  We demonstrate sensitive detection of single charges using a planar tunnel
junction 8.5nm wide and 17.2nm long defined by an atomically precise phosphorus
doping profile in silicon. The conductance of the junction responds to a nearby
gate potential and also to changes in the charge state of a quantum dot
patterned 52nm away. The response of this detector is monotonic across the
entire working voltage range of the device, which will make it particularly
useful for studying systems of multiple quantum dots. The charge sensitivity is
maximized when the junction is most conductive, suggesting that more sensitive
detection can be achieved by shortening the length of the junction to increase
its conductance.
