Organic electronics has been a subject of growing interest recently. Different organic sensors and basic digital circuits were reported and, more recently, rather complex digital circuits and analogue functions, including data converters, have been demonstrated. The vast majority of organic circuits employ p-type-only technologies with a single transistor threshold. Due to the very high input resistance of parameter analyzers, it is trivial to measure the static characteristic of these digital gates without loading them. However, the relatively low input resistance of a commercial picoprobes (1 to 10MO), together with the reduced driving capability of organic digital gates (Iout~1µA) and the large bias voltage traditionally needed for organic circuits, dramatically influences the behaviour of the digital gates when one wants to perform dynamic measurements on wafer. This is a major issue for experimental wafer-level characterization. To solve this problem, a digital buffer was designed (Fig. 1a) to measure digital gates with a commercial picoprobe (Picoprobe 12C by GGB Industries – Rin = 1MO, Cin = 0.1pF). The output stage (M3 and M4) employs two identical transistors (W/L = 2000/5). The signal at the gate of M3 is just buffered to the output, and thus M3 can be connected directly to the input. On the other hand, M4 introduces an inversion; hence its input signal needs to be inverted. This operation is carried out by the first stage (M1 and M2), which is an inverter employing a diode connected load for better speed. The other two schematics in Fig.1 represent state of the art solutions to obtain a buffer, as cascade of two inverters. The two basic topologies of inverter can have either a diode connected (Fig. 1b) or a 0Vgs load (Fig. 1c). The DC transfer characteristics of the three topologies are shown in Fig.2, and a transient simulation of some relevant signals is shown in Fig. 3. The output swing is strongly improved with respect to the solution which does not use the proposed buffer. The circuit discussed is very useful for the dynamic measurements of p-only organic digital gates, which are characterized by a reduced current drive capability. Measurements of this circuit are planned before the conference.
|Publication status||Published - 2011|