Measuring some Zener diodes

19.04.2020 12:05

I've been stuck working on a problem for the past few days. I need to protect an analog switch in a circuit from expected over-voltage conditions. Zener diodes seemed like a natural solution, but the border between normal operation and over-voltage is very thin in this particular case. I couldn't find components with a characteristic that would fit based solely on the specifications given in the datasheets. I've been burned before by overestimating the performance of Zener diodes so I decided to do some measurements and get some better feel for how they behave. The results were pretty interesting and I thought they might be useful to share.

The following measurements have all been done with my tiny home-brew curve tracer connected to a Tektronix TDS 2002B oscilloscope. Unfortunately this model only has 8-bit vertical resolution. This caused some visible stair-stepping on the vertical parts of the traces below. Nevertheless the measurements should give a pretty good picture of what's going on. Before doing the measurements I've also checked the DC calibration of the whole setup against my new Keysight U1241C multimeter. The error in measured voltage and current values should not be more than ±3%. Measurements were done roughly at room temperature and at a low frequency (100 Hz).

First measurement is with SZMMBZ5230BLT11G, a 4.7 V Zener diode from ON Semi in a SOT-23 SMT package. I've only measured a single one of these, since soldering leads to the SMT package was time consuming. The figure shows current vs. voltage characteristic in the reverse direction. The narrow, dark blue graph shows the actual measured values. The black dashed line shows the maximum power dissipation limit from the datasheet. I also made a model for the diode based on the minimum and maximum values for VZ and the single ZZT value given in the datasheet. The light blue area is the range of characteristics I predicted with that model.

Voltage vs. current graph for SZMMBZ5230BLT11G

The relevant part of the datasheet for this diode:

Excerpt from the MMBZ52xxBLT1G datasheet.

Image by ON Semiconductor

This is the same measurement repeated for BZX79C4V7, also a 4.7 V Zener diode from ON Semi, but this time in a sealed glass THT package. I've measured 10 of these. All came shipped in the same bag, which might mean they're from the same production batch, but I can't be sure. All 10 measurements are shown overlapped on the same graph.

Voltage vs. current graph for BZX79C4V7.

The relevant part of the datasheet:

Excerpt from the BZX79Cxx datasheet.

Image by ON Semiconductor

It's interesting to see that both of these parts performed significantly better than what their datasheets suggest. They were both in the allowed voltage range at the specified current (note that one is specified at 20 mA and the other at 5 mA). The differential impedance was much lower however. SZMMBZ5230BLT11G is specified at 19 Ω at 20 mA and I measured around 1 Ω. BZX79C4V7 is specified at 80 Ω at 5 mA and I measured 11 Ω. The datasheet for BZX79C4V7 does say that 80 Ω is the maximum, but SZMMBZ5230BLT11G isn't clear on whether that is a typical or the maximum value. It's was also surprising to me how the results I got for all 10 BZX79C4V7 measurements were practically indistinguishable from each other.

A note regarding the models. I used the classic diode equation where I calculated the parameters a and b to fit VZ and ZZ (or ZZT) values from the datasheets.

I = a ( e^\frac{U}{b} - 1)

As far as I know, a and b don't have any physical meaning here. This is in contrast to the forward characteristic, where they represent saturation current and thermal voltage. I wasn't able to find any reference that would explain the physics behind this characteristic and most people just seem to use such empirical models. The Art of Electronics does say that the Zener impedance is roughly inversely proportional to the current, which implies an exponential I-U characteristic.

From my rusty understanding of breakdown physics I was expecting that a junction after breakdown wouldn't have much of a non-linear resistance at all. I was expecting that a good enough model would just be a voltage source (representing the junction in breakdown) and a series resistance (representing ohmic contacts and bulk semiconductor). It seems this is not so, at least for the relatively low current conditions I've measured here. The purely exponential model also fits my measurements perfectly, which seems to confirm that this was a correct choice for the model.

Update: I found Zener and avalanche breakdown in silicon alloyed p-n junctions—I: Analysis of reverse characteristics (unfortunately pay-walled). It contains an overview of the various mechanisms behind junction breakdown. In contrast to all other references I've looked at it actually goes into mathematical models and doesn't just stop at hand-waving qualitative descriptions. The mechanisms are complicated and the exponential characteristic I've used is indeed just an empirical approximation.

Finally, it's interesting to also look at how the forward characteristics compare. Here they are plotted against a common signal diode 1N4148. Both Zener diodes are very similar in this plot, despite a different Zener impedance and a differently specified forward voltage in the datasheet. Compared to the signal diode they have the knee at a slightly higher voltage, but also steeper slopes after the knee:

Comparison of forward characteristics.

In conclusion, it's interesting to see how these things look like in practice, beyond just looking at their specifications. Perhaps the largest take away for me was the fact that a purely resistive model obviously isn't a good way of thinking about Zener diodes in relation to large signals. Of course, it's dangerous to base a design around such limited measurements. Another batch might be completely different in terms of ZZ and I've only measured a single instance of the SOT-23 diode. Devices might change after aging and so on. After all, the manufacturer only guarantees what's stated in the datasheet. Still, seeing these measurements was useful for correcting my feel for how these parts are behaving.

Posted by Tomaž | Categories: Analog | Comments »