An Introduction to Organic Field-Effect Transistor (OFET)
An OFET as a field-effect transistor where organic semiconductor is the semiconducting material in the channel. Organic semiconductors could be deposited by various methods like vacuum evaporation, spin-coating or dip-coating e.t.c to get an OFET. An OFET can be fabricated on different substrated like: glass, plastics or conventional crystalline doped silicon wafer. OFETs when fabricated on the plastic substrates, it gives an OFET flexiblity for the realization of RFID tags and wearable electronic gadgets. When OFETs are fabricated on the glass substrates, we get a transistor for the realization of a transparent electronic circuits (as depicted in the video from Corning Inc.).
Economically, OFETs eliminate expensive fabrication technique which are relatively low-temperature processable as compared to conventional silicon based FETs. Due to low processing temperature, the OFETs could be easily fabricated on very large area because substrates themself are very cheaper compared to silicon. OFET could be very useful electronic device in the field of large area electronics due to its features like: transparancy, flexibility, cheaper and also stretchable. The same deposition process could be also applied towards the realization of cheaper thin-film OFETs or thin-film solar cells or thin-film light emitting diodes (LEDs).
Basic Electrical Characteristics of a long channel length P3HT OFET
To understand the basic electrical behavior in an OFET, a p-type OFET (organic semiconductor is p-doped) with sufficiently large channel length (orders of 10 micrometres) is depicted in the figure (1), with suitable electrical connections. Source is grounded, at the drain bias potential is applied and at the gate suitable control potential is applied. Output current is measured at the drain terminal. For constant gate potential, Id vs. Vds curve is obtained, termed as the output characteristics. And for constant drain potential, Id vs. Vgs curve is obtained, termed as the transfer characteristics. The output curve and the transfer curve are two major electrical charactersitics which describe the complete electrical behavior of an OFET. In this section, output and transfer characteristics of a typical OFET (L >> tox) would be discussed. For the nanoOFETs with channel length similar to oxide layers are termed as short-channel OFET, which would be discussed in another section (study of the short-channel (L = 70nm) OFET).
Fig (1): OFET with electrical connections.
AFM Images of a short-circuited P3HT OFET
Fig (1): AFM image (top-topography and bottom-phase contrast) of a short-circuited P3HT OFET. The figure depicts that semiconductor (P3HT) has melted inside the channel between the contacts. The pictures has been obtained in the AFM at Jacobs University Bremen.
AFM picture was taken from a sample with L = 240 nm OFET (as shown in figure 1). The sample was short-circuited with potential difference of Vds = +10 V. OFET Interestingly, the picture clearly reveals that the semiconductor inside the channel has melted, which is the result of very high current density between the contacts. Thus, P3HT fails to form grain like structures (which is visible above the contacts) with a long-trail of deposited semiconductor.
Experiment results of field dependent Mobility observed in the P3HT OFETs
On application of electric field (E), carriers flow from higher potential to the lower potential region (electrons towards positive potential). The drift velocity (vd) of the carrier linearly increases with the applied electric field. Mathematically,
vd = μ * E
The constant quantity is the mobility of the carieer (μ-for hole or the electron as the carrier). Larger is the mobility higher is the drift velocity and higher is the current density (J).
J = vd (-e) n
where, -e is single unit charge (-1.6E-19 Coloumbs) and n is the electron density (cm-3).
Experimentally, it is observed that mobity of electron at 300 K in a crystalline Silicon solid is 1500 cm2/Vs for n = 5E22 cm-3 .