Abstract: A sensing element includes a field-effect transistor (FET) with an ultra-thin channel, a reference electrode, a first and a second passivation layer, and a microchannel. The first and the second passivation layer enclose a first and a second portion of the FET, respectively. The microchannel is bonded to the first and the second passivation layer, such that the microchannel is extended across the channel of the ultra-thin channel FET. The ultra-thin channel has a chemically or physically modified surface. When an analyte to be tested passes through the microchannel and is in contact with the modified surface of the ultra-thin channel, it results in changes in the conductance of the ultra-thin channel FET. Trace detection may be conducted on the analyte by observing changes in the conductance. A method for manufacturing the sensing element and a biological detection system employing the sensing element are also provided.