Around us, there are various kinds of sensors, e.g., photo sensor, temperature
sensor, pressure sensor, and so on. Among them, our research focuses on
chare sensors based on a metal-oxide-semiconductor field-effect transistor
(MOSFET) because those merits, i.e., fast sensing, small size, high charge
sensitivity and so on, can be available for various fields. (The basic mechanism of the MOSFET is explained in another page.) The charge sensitivity, which is one of the most important performances
of the sensor, can be enhanced with reduction in the size of the MOSFET.
Fortunately, since the shrinkage of the MOSFET is a trend of electronic
circuits, the trend help us improve the charge sensitivity of the MOSFET
sensor. Another isssue of the sensor is noise, which disturb the target
signal. Actually, noise also disturbs precise operation of electrical circuits.
Therefore, we have been studying sensors, especially, with the considerations
of charge sensitivity and noise from the viewpoint of noise to overcome
noise issue for the MOSFET sensors as well as electrical circuits. As a
result, the sensor based on MOSFETs will be used to detect extremely small
signal, single electrons, single molecules, tiny mechanical motion and
so on, which gives rise to the useful application as well as their microscopic
understanding. That is another reason why I have been studying sensors.
First of all, let me explain a mechanism of a sensor based on a MOSFET.
The MOSFET has a source, drain, and channel through which a lot of charges
travel. Charge flow is detected by ammeter as current flowing through the
MOSFET. Here, I assume that the charge is an electron. When a target with
negative charge gets close to the MOSFET’s channel, the target gives rise
to repulsive force to the electrons flowing through the channel and thus
suppresses the electron flow. As a result, current flowing through the
MOSFET is reduced and such reduction gives information about the target.
The current reduction caused by the target is one of the parameters to
dominate charge sensitivity: lager reduction means higher charge sensitivity.
As easily expected, since larger charge sensitivity is desirable in any
field, the MOSFET sensor requires larger current reduction. The one way
to achieve it is shrinkage of the MOSFET. When the sensor becomes smaller,
the effect of the repulsive force from the target on electrons flowing
through the MOSFET becomes relatively larger and thus reduction in current
flowing through the MOSFET becomes larger, compared to the larger MOSFET.
Another way to improve charge sensitivity is to design the MOSFET structure
so that the target can be closer and closer to the MOSFET’s channel. This
is because the repulsive force from the target can gives larger effect
to electrons flowing through the MOSFET.
How small is the MOSFET to have large charge sensitivity? We demonstrated
that the MOSFET with a tiny channel (<10 nm) allows precise detection
of one electron, which is the minimum charge in the world, at room temperature
(Jpn. Jpn. J. Appl. Phys., v. 47, p. 8305, 2008). Although it is impossible to watch electrons, it is possible to feel one electron by the MOSFET sensor. Experimentally, we are succeeded in monitoring that single electrons flow through the MOSFET one after another. In other words, it is observation of shot noise in the MOSFET and time-domain analysis about shot noise provides us with new physical understanding (Appl. Phys. Lett., v. 98, p. 193502, 2011). In addition, the MOSFET sensor can be used to detect mechanical motion (Appl. Phys. Lett., v. 95, p. 233102 2009) and molecules (Appl. Phys. Lett., v. 94, p. 163106, 2009). On the other hand, since noise also affect charge sensitivity, we have studying noise (Nature Communications, v. 1, DOI:10.1038/ncomms1092, 2010).
