Source and drain

Step 8. The -type source and drain regions are created by As ion implantation. The As can penetrate the thin gate oxide, but not the thick field oxide or the polysihcon gate. The formation of the source and gate does not require a separate resist pattern, thus this technique is called self-aligning. 

Fig. 9. Fabrication sequence for an oxide-isolated -weU CMOS process, where is boron and X is arsenic. See text, (a) Formation of blanket pod oxide and Si N layer resist patterning (mask 1) ion implantation of channel stoppers (chanstop) (steps 1—3). (b) Growth of isolation field oxide removal of resist, Si N, and pod oxide growth of thin (<200 nm) Si02 gate oxide layer (steps 4—6). (c) Deposition and patterning of polysihcon gate formation of -source and drain (steps 7,8). (d) Deposition of thick Si02 blanket layer etch to form contact windows down to source, drain, and gate (step 9). (e) Metallisation of contact windows with W blanket deposition of Al patterning of metal (steps 10,11). The deposition of intermetal dielectric or final...

The MOSEET has three regions of operation. The cutoff region occurs for V g < Up. In this region, the drain-to-source current is the reverse saturation current of the back-to-back source and drain junctions. This leakage current is small but nonzero and allows charge to leak off capacitors which are isolated by cutoff MOSFETs. Because this is how bits are stored in dynamic memory (DRAM) ceUs, DRAMs must be regularly refreshed to retain their memory. 

Fig. 1. Schematic for chemoreceptor-modified ISFET biosensor for detection of acetylcholine where the source and drain are both n-ty e siHcon.

Conventional electronic devices are made on silicon wafers. The fabrication of a silicon MISFET starts with the diffusion (or implantation) of the source and drain, followed by the growing of the insulating layer, usually thermally grown silicon oxide, and ends with the deposition of the metal electrodes. In TFTs, the semiconductor is not a bulk material, but a thin film, so that the device presents an inverted architecture. It is built on an appropriate substrate and the deposition of the semiconductor constitutes the last step of the process. TFT structures can be divided into two families (Fig. 14-12). In coplanar devices, all layers are on the same side of the semiconductor. Conversely, in staggered structures gate and source-drain stand on opposing sides of the semiconductor layer. 

FET is ihe on-off current ratio, which indicates its ability to shut down the current, and is particularly relevant in applications such as active matrix displays and logical circuits. Because of the presence of p-n junctions at both the source and drain electrode, the on-off ratio of MOSFETs is in the I0y range [12J, while that of a-Si H TFT is limited to 106 [13], High mobility ensures high on-current and, hence, also contributes to a high on-off ratio. 

In a MESFET, a Schottky gate contact is used to modulate the source-drain current. As shown in Figure 14-6b, in an //-channel MESFET, two n+ source and drain regions are connected to an //-type channel. The width of the depletion layer, and hence that of the channel, is modulated by the voltage applied to the Schottky gate. In a normally off device (Fig. 14-9 a), the channel is totally depleted at zero gate bias, whereas it is only partially depleted in a normally on device (Fig. 14-9 b). 

A big problem with /t-type materials is their laigc instability in oxygen. This is exemplified by CW), the mobility of which can be as high as 0.08 cm" V 1 s l in ultra-high vacuum, but falls by four or five orders of magnitude upon exposure to air [105]. This could be due to problems of contacts, as illustrated by the fact that modifying the surface of the source and drain electrodes with tetrakisdimethylami-noethylene (TDAE) leads to a substantial increase in the mobility [I05. ... 

CCD detector designers try to increase the signal-to-noise ratio of an amplifier in two ways (1) increase the responsivity, or (2) decrease the random current fluctuation between source and drain. The responsivity can be increased by decreasing the amplifier size. Decreasing the amplifier size decreases the capacitance of the MOSFET. The responsivity of a MOSFET obeys the capacitor equation which relates voltage, V, to the charge Q on capacitance C V = QIC. 

Sources" are formed in the first step and then "Gates" are formed. A silicide is then used to lower the contact resistance between the silicon of the gate, source and drain and the contacting plug. The thickness of gate oxides is only about 40-50 A. They are thermally grown rather than... 

The major steps are labeled 1 to 5. However, there are severed intermediate steps also involved. At "1", the shcdlow trenches cire formed, source and drain implants were accomplished, diffusion barriers were formed from Ti/TiN and a dummy gate made from Si3N4 was deposited on... 

Figure 2. Scanning electron micrograph of a SE transistor with source and drain ( + U/2 and —U/2, respectively) feeding the central island (Insel), which is capacitively coupled to a gate electrode. The size of the central island is about 60 nm x 60 nm.

FIGURE 10.3 A schematic diagram of an ISFET pH measurement system. A potential responding to the hydrogen ions concentration in the solution is established on the pH sensitive gate-solution interface of the ISFET. It electrostatically influences the current (4-d) flow between the source and drain, therefore, the current 4 d change is directly related to the pH change in the solution. 

The addressing of nanoelectronic assemblies metal-molecule (nanocluster)-metal with device-like functions, such as rectifiers, switches, or transistors requires a source and a drain, and one or more localized electronic levels. The roles of source and drain (both as working electrodes WEI and WE2) may be represented by the tip of an STM, combined with an appropriate substrate or, alternatively, a pair of nanoelectrodes see Fig. 3. 

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