Fundamental Electrical Characteristics

Devices of asymmetric S/D configuration can be operated in two different modes, that is to say, forward and reversed modes, respectively. For the forward mode, the Ni-silicided junction is used as the source while the n⁺-doped junction is used as the drain. On the contrary, the reverse mode measurements were carried out by interchanging the source and drain terminals. Fig. 3-1 illustrates the bias configurations of the ASSB-TFT under forward and reverse operation modes. The experimental transfer characteristics of n-type asymmetric Ni-silicided SB-TFTs are discussed in the following paragraphs.

Figs. 3-2 (a) and (b) show the transfer characteristics of n-type ASSB-TFT devices operated under forward mode with nominal channel length of 0.5 μm and 1 μm, respectively. The drain voltage varies from 0.1V to 1.6V with 0.5V voltage steps.

It is obvious that both devices exhibit a two-step subthreshold swing (SS) (dashed line in Fig. 3-2) with increasingly positive gate voltage (Vg), a feature significantly different from that of conventional MOSFETs, i.e., doped S/D-junctions devices. Such a phenomenon originates from the competition of two different carrier injection

mechanisms, thermionic emission current (J_th) and tunneling current (J_tn) [43], which are conceptually illustrated with the band diagrams shown in Fig. 3-3. As can be seen in the figure, in the subthreshold regions, as the applied Vg is low, thermionic emission current (Jth) dominates the conduction, so only carriers with energy greater than the Schottky barrier height (SBH) contribute to the current, as schematically

displayed in blue in the figure. When Vg is increased to the level Vg = V1, the band of the channel near the source junction becomes flat (dashed line in Fig. 3-3). When Vg

further increases over V1, the current now consists of both the J_th and the J_tn components, represented by the red lines in Fig. 3.3. With a sufficiently high Vg, tunneling current dominates the current flow as the Schottky barrier is thinned.

In brief, when the potential barrier is higher than the SBH at the source side, the thermionic current dominates and thus the SS is nearly a constant. However, when the device is turned on, the main increase in current arises from the tunneling through the SB at the source and is strongly dependent on the shape of the SB which is modulated by Vg. The tunneling current is a function of the SBH between the metal and the semiconductor, the gate dielectric thickness and gate voltage.Fig. 3-4 shows transfer characteristics of devices operated under the forward mode with different gate oxide thicknesses. It can be seen that, with the increase in oxide thickness, the drain current is lowered and thus degrades the device performance. On the other hand, the reduction of gate oxide thickness causes an increase of the electric field under the gate which makes the cell transistor more susceptible to gate-induced drain leakage (GIDL). The GIDL current is the tunneling mechanism caused by band-to-band tunneling or trap-assisted tunneling in the gate-to-drain overlap region and can dominate the drain leakage current even at zero gate bias in field-effect transistors (FET) with ultra-thin gate oxide [44].

Fig.3-5 shows and compares the transfer characteristics of an ASSB-TFT operated in forward and reverse modes, respectively, together with a conventional n-type impurity-doped TFT device with the same channel length/width and gate oxide thickness. It is obvious that the n-type ASSB-TFT in forward mode depicts improved on/off current ratio (ION/IOFF) of more than 10⁶ with low off-current of less than 10^-6

μA/μm (normalized to the channel width of 10 μm), where ION is chosen as the maximal ID and IOFF is the minimal one. The lowered off-current is ascribed to the prohibition of hole tunneling current from the n⁺-doped drain side. Conversely, under reverse operation mode, an undesirable off-state current is ascribed to hole tunneling from the silicided-drain junction, resulting in a deteriorated ION/IOFF of around 10⁴. Note that, it is apparently that the ASSB-TFT operated in forward mode suffers from lower on-current, and a poorer SS. The degraded driving capability is ascribed to the high source resistance due to the high Schottky barrier at the silicon/silicide source junction. The presence of NiSi/Si SB junction offers a potential barrier for electrons of about 0.65 eV [45]. The tunneling distance is spatially modulated by the gate voltage and conduction occurs when the tunneling distance is sufficiently small.

Although conventional SB transistors have several advantages, as was described in Chapter 1, the drawback of abnormally high drain leakage current attributed to the GIDL-like effect is a serious problem, especially for memory application. Several studies have reported that ambipolar conduction could cause misidentification of memory logical states [45-49]. Utilization of an asymmetric S/D structure to eliminate off-state leakage and achieve unipolar conduction has been introduced in previous chapters. Figs. 3-6 and 3-7 depict the energy band diagrams of conventional symmetric Schottky-barrier transistors and the ASSB-TFT devices, respectively, operated at various bias conditions. The band diagrams for ASSB structure and conventional SB devices are quite similar except at the drain region where the metal silicide of the symmetrical structure is replaced by the heavily-doped silicon in ASSB structure. Compared with the conventional Schottky-barrier transistors, the ASSB-TFT structures can significantly suppress the gate-induced drain leakage (GIDL)-like off-current and thus mitigate the undesirable ambipolar conduction of conventional SB devices.

Due to the fact that NiSi is a mid-gap material, SB devices with NiSi serving as S/D could be operated in either n- or p-channel, depending on the gate and drain bias polarity. In this study, we have also fabricated p-type ASSB-TFT devices, namely, the P⁺-doped junction was replaced with BF2+ doped junction schematically shown in Fig.

3-8. The definitions of operation modes are the same as those mentioned above. The transfer characteristics of a p-type ASSB-TFT device operated under forward mode and reverse mode are shown in Fig. 3-9. For the forward mode, the GIDL-like leakage current is suppressed by one order of magnitude as compared with that of the reverse mode, although its on-current is also degraded.

Fig. 3-10 shows the transfer characteristics in forward mode for both n- and p-channel operations under proper bias conditions. For n-channel operation, field emission of electrons from the source junction contributes to the on-state current. On the other hand, when the device is operated at p-channel mode, the on-state current is ascribed to the field emission of holes from the source junction. It is interesting to note that both channel operations exhibit comparable drive capability.Although the barrier height of the NiSi/silicon Schottky junction for holes (~ 0.45eV) is less than that for electrons (~ 0.65 eV), the effective mass of holes for Schottky tunneling is higher than that of electrons. Thus, a comparable on-state current is reasonable.