First chapter includes literature survey of structural, electrical and magnetic properties of unsusbtituted (pure) polythiophene and its derivative – poly (3,4- ethylenedioxythiophene) (PEDOT) as well as their applications in electronic devices. Second chapter contains chemical synthesis of polythiophene and PEDOT. It also discusses instrumentation part of various techniques such as elemental analysis, FTIR, UV-visible, XRD, SEM, TGA-DTA, two & four probe method and Guoy’s method used for characterization of polymers. Doping of polythiophene by FeCl3 and iodine for various concentrations and the results obtained using various characterization techniques are discussed in details in chapter three and four respectively. Results obtained due to doping of PEDOT by FeCl3 and CSA using various characterization techniques are discussed in chapter five. Chapter six includes use of polythiophene doped by FeCl3 for fabricating p–n junction temperature sensor and tunnel diode. Various parameters of fabricated tunnel diode are compared with conventional silicon tunnel diode in this chapter. Finally in the chapter seven conclusion and scope for future work is included.