1. Field of the Invention
The present invention relates to a transmission gate circuit, and more particularly, to a transmission gate circuit capable of compensating body effect.
2. Description of the Related Art
In Integrated circuit (IC) field, a transmission gate functions as a switch to determine whether signal passes or not.
Please refer to FIG. 1 illustrating a circuit diagram of a conventional transmission gate circuit 10. The transmission gate circuit 10 comprises a P-type metal oxide semiconductor (PMOS) P1 controlled by a control signal C, and an N-type metal oxide semiconductor (NMOS) N1 controlled by a control signal C′ inversed to the control signal C. As the control signal C is at high voltage level, the transmission gate circuit 10 turns off, so that output Y is floating. Conversely, the control signal C is at low voltage level, the transmission gate circuit 10 turns on, so that input X is delivered to output Y However, a threshold voltage VTHp of the PMOS device P1 is as a function of VTHp=VTHp0+
      γ    ⁡          (                                                                                2                ⁢                                  Φ                  F                                                                    +                                                        V                SB                                                                  -                                                        2              ⁢                              Φ                F                                                                    )        ,and a threshold voltage VTHn of the NMOS device N1 is as a function of VTHn=VTHn0+
      γ    ⁡          (                                                                                2                ⁢                                  Φ                  F                                                                    +                          V              SB                                      -                                                        2              ⁢                              Φ                F                                                                    )        ,where VSB represents voltage difference between source and body, VTHp0, VTHn0 represents threshold voltage measured without body effect, γ is a coefficient associated with the body effect, and ΦF indicates to Fermi potential. Therefore, an existence of voltage difference VSB varies the actual threshold voltage. As an example of NMOS element, a large voltage difference VSB may rise the threshold voltage VTHn from 0.8V to 1.6V, i.e. almost twice. Such body effect, that is a rise of the threshold voltage, degrades conductivity of current as well as slows operation of the MOS circuit.
Although a use of the PMOS device P1 and the NMOS device N1 in the transmission gate circuit 10 stables and balances a turn-on resistor over a wide range of input voltage, respective body effects for the MOS elements P1 and N1 still cause an uneven turn-on resistor under a higher or lower range of input voltage.