NOVELTY - Apparatus for chemical and biological detection of molecules has a source contact (104) and a drain contact (106) attached to gate insulator (108), and semiconductor layer (102) between the contacts. The layer comprises organic monolayer molecules comprising conjugated segment covalently bonded to sensing end group and attaching end group. The attaching end group is attached to the insulator.    USE - For chemical and biological detection of molecules adsorbed on organic self-assembled surface. The transistor is used for chemical and biological sensor arrays and electronic noise applications.    ADVANTAGE - The organic self-assembled transistors have monolayer sensitivity, chemically tailored surface, spectroscopic readout, silicon very large integration compatibility and complementary polarity devices, and is compact and cheap. The detection accuracy and calibration of the transistor are easily maintained by periodic cleaning using relatively high current pulses that heat the organic layer and drive off adsorbed molecular species. The related vapor molecules are easily distinguished from each other using a transistor response such as differential response. A very low concentration of volatile component is also detected easily. Even critical locations are continuously monitored without requiring the individual monitoring the sensor.    DETAILED DESCRIPTION - Apparatus for chemical and biological detection of molecules adsorbed on organic self-assembled layer, has a gate insulator attached to gate contact (110), source contact and a drain contact attached to gate insulator, and semiconductor layer between the contacts. The layer comprises organic monolayer molecules comprising conjugated segment covalently bonded to sensing end group and attaching end group. The attaching end group is attached to the insulator.    INDEPENDENT CLAIMS are also included for the following:    (1) Organic self-assembled transistor (100) comprising gate contact, gate insulator, source and drain contacts and semiconductor layer fabricated on a silicon substrate (112);    (2) Process of making the transistors which involves high temperature processing of silicon to generate substrate, gate, gate insulator, source and drain, and depositing active organic monolayer; and    (3) Chemical and biological sensor array system which comprises silicon circuitry connecting an array of transistor sensors and processing module.    DESCRIPTION OF DRAWING(S) - The figure shows the schematic illustration of organic self-assembled transistor.    Semiconductor layer (102)    Source contact (104)    Drain contact (106)    Gate insulator (108)    Gate contact (110)    Silicon substrate (112)