Nanocrystalline silicon (nc-Si) thin film transistors (TFTs) are well suited for circuit applications that require moderate device performance and low-temperature CMOS-compatible processing below 250°C. Basic logic gate circuits fabricated using ambipolar nc-Si TFTs alone are presented and shown to operate with correct outputs at frequencies of up to 100？kHz. Ring oscillators consisting of nc-Si TFT-based inverters are also shown to operate at above 20？kHz with a supply voltage of 5？V, corresponding to a propagation delay of <10？μs/stage. These are the fastest circuits formed out of nanocrystalline silicon TFTs to date. The effect of bias stress degradation of TFTs on oscillation frequency is also explored, and relatively stable operation is shown with supply voltages >5？V for several hours. 1. Introduction Nanocrystalline silicon (nc-Si) has attracted much interest for use in circuit applications that require low temperature, large-area fabrication, and alternative substrates. Examples of these applications may include neuromorphic architectures for vision sensing [1, 2], flexible and flat-panel displays [3–5], radio frequency identification (RFID) tags [6], solar cells [7], and many more. Many of these applications do not require high performance devices, but rather the viability of a simple and inexpensive fabrication process. Neuromorphic circuits are also defect tolerant to a certain extent, allowing low-yield devices to be employed. Nc-Si thin film transistors (TFTs) can be fabricated using a CMOS-compatible fabrication process [8, 9], which also facilitates three-dimensional integration with high-performance CMOS structures [5, 10, 11]. In such a case, TFT circuitry such as biosensor arrays or neural networks could be fabricated around a CMOS core to improve the overall functionality. The limited use of noncrystalline silicon devices in circuits was historically due to low carrier mobility [12], device degradation under bias stress [13, 14], and the lack of p-channel operation [15]. Use of nc-Si instead of amorphous silicon (a-Si) improves the electron mobility and device degradation to a certain extent. The above drawbacks have been further addressed in the recent years by high purity nc-Si deposition which reduces the incorporation of impurity oxygen in the channel layer and ensures ambipolar operation of the devices [8, 15]. In this context, the term “ambipolar” refers to either n- or p-channel operation depending on the bias conditions. The maximum mobility observed in nc-Si TFTs is 150？cm2/V·s for electrons and 25？cm2/V·s in case of holes
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