In the following we present ring oscillator circuits that employ MESFETs and JFETs based on oxide semiconductors. These experiments show that (i) based on both these device technologies integrated circuits are feasible, (ii) the electrical properties are superior to the traditional concept (MISFET), and (iii) the timedependent properties of the circuits allow to quantify dynamical properties of the involved MESFET and JFET. An n-type channel ZnO, probably the most thoroughly studied oxide semiconductor, is used. We employ SDFLlike circuit technology (see, e.g., ref.[15]) in order to obtain full inverters using only unipolar n-type channels. Our bipolar gates represent a completely novel materials based approach to oxide circuits, possibly allowing a decrease of driving voltage by a factor of ten. Thus, our choice of materials allows a low power oxide device and circuit concept.
Results and DiscussionTypical signals measured at the output of ring oscillators are presented in the inset of Figure 1. At frequencies below ≈500 kHz a rectangular signal is observed, with output voltages from V DD to nearly 0 V. For higher frequencies, V DD is not reached. This might be due to bandwidth limitations of the measurement setup.The oscillation frequency in dependency on V DD is depicted in Figure 1 for three-stage ring oscillators from all samples. The oscillations start at a certain V DD , which can be identified as voltage drop V shift across the level shifter diodes (see Figure 2 for an inverter circuit schematic). For V DD < V shift the inverter output voltage range does not match the input voltage range and oscillations cannot occur. Within the voltage interval V shift < V DD < V shift + 1 V a shift in the frequency can be observed for many devices. For larger V DD the frequency saturates and stays approximately constant within the measured voltage range. This is expected for SDFL based ring oscillators, as the supply currents are limited by the pull-up and pull-down transistors which act as constant current sources. In the following we examine each oscillator at an individual operating voltage V DD = V shift + 1 V.In order to understand the frequencies obtained for different samples an estimation of the gate delay τ D based on easily measurable FET and inverter quantities has been developed.Ring oscillator circuits based on junction field-effect transistors as well as metal-semiconductor field-effect transistors with ZnO channels are presented. Single stage delay times down to 110 ns are observed. The experimental oscillation frequencies are related to easily measurable device properties by a simple analytical model. This work proves the feasibility of low power oxide based circuits with Schottky diode and bipolar (pn-) diode gates since both approaches provide significantly lower operation voltages at similar frequencies compared to previously reported oxide thin film transistors based on insulating gates.
