This paper describes the robust design of the 30 V high voltage NMOS (HVNMOS) structure implemented in a 0.6 µm 5 V standard CMOS processes without any additional masks or process steps. The structure makes use of the field oxide (FOX) and light doping N-well to increase the drain to gate and drain to bulk breakdown voltages, respectively. By varying the six spacing parameters: the channel length, gate overlap FOX, N-well overlap channel length, poly to the active area of the drain (OD2), metal extend beyond the OD2 and N-well extend beyond the OD2 in HVNMOS structure, the breakdown voltage can be improved. The experimental results show that the breakdown voltage of the normal NMOS is 11 V, and the breakdown voltage of the HVNMOS is increased to over 30 V. With the optimized layout parameters of the HVNMOS, it can be increased to 38 V.

MOSFETs can be used as capacitors, but its capacitance can vary by 5 to 7 times as its terminal voltage varies. To reduce the voltage dependence of the capacitance, this paper proposed two types of devices: one is called accumulation MOSFET (AMOS) and the other is formed by two conventional PMOS connected in anti-parallel. These two devices are readily available in the standard digital CMOS processes. The proposed capacitors were implemented in three different CMOS processes. The measured results show that the capacitances of both devices have less voltage dependence than a single PMOS. The voltage dependence of the AMOS capacitance can be as small as 17%. The minimum capacitance per unit area of the AMOS is 1.8 times that of the double-poly capacitor in an analog/mixed-mode CMOS process. To verify the usefulness of these two types of capacitors, they are used as compensation capacitors in a conventional two-stage amplifier. The measured results show that the amplifier compensated by the AMOS capacitor has little variation (6%) of the unity-gain frequency over the input common-mode range. Due to its smaller die area and cheaper digital process, AMOS can be used as compensation capacitor without resorting to more expensive analog process.