As the interest in personal health care is increasing, research on Bio-Information Technology (BIT) continues to be as vigorous as ever. Particularly, efforts are being made to develop products using bio-sensors capable of free movement and prompt detection. Therefore, a health care device with SOG (System-on-glass) technology is required due to the advantages of miniaturization, low cost, and simple process. These bio-sensor platforms must be consisting of the fully integrated circuit in order to be in contact with or embedded in the human body.

To display the sensing signal, we need analog-to-digital converter (ADC) circuit and 7-segment decoder which can be integrated using n-type amorphous indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) on a substrate. These bio-sensor platforms consist of an integrated circuit that converts the analog signal to a digital signal and the 7-segment decoder to show the signal level.

TFTs are required to realize the SOG which integrates all devices on substrates like a glass and plastic etc. Oxide semiconductor TFTs such as a-IGZO TFTs show better performance than amorphous silicon TFTs. Owing to their transparency and higher mobility compared to the amorphous silicon TFTs, a-IGZO TFTs have been studied widely.

Figure shows 9 comparators for digitizing, 9-to-4bit priority encoder and 7-sement decoder for display. An OP amp-based comparator includes two inputs, one is an analog input (V_in) and the other is a reference voltage (V_ref) generated by a serially connected resistor array. The input signals are compared with the reference voltages by the comparators, which output “0” or “1” according to the analog input level. Then the binary outputs of the comparator are converted to the binary format (A3, A2, A1, and A0) by a digital encoder. The 4bit outputs (A3, A2, A1, and A0) of the ADC are the inputs of the 7-segment decoder. Among the four outputs of ADC, ‘A0’ is the least significant bit and ‘A3’ is the most significant bit. The outputs of 7-segment decoder are labelled as a, b, c, d, e, f, and g which correspond to each segment of 7-segment display.

The 4bit binary codes from ADC are converted to input signals (a to g) of a 7-segment display. Those circuits with a-IGZO TFTs can be used in applications such as the fully integrated bio-sensor platform and wearable device which are transparent and flexible.

The most complex one in that platform is OP Amp which is comparator to generate binary code and needs high gain to accomplish good performance. The comparator is a circuit that plays an important role in the ADC, and the operation of the ADC is difficult to expect without the correct operation of the operational amplifier. Thus, we proposed OP Amp type comparator with oxide TFT for bio sensor platform.