The Effect of Bias Polarity Dependence on the DC Electrical Characteristics and Sensitivity of Silicon Nanowires for Biosensing Application

This thesis submitted in partial fulfillment of the requirements for the degree of B.Sc in Electrical and Electronic Engineering of East West University, Dhaka, Bangladesh.We investigate the effect of bias polarity on the DC electrical characteristics of p-type silicon nanowires and its effect on the gate sensitivity for possible application as biosensors. A 75 nm thick nanowire with doping concentration of 1014 cm-3 is investigated for different channel lengths. It is found that when drain and gate voltage both are positive nanowires ID-VD characteristics typically exhibit non-linear diode like characteristics with no appreciable conduction up to a certain level of drain bias. The gate sensitivity of nanowires at this mode of conduction is estimated and found to be 18.9%/V for a channel length of 1 μm which reduces to a value of 12.5%/V for the channel length of 100 nm. When gate voltage is positive and drain voltage is negative no appreciable conduction is observed and the gate effect is diminished at short channel lengths which implies that at this mode of conduction p-type Si NW might not be suitable for biosensors. When gate voltage is negative and drain voltage is positive nanowires exhibit less non-linearity and with the increase of negative gate voltages perfectly linear characteristics is achieved through the accumulation of holes in the nanowire by the gate effect. The sensitivity of nanowires at this mode of operation is found to be 11.318%/V and 7.549%/V respectively for 1 μm and 100nm channel length for a gate voltage change from -2v to -1.5V with a drain bias set to 8V. These levels of sensitivity agree very well with the reported sensitivity of silicon nanowire biosensors. However, when both gate and drain voltages are negative a drastic change of the nanowire characteristics is observed and the 75nm thick nanowires NWs are behaving perfectly as a transistor. It is observed that in this mode of operation sensitivity is very high compare to the other modes of operation for all channel lengths. For 1μm channel length, sensitivities obtained are 199.866%/V (when VG =1-1.5), 154.373%/V (when VG =1.5-2) and 103.604%/V (when VG =2.5-2). For 100nm channel length, sensitivities obtained are 35.019%/V (when VG =1-1.5), 42.058%/V (when VG =1.5-2) and 38.333%/V (when VG =2.5-2). This result indicates that for p-type Si NW application of negative voltages to both VG and VD are the most viable mode for biosensor operation. We also investigated sub threshold characteristics of nanowires and it is observed that nanowires can be set to exhibit excellent sensitivity with an appropriate VG range if it can be ensured that NWs fall within the subthreshold region of operation. Sensitivity values of 285.713%/V and 174.187%/V are achieved respectively for 1 μm and 100 nm channel lengths with positive drain voltage and negative gate voltages and sensitivity values of 307.6 %/V and 285.7 %/V are achieved respectively for 1 μm and 100 nm channel lengths with negative voltages both at drain and gate by ensuring sub-threshold regime of operation. This investigation reveals the requirement of appropriate biasing scheme for highly sensitive bio sensor operation and is very significant for nano wire based bio sensor applications as transistor behavior can be set by choosing appropriate bias conditions which would allow large conductance charge upon attachment of Bio molecules and a highly sensitive biosensor could be realized