Printed transistors can have the controlled current pass horizontally in the traditional geometry invented forty years ago and used in silicon chips. Alternatively, a vertical geometry can be used, so the channel length is no longer determined by the printing resolution. This gives a smaller transistor that works at higher frequency and is capable of managing higher currents.
Cambridge University in the UK has shown how their manufacture can be simplified and ORFID in the USA is commercializing them. However, although Matsushita and many other Japanese companies have researched vertical printed transistors and they are even used in a minority of silicon chips, only a minority of companies in the printed transistor business are developing them, the sticking point being the difficulties of manufacture, which reflects in yield and cost.
However, there is some return to the subject and they are even considered interesting for light emitting transistors and OLEDs that incorporate drive transistors. They usually attempt to use organic semiconductors in an attempt to overcome their performance limitations while keeping the benefits such as printability and avoidance of precious metals. These devices are usually referred to as Vertical Field Effect Transistors VFETs or Vertical Organic Field Effect Transistors VOFETs. Potentially, they are therefore a route to such things as driving current driven devices like OLEDs, getting high speed video, circuits that are not embarrassingly large in area and making RFID labels that work at UHF and above. IDTechEx has tracked many patents on the subject, notably from the year 2000 onwards.
Chiba University
Recently, Chiba University has been working with DaiNippon Printing to progress these aspects for flexible displays and here we share some of their slides presented at the IDTechEx Smart Labels Asia conference September 10-11 in Tokyo Japan. A part of this work belongs to "Advanced Organic Device Project" which OITDA contracted with New Energy and Industrial Technology Development Organization (NEDO) in Japan.
They find that flexible Organic Static Induction Transistors OSITs - a form of vertical transistor - show stable electrical characteristics under bending conditions. Organic Light Emitting Transistors OLETs, in the form of flexible & printable Metal Insulator Semiconductor ie MIS-OLETs, show maximum luminance of 500cd/m2 and on/off ratio of 1000. Their organic logic devices based on OSITs have an operational voltage is lower (-2 V to +2 V) than that of organic inverters based on horizontal versions usually referred to simply as OFETs. Organic logic devices based on pentacene and ZnO TFTs were fabricated, which show supply voltage of -5 to -20V and gain of 8.
Their OSITs exhibit stable electrical characteristics at compressive and tensile strains up to a bending radius of 5 mm, suggesting possible application as transistors in flexible electronics (see top image).
Luminance modulation by low gate voltage as low as ±1V was obtained.
The obtained results indicate that the operational voltage of organic inverters based on OSITs is lower (-2 V to +2 V) than that of organic inverters based on OFETs (-40 V to +40 V).
For more information read Printed and Thin Film Transistors and Memory 2007-2027.
