A method of fabricating a monolithic sensor unit comprising a sensor and field-effect transistor formed within a layered semiconductor stack, the layered semiconductor stack comprising a substrate, a first semiconductor device layer for forming the field-effect transistor deposited on the substrate, a second semiconductor device layer deposited on the first semiconductor device layer, and a common contact layer for the first semiconductor device layer and the second semiconductor device layer, wherein the second semiconductor device layer provides an active region for the sensor, wherein the common contact layer connects the active region for the sensor in series with the field-effect transistor, and wherein the method includes the steps of: selectively etching a portion of the second semiconductor device layer to expose a surface of the first semiconductor device layer; and performing a wet etch process on a portion of the first semiconductor device layer to form a gate recess in the first semiconductor device layer, wherein the wet etch process comprises: applying a native oxide removal solution to remove native oxide from the exposed surface of the first semiconductor device layer, and applying an etchant solution to remove material from the first semiconductor device layer and thereby form the gate recess, wherein the sensor unit consists of three electrodes, the three electrodes being formed by: depositing a gate electrode in the gate recess to form a gate electrode of the field-effect transistor; depositing a first contact electrode on the surface of the first semiconductor device layer; and depositing a second contact electrode on a surface of the second semiconductor device layer; wherein the common contact layer and the first contact electrode act as the source and drain of the field-effect transistor, wherein the active region for the sensor is disposed between the common contact layer and the second contact electrode, and wherein the first contact electrode and the second contact electrode are arranged such that a current between the first contact electrode and the second contact electrode is controllable based on a bias voltage applied to the gate electrode. selectively etching a portion of the second semiconductor device layer to expose a surface of the first semiconductor device layer; and performing a wet etch process on a portion of the first semiconductor device layer to form a gate recess in the first semiconductor device layer, wherein the wet etch process comprises: applying a native oxide removal solution to remove native oxide from the exposed surface of the first semiconductor device layer, and applying an etchant solution to remove material from the first semiconductor device layer and thereby form the gate recess, applying a native oxide removal solution to remove native oxide from the exposed surface of the first semiconductor device layer, and applying an etchant solution to remove material from the first semiconductor device layer and thereby form the gate recess, wherein the sensor unit consists of three electrodes, the three electrodes being formed by: depositing a gate electrode in the gate recess to form a gate electrode of the field-effect transistor; depositing a first contact electrode on the surface of the first semiconductor device layer; and depositing a second contact electrode on a surface of the second semiconductor device layer; depositing a gate electrode in the gate recess to form a gate electrode of the field-effect transistor; depositing a first contact electrode on the surface of the first semiconductor device layer; and depositing a second contact electrode on a surface of the second semiconductor device layer; wherein the common contact layer and the first contact electrode act as the source and drain of the field-effect transistor, wherein the active region for the sensor is disposed between the common contact layer and the second contact electrode, and wherein the first contact electrode and the second contact electrode are arranged such that a current between the first contact electrode and the second contact electrode is controllable based on a bias voltage applied to the gate electrode.