Hybrid TFT Technology for AMPLED
Publication Date: 2004-May-24
The IP.com Prior Art Database
Conventional a-Si TFT's cannot be used for the PLED drive transistor in an AMPLED display due to inherently poor threshold voltage stability under the high duty cycle stress necessary when driving AMPLED,s. However al! the additional low duty cycle switching. transistors necessary in a pixel, can be real1sed satisfactorily using standard low leakage bottom gate a-Si TFT's.
Microcrystalline Si TFT's have good Vt stability but (currently) suffer from a relatively high leakage current. Leakage current, however, is not critical to the operation of an AMPLED pixel drive transistor.
Therefore it is proposed that a hybrid technology be used in which both a-Si and micro-crystalline TFT's are fabricated together in an AMPLED display, a-Si for the low duty cycle switching transistors and micro-crystalline Si for the high duty cycle drive transistor. If a bottom gate a-Si TFT structure is combined with a co-planar micro-crystalline TFT structure, then AMPLED pixels could be realised with just one additional mask.
A, B) AMPLED devices are current driven, a constant current supply is required by the organic LED device to produce a constant light output. Currently the favoured material for fabrication of the TFT’s, in the active matrix pixels of the AMPLED, is polysilicon as this is a low reverse leakage technology and is electrically stable allowing an accurate current to be supplied through the LED via a known gate voltage on the pixel TFT. TFT’s fabricated from a-Si would be preferred, due to the simpler/cheaper processing, but for the fact that they are not electrically stable and therefore there is not a reliable relationship between current delivered and voltage applied to the gate over time.
Attempts are being made to address this problem by designing complex in-pixel circuits that seek to measure, whenever the a-Si TFT is addressed, the TFT threshold voltage and automatically compensate so that the correct light levels are output. However the complexity extends to the driving scheme and has the effect of reducing significantly the pixel aperture.
TFT’s fabricated from PECVD deposited micro-crystalline silicon (m-Si) would meet the stability requirements required to drive the organic LED device and this technology is similar to a-Si TFT’s i.e. simple and cheap. However m-Si TFT’s cannot currently be made with a low reverse leakage and s...