Electronic – Diode forward current with µV..mV forward bias

diodesleakage-current

When running LTSpice simulations on forward biased diodes I get plots like the following:

enter image description here

This example is for a BAS116 and a 1N4001 – the current in the sub mV region is about 5 orders of magnitude different.

  1. I am wondering if there is a reason for this huge difference in current given that the Silicon diodes should obey more or less the same principles in the "sub-threshold" region ?! Or is this only an inaccuracy in the models ?

  2. I have read about the Shockley Diode equation which predicts an exponential dependence. In Spice the V-I dependence is linear below about a few 10s of mV. Is this accurate ?

I am interested in this regime of operation as I want to use antiparallel diodes to clamp a fault voltage of a node to within +/- 1 V of another node, but present as little leakage as possible (pA) between the two nodes in normal operation, where both nodes are within about 1 mV of each other.

EDIT

I realized now, that the Shockley equation does indeed predict the linear region for \$|V_D|\ll V_T\$. Thanks to Enrico's answer I became aware of the large differences in Diode Reverse Saturation Current that determines the leakage in this bias regime.

For my application, I need a low leakage, but also large current capability for a few µs in the fault case. After comparing several diodes, it looks like unidirectional ESD diodes are best for this job. They will clamp in forward direction to about 1 V, so antiparallelling two of them does a good job. Their actual Zener Voltage does not matter in this application, so one could use whatever is on hand. Their leakage is also much better than rectifier diodes with a similar current rating. The latter seem to favor low forward drop.

There was also the comment of just measuring it. I will try to add results, if I still remember when I get around to visit the lab and find the time.

Best Answer

Diodes Spice models are usually pretty accurate especially those written by the manufacturer.

The difference lies in how the 2 diodes are designed.

I'm focusing my attention on Is, the saturation current.

It seems to me that the saturation currents Is of the 2 diodes are different.

See this Wikipedia formula which is the Schottky model of diodes:

enter image description here


Is depends, among other factors, on:

  1. The cross-sectional area A of the pn junction

  2. The donor and acceptor densities ND and NA

See this Wikipedia formula:

enter image description here


Happens that the 2 diodes have different cross section and different donors and acceptors densities.