Inverted thin-film transistors with a simple self-aligned lightly doped drain structure

The I-V characteristics of inverted thin-film transistors (TFT) are studied. A simple lightly doped drain (LDD) structure is utilized to control the channel electric field at the drain junction and to improve the performance of the TFTs. The LDD region is self-aligned to the channel and the source/drain regions. It is created by a spacer around an oxide mask which exclusively defines the channel length L_ch. Experimental data show that the leakage current, subthreshold swing SS, saturation current, and on/off current ratio of the inverted TFTs are closed related to L_ch, L_LDD, the drain bias, gate voltage, and LDD dose. With a gate deposited at low temperature, a saturation current of ~1.25 μA at 5 V and a leakage current of ~0.03 pA per micrometer of channel width were achieved. The current ratio therefore exceeds seven orders of magnitude, with an SS of 380 mV/decade. At 3.3 V, the current ratio is ~7×10⁶