M (Figure 8). It is seen that the maximum and a sign reversal of I_sc(P) at high microwave power are manifested a somewhat peculiarly: at first, the pulse amplitude increases as microwave power, then a dip occurs between the leading and trailing edges of the pulse. This dip comes deeper as power increases, followed by the polarity reversal of the signal in the dip, but positive teeth remain, corresponding to the rise and fall of the microwave pulse. It follows from here that the settling and decay time of the detected signal does not exceed the duration of the rise and fall of the microwave pulse, i.e. no more than 0.1 μs.

Researchers often use a small-signal approach to describe high-frequency detector (rectifier) properties of semiconductor and MIM-diodes [11] [13] [15] . This approach is based on the expansion of the CVC of the diode in powers of a small high-frequency signal V₀cosωt (ω = 2πf is angular frequency), followed by averaging over the period T = f⁻¹ of this signal. As a result, in the first approximation, a quadratic dependence of the detected voltage on the signal amplitude V₀ or a linear dependence on the power P ∝ V 0 2 (quadratic detection) is obtained. Further increase in P changes this dependence to V o c ( P ) ∝ P 1 / 2 ∝ V 0 (linear detection). The disadvantage of this approach is that the CVC is a secondary (derivative) characteristic of the electronic processes occurring in the diode, and therefore is not physical. As a result, it is not possible to explain, for example, the manifestation of detector properties at X-band frequencies for diodes whose cutoff frequency is 20 kHz - 100 MHz [20] .

In some cases, for modeling of rectifiers performance a diode is considered to work as an ideal switch-namely, zero forward voltage, zero on-resistance, infinite off-resistance, and zero switching time [24] , whereas current-voltage characteristics and switching timeof real diodesare far from this model (Figure 11.14 in ref. [13] and Figure 21 in ref. [25] ).

As it is known from the Physics of semiconductor diodes [26] , a direct external bias (forward or reverse) changes the height and shape of the potential barrier in the space charge region of the diode, creating an asymmetric and nonlinear current-voltage characteristic (Figure 21 in ref. [26] ). Therefore, it can be argued, based on the similarity of Figure 6 and Figure 7 that the shape of the V_oc(P) and I_sc(P) are indeed related to details of the potential barrier. However, the detection (rectification) mechanisms proposed for MIM (MOM) diodes (quantum mechanical tunneling through an asymmetric barrier, enhancement of thermal field emission) [15] [16] [17] [18] [19] do not explain our experiment, since in GA402 diodes the width of the space charge region is about 1 μm, which excludes the electron tunneling.

There are old contradictory points of view in literature on the polarity of direct voltage, generated in diodes under microwave or optical radiation [26] [27] [28] . To solve the problem, we carried out additional experiments by a deliberate breakdown of all type diodes. Diodes were broken through in different ways - by passing direct current (>1A), high microwave power (more than 10 W), and in MIM diodes also by mechanical force. After the breakdown, the CVC of the diodes became linear and symmetrical, i.e. rectification is absent, the DC resistance of the diodes is several Ohm. These experiments showed that the polarity of the open-circuit voltage (of an operable diode) in all cases is opposite to the sign of the thermoEMF of a punched diode (Figure 8). The reasons for this remain to be seen. Note that the most characteristic results only are showed in Figure 8(a) and Figure 8(b) (curves 2), bearing in mind that it’s similarity in all other diodes.

Most of the known studies of rectennas are devoted to the rectifying features of MIM diodes [1] - [7] [14] - [19] . Such interest in MIM diodes (mainly point- contact, as well as small area Schottky diodes) is due to a purely technical approach - the lower the resistance and capacitance of the diode, the higher the cutoff frequency. Quantum mechanical tunneling of electrons through a thin (10 - 100 nm) potential barrier, which has the simplest trapezoidal form, is considered to be the main detection mechanism of MIM-diodes. But this mechanism cannot explain polarity reversal of V_oc(P) or I_sc(P) at higher incident power [18] [19] [21] as well as correlation of pairs of contacting metals and shape of I_sc(P) [19] . The mechanism of such a correlation has not been established, and it can be assumed that it is due to the properties of the potential barrier created by the dielectric (oxide) layer between the metals.

The situation forces researchers to recognize [29] that “… once stabilized, the output voltage V_out only depend on static parameters (i_d0, R, T…). In particular, it becomes independent of the geometrical capacitance. Consequently, the drastic drop of V_out observed in the high frequency regime cannot be induced by the diodegeometrical capacitance.”

Analyses of experimental results on rectennas and rectifiers as well as theories of MIM-diode working led Zhao et al. [2] to conclude: “It’s worth noting that there is no suitable rectifier for solar rectenna system to date. MIM diodes, including metal-oxide-metal (MOM) diodes and metal-vacuum-metal (MVM) diodes, are assumed as potential solutions, but the high impedance and low responsivity do not meet the requirements for visible and NIR rectification.”

Problems caused by uncertainty of physical mechanism for generation of direct voltage or current take place also for thermoelectric and photoelectric converters [30] [31] . This makes us think that some unidentified yet physical mechanism is situated behind the reported above phenomena in diodes of various types. Due to this mechanism a spatial distribution of the charge carrier’s density in diode and, as consequence, microwave or optical field distribution come uniform at high microwave power, so the rectified voltage disappears and thermoEMF comes the prevailed one. Same uniform distribution of charge carriers in a diode can be generated by initial direct forward current passing through the diode (Figure 6, curves 2 - 5). It may be considered that the hysteresis on the power dependence of open-circuit voltage in some types of p-n-junction diodes arises from this mechanismas well owing to peculiarities of the potential barrier of the junction.