Body biasing is the change in transistor threshold voltage because of voltage difference between the transistor source and body. As the voltage difference between source and body effect the threshold voltage of any transistor, so body can be considered as a second gate to determine when transistor will turn on or off. Body coefficient “gamma” determines the strength of body effect. Body effect is diminished with transistor scaling, instead transistor body is connected to power (VDD) for p-channel transistor and to ground for n-channel transistor.
It involves connecting the body of transistor either to ground or power supply. The body biasing can be applied to transistor either by using off-chip (external) source or by using on-chip (internal) source. In on- chip approach, reverse body bias voltage is provided by charge pump circuit and forward bias voltage is provided by voltage divider circuit. In reverse body biasing we apply negative body-to-source voltage to an n-channel transistor which raises transistor threshold voltage but on the other hand make transistor both slower and less leaky. In forward body biasing we apply positive body-to-source voltage to a p-channel transistor which lowers transistor threshold voltage but on the other hand make transistor both faster and leakier.
Body Bias Methodologies:
· Fixed Body Bias
In this methodology a fixed body bias voltage is applied to all chips in which body bias value is set during design. In power gating transistor a fixed forward bias voltage is applied during the on state to reduce the on-resistance of the transistor switch and a fixed reverse bias voltage is applied during the off state to reduce the remaining leakage in the power-gated block.
· Adaptive Body Bias
In this methodology different fixed body bias voltage is calibrated at production test. When forward body bias voltage is applied to slow chip it lowers the threshold voltage and speeds up the chip and when reverse bias voltage is applied to a fast chip it increases transistor threshold voltage and reduces the excess leakage current (and leakage power consumption) of the chip.
· Dynamic Body Bias
In this methodology the body bias voltage is changed multiple times while the chip is operating rather than setting the body bias just once either during design or at production test. This methodology is also used to reduce temperature and aging effects and it will manage power management modes more effective so that it can optimize very low power operation.
Dynamic body bias can adjust the transistor threshold voltage to compensate for changes in the transistor as the product ages and also adjust threshold voltage of transistor to compensate for temperature related changes in the threshold voltage for transistor as the part heats up and cools down, maintaining more uniform power and leakage.