Polymer grid triodes

Various structures of vertical-type organic transistors, (a) PGT, polymer grid triode (b) VOFET, vertical organic field effect transistor and (c) MBOT, metal-base organic transistor. 

The Tlastic Retina Image Enhancement Using Polymer Grid Triode Arrays... 

The local contrast enhancement algorithm (Eqs. 2 and 3) can be implemented with a simple device made from layers of conducting polymers (2). The device consists of an array of polymer grid triodes (3, 4) connected through a common grid which serves as a resistive network. 

Figure 2 Structure of the polymer grid triode with the various layers. (1) and (5) are the cathode and anode (pixel) arrays, respectively. The other layers are continuous films common to all the PGTs within the array (2) and(4) are semiconducting layers, poly(2-methoxy-5-(2 -ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, and (3) is the common grid network filled with semiconductor (3 ).

Polymer grid triode arrays, with four triodes on a single substrate all with a common grid, have been fabricated. The structure of this linear array is shown schematically in Figure 4. The fabrication process for the triode array is similar to that of a single PGT (5, 4) the principal difference is lhat there are separate contact pads for each device in the array. For the arrays fabricated in this initial study, the... 

Figure 4. Four polymer grid triodes in an array with a common grid.

The final step required to demonstrate the validity of the polymer grid triode implementation of local contrast enhancement is to show that it computes the center-surround difference in Eq. 2. The equivalent circuit of the PGT demonstrated previously (3, 4) is that of two coupled diodes connected back-to-back, like that of a bipolar transistor (5). This is achieved by using semiconducting polymer in layers (2), (3 and (4). For the prototype array sketched in Figure 2c, layer (2) was fabricated with a material with sufficient conductivity to make an ohmic contact to the grid so that the equivalent circuit is simplified to a diode in series with a resistor. In the initial experiments, polyvinylcarbazole (PVK) was used for this resistor layer (2). In forward bias,... 

To build a full plastic retina, the polymer grid triode image enhancement array would be fabricated directly onto the output (back) side of a photodetector array (for example an infrared detector array) using each detector output pad as the anode or cathode of the PGT at that node. The semiconductor layers would be cast sequentially from solution and applied onto the detector array much like an antireflection coating. The final contrast enhanced output would be connected to a demultiplexer by bump-bonding that is by cold-welding indium bumps arrayed reciprocally on die PGT array output and on e demultiplexer input. 

Earlier efforts toward device applications using the polythiophenes can be seen in the literature [201]. A recently proposed device of polymer grid triodes by Yang and Heeger is also worth noting [202]. 

In 1994, a polymer-based transistor [237] constructed in a geometry closely related to that of a vacuum tube triode was demonstrated [237]. The structure consists of a thin MEH-PPV polymer film sandwiched between two electrodes, with a third electrode (PANI network) [209, 238-241] embedded within the MEH-PPV film (Fig. 13). This third electrode plays a role similar to that of the grid in a vacuum tube, controlling the current between the two outermost electrodes. Using a generalized analysis based on field-assisted carrier injection by tunneling, the carrier injection and transport in the polymer grid triode could be modeled [242]. 

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