Over/Undershooting Effects in Accurate Buffer Delay Model for Sub-Threshold Domain

Scaling down the supply voltage (V_dd) below the transistors threshold voltage (V_th) has become a very popular technique in designing Ultra-Low-Power circuits whose demand has dramatically increased in the last few years. Designing these kinds of circuit is still a challenge, especially when the latest advanced process technologies are employed. The well-known design methodology used in the typical super-threshold domain (V_dd > V_th) cannot be applied to the design of a sub-threshold circuit due to the different transistor current-voltage relationships that hold when V_dd <; V_th. For this reason, designers need supports suitable for the sub-threshold domain. This paper proposes a complete mathematical model able to predict the output behavior of a sub-threshold CMOS inverter. The model proposed here takes into account the effects of the transient variation of the transistor on-current during the gate switching. Moreover, for the first time, over/undershoot effects due to the input-to-output coupling capacitance are taken into account. The proposed model is formed by closed-form expressions able to predict the over/undershoot position, its amplitude and the inverter delay with great accuracy. Furthermore, it can be easily exploited in predicting the delay of cascading inverters, usually used to realize clock buffers. Under Process-Voltage-Temperature variations, the delay of a single inverter realized using a commercial CMOS 45 nm process technology is predicted with a maximum error lower than 16%. Even better results are obtained when the model is applied to inverter chains.