In RF frequencies, the PIN Diode can be seen as a circuit diagram as shown on the right hand side. The topology may differ slightly under different biasing conditions. When the diode is forward biased, the upper right equivalent circuit can be used. In here, the variable resistance, which is as a function of forward current flowing through the diode, expressed in formula form. Rs is the contact and metal resistance and the Cp is the package capacitance. The parameters k and x are empirical constants. When zero or reverse biased, the lower right equivalent circuit is valid. Here the package has an important contribution to the high frequency behavior of the device.