  Photodetectors based on two-dimensional (2D) atomically thin semiconductors
suffer from low light absorption, limiting their potential for practical
applications. In this work, we demonstrate a high-performance MoS2
phototransistors by integrating few-layer MoS2 on a PN junction formed in a
silicon (Si) substrate. The photovoltage created in the PN junction under light
illumination electrically gates the MoS2 channel, creating a strong
photoresponse in MoS2. We present an analytical model for the photoresponse of
our device and show that it is in good agreement with measured experimental
photocurrent in MoS2 and photovoltage in the Si PN junction. This device
structure separates light absorption and electrical response functions, which
provides us an opportunity to design new types of photodetectors. For example,
incorporating ferroelectric materials into the gate structure can produce a
negative capacitance that boosts gate voltage, enabling low power, high
sensitivity phototransistor; this, combined with separating light absorption
and electrical functions, enables advanced high-performance photodetectors.
