Off-state resistance

It should be noted that both Feldman et al. and Dey and Fong used a-Si films deposited by vacuum evaporation. This probably accounts for the relatively low OFF-state resistances which they both found (100-100 k 2). It is now well established that vacuum-evaporated a-Si is a very different material from the hydrogenated form of a-Si obtained, for example, by the carefully controlled glow-discharge decomposition of silane (e.g., Spear, 1977). 

To be used as the addressing matrix switches for individual sensor elements, the on/off state current ratio is the parameter to be considered. For a biosensor array with thousands of sensor elements the off-state resistance must be at least 3 orders of magnitude larger that of the on-state to secure precise measurement of data. This can be easily achieved by the use of a single FET based on either CMOS [5], poly-Si TFT [15], or amorphous-Si TFT [59]. 

Diarylethenes are promising synthetic photoswitchable molecules because of their outstanding fatigue-resistance in reversible light-induced transformation between two isomers with different absorption spectra.21,22 The tt-conjugation extends over the entire molecular in the closed form, whereas it is restricted to each half of the molecule in the open form. As a consequence, the closed form is expected to exhibit better electrical conductance than the open form. The two forms are referred to as the on and off states of the switch, respectively. The UV-vis spectra of this molecular switch in toluene show that the wavelengths that can be used for on to off switching are 420 < A< 650 nm, and 300 < A< 350 nm for the reverse operation. 

In the OFF state, the FET appears as a resistor in parallel with RD. If we arbitrarily specify that the FET should not degrade the R-C product by more than 10%, we obtain for the minimum resistance in the OFF state, Roff-... 

The electro-optic properties of PDLC films are controlled by the types of materials used, the droplet morphology and the method of film construction. Desireable properties include high clarity and transmission of the film in the ON and OFF states, low driving voltage, low power consumption, fast switching times and high film resistance. Since these properties are related, it is usually not possible to change them independently. 

I-V curve of the OFF state generally exhibits three different regions of resistance behaviour as the voltage increases. In the first portion, the behaviour is Ohmic, while in the second it exhibits Poole-Frenkel... 

Case (c). The switching device has no stable operating point between the high resistance OFF state and the conductive ON state to which the device switches when the voltage exceeds the threshold voltage V. The device switches to its original OFF state when the current is decreased below the holding current Ij. The characteristic is essentially symmetric. The threshold switch described by Ovshinsky (1967, 1968) is an example of this device. 

Case (d). The switching device with memory also has two stable states. The high resistance state and the mode of switching resemble those of case (c). The conducting ON state, which is established after switching by means of a setting current, remains even if the voltage is removed entirely. The OFF state can be re-established by a short current pulse of either polarity. 

Fig. 6.2. Sketch of the time response to a voltage pulse Vp > and to interrogating voltage pulses V < for (a) a threshold switch and (b) a memory switch. Switching from OFF to ON occurs after a delay time tj. The memory is SET after a lock-on time interval LO after switching. The threshold switch (a) returns to OFF after the end of the Vp pulse. The memory switch (b) requires a RESET current pulse to return to the high resistance OFF state.

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