While looking at a Zener diode datasheet, visit
I scrolled past a temperature coefficient plot and became puzzled for a few moments. Why was the temperature coefficient negative for low breakdown values and high for others? I then remembered that, unfortunately, the term Zener diode is used describe diodes designed to operate in the reverse breakdown region, both due to Zener and avalanche multiplication effects. For small (<5V) breakdown voltages, the Zener effect usually dominates and has a negative temperature coefficient. For larger breakdown voltages, the breakdown mechanism is typically avalanche multiplication.
(alpha is the temperature coefficient, image is from Wikipedia)
In a true Zener diode, the p-n junction is heavily doped so that the depletion region is very thin. This can cause appreciable tunneling through the depletion region barrier. The probability is inverse exponentially proportional to the square root of the effective carrier mass and the (3/2)nd power of the bandgap energy (reference). It can be shown that the effective carrier mass increases and the bandgap energy decrease with temperature (reference) is the dominant effect. The net result is a negative temperature coefficient. That is, the probability of tunneling across the depletion region energy barrier increases with an increased temperature and therefore the effective Zener breakdown voltage is made less negative.
The story is quite the oposite with avalanche multiplication breakdown. The idea behind this process is that a electron in the depletion region experiences significant acceleration due to the applied electric field accumulating enough momentum to create an electron-hole pair upon impact with an atom in the crystal lattice. The newly liberated electron then also accelerates to required velocity to liberate another electron. Now there are two electrons with the required velocity, and so the current multiplies. What started as just one electron traveling quickly has suddenly turned into an appreciable current given a strong enough applied field. This is where the name “avalanche multiplication” comes from. Typically, in order to have high electron velocity, a thick depletion region is desired so the p-n junction is relatively lightly doped. The reason that this process has a positive temperature coefficient, or the “Zener” breakdown voltage becomes more negative with increased temperature, is that the probability of an electron colliding with an atom is decreased due to the increased thermal jitter. In this sense, the atom becomes more of a moving target so a direct collision is more difficult.
Although this may seem like a trivial matter, it is important in various designs. Beyond ESD protection circuits, “Zener” diodes are frequently used as band-gap voltage references. A good voltage reference should have near-zero temperature dependance over the operating range so a single “Zener” diode may not be good enough. Aditionally, one type of breakdown may be more compatible than the other in a certain manufacturing process.