A novel implantless MOS thin-film transistor with simple processing, excellent performance and ambipolar operation capability

Abstract

A novel thin-film transistor (TFT) device that requires no implant step and capable of ambipolar operation is proposed and successfidly demonstrated. The new structure (Fig. 1) features an undoped Si active channel, a top metal field-plate (i.e., the sub-gate), and Schottky sourddrain. The equivalent circuit of the device is as shown in Fig. 2. During device operation, a high fixed voltage is applied to the sub-gate to form a field-induced sourddrain layer under the sub-gate region. Depending on the polarity of the sub-gate bias, the device can be set for n-channel operation with positive sub-gate bias, and p-channel operation with negative sub-gate bias. The new device is similar to conventional Schottky barrier (SB) , with the exception of a field-induced sourddrain region between the channel and Schottky sourddrain.The existence of the field-induced sourddrain region serves to supply abundant channel carriers during on-state, while reducing the notorious off-state leakage that have plagued all previous SB MOSFETs. The new device is also similar to MOSFETs with field-induced drain (FID)[5] [6], except that the heavily-doped sourddrain region is replaced by Schottky sourddrain. While retaining all the advantages of FID such as low off-state leakage and low junction leakage, the use of Schottky sourddrain not only reduces processing steps (i.e., implant and annealing), but also allows ambipolar operation, thus greatly simplify processing steps especially for CMOS process integration. Figure 3 shows the key fabrication steps for the proposed device. It should be noted that neither channel nor source/drain doping was used, so no subsequent annealing step was necessary. The sub-gate was formed during regular metal patterning, thus no extra steps were required. m e r metal patterning, wafers received further plasma treatment in NH, for 1 hour. Figure 4 shows the effect of sub-gate bias on the subthreshold characteristics of the fabricated device. Superior p-and n-channel device characteristics are simultaneously realized, for the first time, on the same device when a proper bias is applied to the sub-gate. It should be emphasized that the superior p-and n-channel characteristics are obtained on the same device by simply changing the polarity of the sub-gate bias. Figures 5 and 6 compare the subthreshold characteristics of a device with subgate bias of k 50 and f 30 V, respectively. The associated output characteristics are shown in Figs. 7-10. The effects of sub-gate bias in improving the o d o f f current ratio and the device drive capability are clearly evident. The odoff current ratios in Fig.5 for both n-and p-channel operations are around lo6. More importantly, no GIDL (gate-induced drain leakage)-like leakage is found in the off state (i.e., the drain current remains flat under off-state gate bias). In Fig.7, negative differential conductance (NDC) characteristics are observed when V, and V, are high. This could presumably be due to the trapping of electrons near the source side caused by hot electron injection...

Reconfigurable SOI-MOSFET: Past, Present and Future Applications

“…Ambipolarity is sometimes treated as an undesired effect, since it increases the leakage current (high off-state currents). In a study on Schottky barrier Thin-Film Transistors (TFT) [8] it was proposed the use of a second gate for the device (Fig. 2).…”

“…This second gate is currently called the programming gate and is another important concept for today's reconfigurable transistors. These works ( [5] and [8]) provide the bases for RFETs independently, from structures that share a thin layer of semiconductor on insulator, a programming gate to select the carriers to be used (electrostatic doping) and contacts Schottky with the semiconductor. In other words, the concept of reconfigurable transistors emerges naturally from the analysis of devices with these basic properties.…”

Ambipolar Schottky-barrier TFTs

“…This is due to the constraint imposed by the fixed barrier height of the Schottky contact of a given metallic layer. In this work, we have demonstrated a novel poly-Si Schottky barrier thin-film transistor (SBTFT) device that can effectively improve the on/off current [8]- [11]. The new device employs a field-plate (or subgate) to induce an electrical drain extension in the active poly-Si layer.…”

Schottky barrier thin-film transistor (SBTFT) with silicided source/drain and field-induced drain extension

“…In this work, we proposed and demonstrated, for the first time, a novel poly-Si TFT device that can effectively alleviate the aforementioned shortcomings [7]. The new device features a field-plate (or subgate) that can induce an electrical drain extension in the active poly-Si layer.…”