A system for sensing at least one type of molecules in a gas or liquid sample, comprising: at least one multi-gate field effect transistor, comprising: a piece of semiconductor with an active region extending between a source region and a drain region, and left and right lateral regions (110, 112) extending along the active region on different sides, wherein the source region, drain region and active region are doped with dopants of a same sign, and the left and right lateral regions are doped with dopants of an opposite sign to the source region, drain region, and active region; left and right lateral gate electrodes (124, 126) that respectively produce an electric field in the left and right lateral regions (110, 112), creating a conducting channel connecting the source and drain regions (106, 108) in the active region when appropriate voltages are applied to them, a position of the conducting channel depending on the applied voltages; and a sensing surface adjacent to the active region, that molecules of the at least one type adhere to when the sensing surface is exposed to the molecules, the conductivity of the conducting channel being measurably affected by a local concentration of the adhering molecules near the position of the conducting channel; and a controller adapted to successively apply different voltages to the lateral gate electrodes (124, 126) of the transistor, such that depletion regions are produced adjacent to the left and right lateral regions (110, 112) leaving the conducting channel in between, the conducting channel moving to a plurality of different positions when the different voltages are applied by changing the voltage of the left lateral electrode relative to the voltage of the right lateral electrode, and at each position to measure its conductivity. at least one multi-gate field effect transistor, comprising: a piece of semiconductor with an active region extending between a source region and a drain region, and left and right lateral regions (110, 112) extending along the active region on different sides, wherein the source region, drain region and active region are doped with dopants of a same sign, and the left and right lateral regions are doped with dopants of an opposite sign to the source region, drain region, and active region; left and right lateral gate electrodes (124, 126) that respectively produce an electric field in the left and right lateral regions (110, 112), creating a conducting channel connecting the source and drain regions (106, 108) in the active region when appropriate voltages are applied to them, a position of the conducting channel depending on the applied voltages; and a sensing surface adjacent to the active region, that molecules of the at least one type adhere to when the sensing surface is exposed to the molecules, the conductivity of the conducting channel being measurably affected by a local concentration of the adhering molecules near the position of the conducting channel; and a piece of semiconductor with an active region extending between a source region and a drain region, and left and right lateral regions (110, 112) extending along the active region on different sides, wherein the source region, drain region and active region are doped with dopants of a same sign, and the left and right lateral regions are doped with dopants of an opposite sign to the source region, drain region, and active region; left and right lateral gate electrodes (124, 126) that respectively produce an electric field in the left and right lateral regions (110, 112), creating a conducting channel connecting the source and drain regions (106, 108) in the active region when appropriate voltages are applied to them, a position of the conducting channel depending on the applied voltages; and a sensing surface adjacent to the active region, that molecules of the at least one type adhere to when the sensing surface is exposed to the molecules, the conductivity of the conducting channel being measurably affected by a local concentration of the adhering molecules near the position of the conducting channel; and a controller adapted to successively apply different voltages to the lateral gate electrodes (124, 126) of the transistor, such that depletion regions are produced adjacent to the left and right lateral regions (110, 112) leaving the conducting channel in between, the conducting channel moving to a plurality of different positions when the different voltages are applied by changing the voltage of the left lateral electrode relative to the voltage of the right lateral electrode, and at each position to measure its conductivity.