An accurate model to characterise a PIN diode as a variable resistor driven by high speed digital data and RF pump signals is presented. The model extracts the electron density function resulting from these signal drives. This is used to obtain a closed form expression for the PIN diode resistance. The residue theorem and complex inversion formula are used to solve the PN junction equations. A high speed reflection pulse amplitude modulator, which is an essential block for implementing more complex digital modulators, is also examined using this model. The results may be applied to realize a high speed digital modulator using silicon monolithic microwave integrated circuits.