More about diodes!

Need a pn junction diode that has a high reverse breakdown, very low capacitance ( so its fast) and low resistance in the forward direction? If this is the case then a simple pn junction diode may not provide the answer. The reason is, that as the breakdown voltage goes up, the forward resistance goes up, capacitance goes down. If you a need a lot of current then this diode will not provide it. As the resistance goes down the breakdown goes down and the capacitance goes up. So sometimes a simple pn junction diode cannot meet specifications.

Yet if this type of performance is required, either for purposes of high current
(read low resistance ) or high frequency applications then a different type of diode is needed.

This is the P-I-N or N-I-P diode. The I stands for "intrinsic". This diode has a heavily doped p region and n region, just like in a ordinary pn diode. However, the resemblance ends there. In a P-I-N diode there is a high resistivity ( or
"intrinsic" region) sandwiched between the n and p heavily doped regions. The inclusion of the intrinsic or high resistivity region imparts some very useful characteritics to this structure. These characteristics are explored heuristically in this post.

Resistance: The resistance of the P-I-N diode is inversely proportional to the forward current through the diode and can be controlled by it. Very flat resistance characteristics can be generated this way. The reason for the low resistance with current is that as the high resistive region has very few carriers for recombination, any injected minority carriers coming from the heavily doped p and n regions do not die quickly but persist for "long" lifetimes in the I region. Thus the higher the current, the more free carriers in the I region and the lower the resistance. In the ultimate limit the forward resistance reaches the contact resistance which can be made very low.

Capacitance: The pn junction zero bias capacitance in the P-I-N diode is very low ( or relatively low compared to the ordinary pn junction diode). The reason is that the depletion region ( the region that is completely depleted of carriers with increasing reverse bias or zero bias) forms the "insulator" of a parallel plate capacitance. The parallel plates are, of course, the heavily doped p and n regions of the diode. The higher the resistivity of the I region the wider the depletion region and the lower the capacitance. Also the capacitance is very flat over a wide band of high frequencies so matching with other circuits becomes easier. As a result of the low capacitance the P-I-N diode can switch very fast and can be used in high frequency applications.

Reverse breakdown voltage: The breakdown voltage is high since the breakdown electric field drops voltage across a wider depletion region. As the depletion region becomes wider and wider with reverse voltage the breakdown increases.

Thus if one wants to reconcile high breakdown with low resistance and low capacitance then a P-I-N diode is a great choice. Both power diodes and RF diodes can be made with this technology.

Some disadvantages in the usage of the P-I-N diode are that (a) Its performance can only be predicted accurately if the lifetime of the minority carriers in the I region are known accurately. There are not a lot of analytical techniques to calculate this, therefore for precise usage, measurements need to be made. ( See the previous posts). (b) Most circuit simulator programs such as PSPICE do not provide a mathematical model ( empirical or physics based) so circut simulation is difficult. (c) The fabrication of the diode is slightly more complex. However most vendors provide the parameters and application notes for their P-I-N diodes so usage is made fairly easy. However, designing one from scratch can be quite involved because of the above factors.