References Voltage from PN junction till Band Gap - Zener Diodes

In the previous blog, we refreshed ourselves about References in general and then we initialize the journey by showing voltages built from forward biased PN junctions that include standard diodes, LEDs and BJTs. We go further here with more sophisticated devices providing precision voltage references:

4. Zener Diodes

Any PN junction exhibits an avalanche breakdown phenomenon when biased with a reverse voltage. It is called the Zener effect.

That reverse voltage creates a large electric field and acts as an accelerator for the few free electrons: they receive enough energy to hit other (not-free) electrons of the structure.  And a runaway phenomenon occurs rapidly: current increases exponentially in a suddenly.

Quickly, the whole structure collapses and there is no barrier anymore for the current: this happens at the Zener voltage (V_Z).

This Zener voltage occurs, for Silicon, in the range of 6V.

Compared to the forward-biased junction voltage of 0.6 volt, the Zener voltage appears at higher values (around 6 volt) and has a more vertical profile: the Zener voltage is less sensitive to current.

5. Zener avalanche on Bipolar junction transistor (BJT)

Since PN junctions are also found in BJTs, one can apply reverse voltage on the base-emitter junction. It will exhibit also a Zener runaway breakdown phenomenon. It occurs at around 6 Volt.

Such “BJT based” Zeners are easy to obtain since they can be integrated in IC together with the other devices; using the same wafer process; reducing cost.

However, such structures suffer also from an undesired perturbator, that is temperature.

6. Diode and Zener in series improve combined Tempcos.

The reverse-biased avalanche Zener voltage decreases when temperature increases. They have a negative TempCo (temperature coefficient).

We have seen in the previous blog, the forward-biased diode VBE has Tempco around – 1.8 mV/°C.

For a circuit operating in the military conditions, the temperature varies from -55°C to +125°C: in such case the VBE will vary by 324 mV! VBE of 0.8 volt at -55°C will decrease to less than 0.5 volt at +125°C! This is an “imprecision” of more than 65%!

Zener voltage Tempco becomes in general positive when VZ is above 5 volts.

But the beauty is it has a positive Tempco at around 1 to 2mV/°C; ideal to cancel or attenuate the -1.8 mV/°C of VBE!

This is why one can often see a forward-biased diode in series with a Zener: their opposite TempCos tend to mutually compensate (at a first order).

In doing so, the negative Tempco of the forward diode (appox -1.8mV/°C) is partially reduced by the Zener positive Tempco (approx. from +1mV/°C to +2mV/°C). It’s not a perfect match, but it’s a significant improvement.

7. Smart Processing - Buried Zener

Current wafer processing in which active components are created by piling various layers on the wafer surface, exposes the Zener on the surface. The technique of putting active devices deep in the wafer body gives a cleaner voltage.

8. Super Zener

We have seen temperature effect is the major perturbator for the reference stability. ADI has a special technique to combat temperature effect by integrating a “heater” in the package. The control of that heater is made via an integrated thermal sensor.

In doing so the temperature of the Zener junction can be adjusted.

The ADR1000 combines the 3 techniques:

1. The Zener is buried in the silicon structure; the active PN junction zone is not exposed.
2. The integrated heating resistance is associated with a driver and a temperature sensor. The system maintains the ambient temperature at the value for which the reference voltage is defined.
3. There is a forward-biased junction (VBE of Q1) put in series with the Zener.

Of course, this technique can work only to increase the temperature. If the ambient temperature becomes higher, there is no way to cool the package. The right Vref value must be then set at a high temperature.

In summary, we learned in this second part the Zener diodes family generate more stable and robust voltages than direct biased PN junctions. The temperature is the most perturbant parameter. It can be partially compensated by putting in series a normal diode and a Zener diode.

Heating elements to maintain the operational temperature constant inside the device or by burying the structure in the wafer structure are possible solutions.

Analog Devices ADR1000 includes all three techniques mentioned in this blog.