Characterisation and modeling of mismatch in MOS transistors for precision analog design

A characterization methodology is presented that accurately predicts the mismatch in drain current over a wide operating range using a minimum set of measured data. The physical causes of mismatch are discussed in detail for both p- and n-channel devices. Statistical methods are used to develop analytical models that relate the mismatch to the device dimensions. It is shown that these models are valid for small-geometry devices only. Extensive experimental data from a 3-μm CMOS process are used to verify the models. The application of the transistor matching studies to the design of a high-performance digital-to-analog converter (DAC) is discussed. A circuit design methodology is presented that highlights the close interaction between the circuit yield and the matching accuracy of devices. It has been possible to achieve a circuit yield of greater than 97% as a result of the knowledge generated regarding the matching behavior of transistors and due to the systematic design approach.