For PIN Diodes, the most important physical parameters are rD (the forward diode resistance), CD (the total diode capacitance) and the Tau (the carrier life time). The forward diode resistance (rD) consists of two parts a constant part RS, and a variable part RJ. RS is determined by contact and material resistance, while the RJ is determined by the relation: RJ = k/Ifx , where k and x are empirical constants. IF is the diode forward current. The total diode capacitance (CD) consists of two parts. CPACKAGE, is the contribution of the package and CJUNCTION, is the junction capacitance. For the carrier life time, Tau (t ), we need to provide a little more insight. The resistance of a PIN diode is controlled by the conductivity (or resistivity) of the I-layer. This conductivity is controlled by the density of the carriers (charges) in the I-layer which is, in turn, controlled by the DC bias. Minority carrier lifetime, indicated by the Greek symbol t, is a measure of the time it takes for the charge stored in the I-layer to decay, when forward bias is replaced with reverse bias, to some predetermined value. Lifetime has a strong influence over a number of PIN diode parameters, among which are distortion and basic diode behavior. At frequencies which are ten times fc (fc = 1/t), a PIN diode does indeed act like a current controlled variable resistor. At frequencies which are one tenth (or less) of fc, a PIN diode acts like an ordinary PN junction diode. Finally, at 0.1 fc ≤ f ≤ 10 fc, the behavior of the diode is very complex, which is beyond the scope of this training.