Kuan-Lin Yeh

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

A novel Schottky barrier thin-film transistor (SBTFT) with silicided source/drain and field-induced drain (FID) extension is proposed and demonstrated. In the new device configuration, a metal field-plate (or sub-gate) lying on the passivation oxide is employed to induce a sheet of carriers in a channel offset region located between the silicided drain and the active channel region underneath the main gate. The new device thus allows ambipolar device operation by simply switching the polarity of the bias applied to the field plate. In contrast to the conventional SBTFT that suffers from high GIDL (gate-induced drain leakage)-like off-state leakage current, the new SBTFT with FID is essentially free from the GIDL-like leakage current. In addition, unlike the conventional SBTFT that suffers from low on-off current ratio, the new device exhibits high on/off current ratio up to 10/sup 6/ for both n- and p-channel modes of operation. Moreover, the implantless feature and the ambipolar capability of the new device also result in extra low mask count for CMOS process integration. These excellent device characteristics, coupled with its simple processing, make the new device very promising for future large-area electronic applications.

Ambipolar Schottky-barrier TFTs

A novel Schottky-barrier metal-oxide-semiconductor thin-film transistor (SBTFT) was successfully demonstrated and characterized. The new SBTFT device features a field-induced-drain (FID) region, which is controlled by a metal field-plate lying on top of the passivation oxide. The FID region is sandwiched between the silicided drain and the active channel region. Carrier types and the conductivity of the transistor are controlled by the metal field-plate. The device is thus capable of ambipolar operation. Excellent ambipolar performance with on/off current ratios over 10/sup 6/ for both p- and n-channel operations was realized simultaneously on the same device fabricated with polysilicon active layer. Moreover, the off-state leakage current shows very weak dependence on the gate-to-drain voltage difference with the FID structure. Finally, the effects of FID length are explored.

A novel thin-film transistor with self-aligned field induced drain

In this letter, a novel thin-film transistor with a self-aligned field-induced-drain (SAFID) structure is reported for the first time. The new SAFID TFT features a self-aligned sidewall spacer located on top of the drain offset region to set its effective length, and a bottom gate (or field plate) situated under the drain offset region to electrically induce the field-induced-drain (FID). So, unlike the conventional off-set-gated TFTs with their effective FID length set by two separate photolithographic masking layers, the new SAFID is totally immune to photomasking misalignment errors, while enjoying the low off-state leakage as well as high turn-on characteristics inherent in the FID structure. Polycrystalline silicon TFTs with the new SAFID structure have been successfully fabricated with significant improvement in the on/off current ratio.

A novel methodology for extracting effective density-of-states in poly-Si thin-film transistors

A novel methodology that greatly simplifies the procedure of extracting the effective density-of-states (DOS) in polycrystalline-Si thin-film transistors (poly-Si TFTs) is proposed and demonstrated. The characterization is performed on a Schottky barrier (SB) TFT with electrical source/drain extensions induced by a field-plate. Only one single device and two simple subthreshold I-V measurements at room temperature are needed for full band-gap DOS extraction. Impacts of different process treatments are clearly resolved using this methodology.

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

A novel thin-film transistor (TFT) device that requires no implant step and is capable of ambipolar operation is proposed and successfully demonstrated. The new structure features an undoped Si active channel, a tap metal field-plate (i.e., the sub-gate), and Schottky source/drain. The equivalent circuit of the device is given. During device operation, a high fixed voltage is applied to the sub-gate to form a field-induced source/drain 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) MOSFET devices, with the exception of a field-induced source/drain region between the channel and Schottky source/drain. The existence of the field-induced source/drain region serves to supply abundant channel carriers during on-state, while reducing the notorious off-state leakage that has plagued all previous SB MOSFETs. The new device is also similar to MOSFETs with field-induced drain (FID), except that the heavily-doped source/drain region is replaced by Schottky source/drain. While retaining all the advantages of FID such as low off-state leakage and low junction leakage, the use of Schottky source/drain 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.

Conduction mechanisms for off-state leakage current of Schottky barrier thin-film transistors

Conduction mechanisms for the off-state leakage in Schottky barrier thin-film transistor were explored. It was found that the field-emission process dominates the leakage conduction of the device with the conventional structure as the field strength in the drain junction becomes high, and results in the strong gate-induced drain leakage (GIDL) like phenomenon. In contrast, for the device with a field-induced-drain structure, the high-field region is pulled away from the silicided drain. As a result, the field-emission conduction is eliminated, so the GIDL-like leakage current is effectively suppressed.

Reduction of Off-State Leakage Current in Schottky Barrier Thin-Film Transistors (SBTFT) by a Field-Induced Drain

Detailed conduction mechanisms in a conventional Schottky barrier thin-film transistor (SBTFT) and a recently proposed novel SBTFT with field-induced drain (FID) extension have been studied. The new SBTFT device with FID extension shows excellent ambipolar performance with effective suppression of gate-induced drain leakage (GIDL)-like off-state leakage that plagues conventional SBTFT devices. By characterizing the activation energy of the conduction process in the off-state for conventional SBTFT devices, it is suggested that field emission of carriers from the drain junction is the major conduction mechanism. While for the FID SBTFT devices, owing to the effect of Fermi level pinning in the FID region, thermionic emission rather than field emission becomes the dominant conduction mechanism, resulting in the effective suppression of the undesirable GIDL-like leakage current.

Fabrication and Characterization of Schottky Barrier Polysilicon Thin-Film Transistors with Excimer-Laser Crystallized Channel

Schottky barrier thin-film transistors (SB TFT) with field-induced drain (FID) have recently been demonstrated to exhibit ambipolar capability with low off-state leakage current. In this study, we investigate and compare the characteristics of poly-Si SB TFTs with channel layer prepared by excimer laser crystallization (ELC) and solid-phase crystallization (SPC). It is shown that the use of ELC could greatly improve the device characteristics, comparing to the SPC counterparts. Excellent device performance with steep subthreshold slope and on/off current ratio higher than 108 for both p- and n-channel operations are demonstrated on a single device with ELC channel. The effects of sub-gate bias, channel length, and channel offset length, on device characteristics are also explored.

Excellent Ambipolar Characteristics on Schottky Barrier Thin-Film Transistors with Excimer Laser Annealing Treatment

l.Introduction Recantly we have proposed and demonsfrated a novel Schottky barrier (SB) poly-Si TFT with field-induced-drain (FD) extension tll-ts]. Otr experimental results indicate that such device is capable of ambipolar operation and does not suffer from GIDL-like off-state leakage curent tll-ts]. Poly-Si films prepared using solid-phase crystallization (SPC) technique were employed as the active channel layers in our previous sfudy. Such materials, however, are known to suffer from the small grain size as well as large amount of defects both at grain boundaries and inside the grains. In this worlq in order to ftrttrer enhance the device performance, we fabricated and charactdzed, FID SB TFTs having poly-Si channel layers prepared by excimer laser annealing (ELA).

Fabrication and Characterization of Poly-Si Schottky-Barrier Thin-Film Transistors

Poly-Si Schottky-barrier thin-film transistors (SB-TFTs) were fabricated and characterized. In this study, SB-TFTs were first fabricated by using a conventional sidewall spacer to isolate the gate and S/D regions during salicidation. However, it was found that these SB-TFTs depict very poor on/off current ratio ( 3 ) as well as severe GIDL (gate-induced drain leakage)-like leakage current. To overcome these shortcomings, a novel SB-TFT structure is also fabricated in this study to improve the device performance. The new device consists of a field-induced-drain region (FID), which is an offset drain region controlled by a metal field-plate lying on top of the passivation oxide. The FID region is sandwiched between the silicided drain and the active channel region. Carrier types and the conductivity of the transistor are controlled by the metal field-plate. Since the metal field plate is formed simultaneously with the regular metal patterning, no additional processing steps are required. Our results show that the new device can significantly improve the on/off current ratio to over 10 6 for both p- and n- channel operations, while effectively eliminating the GIDL-like leakage.