Substrate effects on metal-oxide single electron transistors electrometer measurements.

Single electron tunneling transistors (SETs) are the most sensitive electrometers available, capable of measuring small fractions of electron charge, with sensitivities down to 10−6 e/√Hz [1]. This makes the SET the best device for applications which require sensing of small changes of charge in nanostructures. Typically, an SET is used to measure the change in charge within a system while some external voltage Vin is swept, Fig. 1. Since there is a parasitic capacitance, Cp, between the sweeping voltage and the SET, to keep the operating point of the SET stable, a compensating voltage, Vcanc, is added to the SET gate in order to cancel the effect of the sweeping voltage. Ideally, this makes the SET immune to the changes in sweeping voltage so that it only responds to other charge fluctuations in the surrounding area. This cancellation scheme is used in many applications where charge transfer events (Fig. 1) are induced by large external electric fields such as the charging of a memory node in single electron memory devices [2]. The majority of metal based SETs to date are fabricated on oxidized silicon substrates. However, for devices fabricated on oxidized Si substrates, the charge cancellation scheme breaks down once a certain threshold is exceeded and charging effects on the SET island are not controlled (Fig. 2a). These charging processes are revealed as Coulomb blockade oscillations (CBOs), which continue even when the gate voltage ramp is stopped. We believe that inversion/accumulation regions are created under the gate electrodes in the semiconductor substrate while the device is operating. This effect will mask the charge transfer events of interest that the SET is intended to detect; instead, the SET detects the parasitic charging from the semiconducting substrate.