ABSTRACT
The current is proportional to the area of electrodes in the high-resistance state, but in the low-resistance state, it does not depend on the area of electrodes. This means that a current channel (filament) with small cross section and with high current density is formed in the low-resistance state. Since the current flows through the small cross-section channel in the low-resistance state, the maximum current of the switch, as a rule, does not exceed several mA. Holding the voltage of low-resistance state (Uh) does not depend on the thickness of the semiconductor film (Fig. 8.2). This indicates that most part of the voltage applied to the device drops at the contacts, but not in the bulk of the semiconductor. As one can see from Fig. 8.2, there is a critical thickness of the semiconductor film (approximately 0.2 µm) at which the threshold voltage and the holding voltage are equal to each other. Consequently, when film thickness is smaller than this value, switching is impossible in this type of devices. It should be added that the threshold voltage decreases with increasing temperature and increasing pressure, but does not depend on the illumination. Under illumination, only the threshold current increases at a constant value of the threshold voltage. An important characteristic of switches is their switching speed. The time response of the threshold switch to a voltage pulse is shown in Fig. 8.3. The time of switching from high-resistance in the low-resistance state (switch-on time) consists of two components: delay time (td) and time of the switching process (ts). The delay time is the time that elapses from the applying of a voltage pulse equal or greater to the threshold value and before a sharp increase in current through the device. The delay time depends strongly on the applied voltage, varying from 10−6-10−7 s with U = Uth to about 10−9s with voltage increasing. The time of the switching process does not exceed 10−9-10−10 s. The renewal time of the high-resistance state (switch-off time tsb) is defined as the minimum time that has elapsed since the end of the pulse that switched the device into the low-resistance state, after which a pulse with amplitude of 10% less than the threshold voltage does not switch the device into the low-resistance state. This time is usually about 10−6 s and increases with the thickness of the chalcogenide glassy film.
