As the field effect transistors (FETs) in complementary metal-oxide semiconductor (CMOS) technology become smaller, there is an increasing possibility of undesired differences in the electrical characteristics of the FETs, or other devices, which are intended to be identical. This can occur due to uncontrollable fabrication biases, or due to randomness in the impurity implants which set threshold voltage. The variations can occur on various length scales, such as across the wafer variations, across the chip variations, or as variations between adjacent devices. Even under perfect processing conditions, the discrete nature of the impurity implants required to establish threshold voltage leads to random variations.
Such variations adversely affect the circuits which are formed from the devices. The most commonly cited example is that of static random access memory (SRAM), which is subject to significant yield loss as threshold variations become large. Analog circuits, such as current mirrors, become inaccurate if devices are not identical. Similarly, critically timed digital circuits may be prone to timing error if the devices are not all identical.
Thus as semiconductor processing technology continues to result in FETs of smaller and smaller dimensions, there is an increasing need to monitor device variations, particularly for very small, very closely spaced individual devices. Presently, device variations are measured by using external test equipment, measuring suitable current-voltage (I-V) curves of individual devices, and analyzing the results afterwards. This is not only time consuming; it also consumes a good deal of silicon area per device, since the probe pads for this method are much greater in area than are the devices being tested.
Other methods of assessing the device variability have been proposed, such as ring oscillators, which measure the average threshold voltage of a number of devices, and memory operation, which measures the threshold variation of 4 or 6 devices.
It would be desirable to overcome one or more of the limitations in previous approaches.