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Electrical Characteristics of P3HT nanoOFET (L = 70 nm)

Conventionally OFETs with channel length less then 1 um (L < 1 um) is termed as nanoOFETs. When transistor size is sufficiently small i.e. channel length is in the orders or smaller than the oxide thickness (L <= tox), transistor deviates from its normal operation and exhibits different short-channel behaviors, like: failure of drain current saturation at higher drain potential, difficult in switching OFF the transistor, higher threshold voltage, lowering of gate-effect and drain induced barrier lowering (DIBL). Such transistors are also termed as short channel OFETs. Typical output curve of the short channel L = 30 nm OFET as observed by Japanese Research Group [1] is depicted in the figure. Now, before going into detail discussion on nanoOFETs, it is necessary to understand the transport behavior in long channel OFETs. OFETs with channel length (L) less than 1 um is termed as nanoOFETs. In such OFETs, when gate-potential is sufficiently larger than the drain-potential, transitor output current saturates. However, in nanoOFETs, transistor fails to produce saturated output current. In other words, output current changes non-linearly with drain-potential. In this section, two major short-channel behavor in L = 70 nm OFET is discussed in detail, which includes: non-saturating behavior and lowering of gate effect in nanoOFETs.

 

Figure (1): Output curve of the L = 30 nm short channel OFET depicting non-linear dependence of output current with drain potential, observed for different gate potentials [1].

 

Figure (2): Structure of a simple short-channel OFET (top) with channel length similar to oxide thickness. Schematic diagram exhibiting the space-charge denstiy (for holes) along the channel in the short-channel OFET (bottom).

 

Figure (3): Output characteristic of sample DDE121: L= 70 nm and W = 60 um fabricated on tox = 58 nm insulator. Measurements are inside the N2 glovebox.

 

Figure (4): Transfer characteristic of sample DDE121_L = 70 nm and W = 60 um fabricated on tox = 58 nm insulator. Measurements are inside the N2 glovebox. Id is plotted in the logarithmic scale.

 

The non-linear current behavior in nanoOFET is described by the term failure of gradual channel approximation. The gradual channel approximation model has assumed that channel length is much longer than the oxide thickness. When channel length if much longer than the oxide layer, then the change of electrical field along the lateral direction is much smaller compared with change in vertical direction. At this situation, there would be no influence of source electric-field on the drain contact. However, in nanoOFETs, channel length is smaller than the oxide thickness. Hence, the assumption that the electrical field at gate-drain junction in the OFET is independent on the electrical field from the source-drain junction is invalid. Channel potential at the source-drain junction would also be influenced by the source-gate potential. Channel hence fails to pinch-off(depletion of holes) producing non-saturating output curves (as shown in figure (1)), for different gate-potentials.

Further in organic semiconductor based OFETs, mobility is quite low. Thus charge injected into the channel from the contacts concentrate forming space-charge carrier region around the contact (source in p-type OFET). In nanoOFETs, with decreasing chanel length the probability of such space-charge carriers reaching the second contact (drain) is much higher. Failure of gradual channel approximation and presence of high space-charge carriers would ultimately break the saturation in nanoOFETs. Such nanoOFETs are also termed as short-channel OFETs. Short-channel behavior was observed in P3HT OFETs with L/tox ratio is less than 10.

Short-channel effect is also observed in the transfer characteristic of the nanoOFET. Firstly, transistor fails to switch OFF and secondly, OFF current dramatically increases with increasing Vds, as shown in figure (4). ON-OFF current ratio at Vgs = +/- 3 V is only 2.5 when measured at Vds = -1 V. Increment in OFF current would directly influence the threshold voltage. Which follows the fact that 'Short-channel devices are difficult to switch-OFF'.

It must be noted that, short-channel behavior in crystalline silicon based short-channel nanoFETs is due to velocity saturation [2].

Note: Shadow deposition was opted for the fabrication of L = 70 nm device.

 

Refrences

[1] T. Hirose, T. Nagase, T. Kobayashi, R. Ueda, A. Otomo, and H. Naito, "Device characteristics of Short-Channel Polymer Field-Effect Transistors," Appl. Phys. Lett., vol. 97, pp. 83301-83303, 2010.

[2] S. M. Sze, Physics of Semiconductor Devices. Singapore: John Wiley and Sons, 2007.

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Last Updated on July 14th, 2012 at 19:00 pm