Table of Contents
Introduction
Zener diode is uniquely engineered to allow current to flow in reverse once a specific voltage, called the Zener breakdown voltage, is reached. This property makes Zener diodes indispensable in critical applications such as voltage regulation, surge protection, voltage reference circuits, and switching circuits. Zener diodes are available in a variety of voltage ratings, power ratings, and tolerances, making them suitable for everything from low-power integrated circuits to high-voltage industrial equipment. In this article, we will explore the construction, working principles, breakdown mechanisms and material choices related to Zener diodes, aiming to deliver a comprehensive understanding tailored for professional applications.
Principle of Operation
As we have learned in this blog, in a reverse-biased diode, the majority carriers are pulled away from the junction, leaving behind fixed ionized atoms at the depletion region and causing it to widen significantly. This widening prevents significant current flow, and the diode essentially acts as an open circuit. However, minority carriers near the junction are still present and are responsible for a small leakage current as they neutralize the fixed charges. As the reverse voltage is increased, the minority carriers gain more kinetic energy, increasing their velocity. Eventually, these high-energy carriers collide with lattice atoms, knocking loose tightly bound electrons, a process known as ionization. This releases additional free carriers, leading to a rapid increase in reverse current known as avalanche current. On the I-V characteristic curve, this is observed as a sudden breakdown at a particular reverse voltage, typically with a slight slope.
This breakdown voltage can be lowered and brought closer to the negative current axis by increasing the doping concentration in the semiconductor material. Heavy doping increases the number of majority carriers, which results in the formation of a narrower depletion region even under zero bias. When reverse voltage is applied, the electric field across this narrow depletion region becomes extremely strong, as defined by the relation:
\[Electric Field = \frac{Voltage}{ Distance}\].
With a reduced depletion width (Distance), the electric field strength increases sharply even at relatively low voltages. Under these conditions, the electric field itself becomes strong enough to pull electrons directly out of their atomic bonds, without needing collisions, a process that bypasses traditional ionization. This phenomenon is known as Zener breakdown.
Unlike avalanche breakdown, Zener breakdown does not rely on impact ionization but on direct quantum mechanical tunneling of electrons across the depletion region. As a result, in reverse bias, two distinct mechanisms can occur depending on doping levels and applied voltage: avalanche breakdown in lightly doped diodes and Zener breakdown in heavily doped diodes. The Zener effect results in a sharp and controlled increase in current at a well-defined voltage, forming the basis of operation for Zener diodes.
Resistance Level of Zener Diode
Static Resistance
When a DC voltage is applied to a diode, the resistance at a specific operating point is called static resistance. It is given by the formula: \[R_D = \frac{V_D}{I_D}\] where \(V_D\) is the voltage across the diode and \(I_D\) is the current through the diode. On the I-V characteristic curve, the behavior of static resistance is easy to observe In the forward bias region, during the vertical rise of the curve (after the knee voltage), the diode exhibits low resistance because a large amount of current flows with little voltage increase. In the knee region and below (near zero current), the diode exhibits high resistance due to very small current flow, even though some voltage is applied. In reverse bias before breakdown, resistance level is quit high. In reverse Bias Zener Region, even though voltage is nearly constant, small increases in voltage cause big increases in current so static resistance becomes very small.
Dynamic Resistance
Dynamic resistance, also known as AC resistance, becomes important when an AC voltage is applied across the diode. Because the AC input continuously varies sinusoidally, the operating point of the diode moves up and down along the I-V curve.This movement defines small changes in voltage \(\Delta V_D\) and current \(\Delta I_D\) around a specific bias point.
If we mark a quiescent point (Q-point) on the diode’s I-V curve (determined by the applied DC voltage), and draw a tangent line at this point, the slope of this tangent represents the dynamic resistance.
The formula for dynamic resistance is \[r_d = \frac {\Delta V_D}{\Delta I_D}\]
where \(\Delta V_D\) is the small change in voltage and \(\Delta I_D\) is the small change in current around the Q-point.
\(\Delta\) (delta) signifies a change in quantity.
As we know the slope of the curve at a point is mathematically equal to the derivative of the function at that point.
If we take derivative of shockley’s equation with respect to \(\Delta V_D\) and invert the result we will get the slope of the curve at point Q.
\[\frac {d}{dV_D} I_d = \frac{d}{dV_D} [I_s(e^{V_d/n.V_T}-1)]\]
\[\frac {d}{dV_D} I_d = I_S \cdot \frac{1}{n.V_T}\cdot e^{V_d/n.V_T} -\frac {dI_S}{dV_D}\]
\[\frac {d}{dV_D} I_d = I_S \cdot \frac{1}{n.V_T}(\frac{I_D}{I_S} +1) – 0 \]
Because, \[ I_D = I_S \cdot (e^{V_D/n.V_T} – 1) \]
\[\frac {I_D}{I_S} + 1 = e^{V_D/n.V_T}\]
so,
\[\frac{dI_D}{dV_D} = \frac{1}{nV_T}(I_D + I_S)\]
As we know \(I_D\) is far greater than \(I_S\) we can eliminate \(I_S\) from above equation and flip the equation to define dynamic resistance
\[r_d =\frac {dV_D}{dI_D} = \frac {nV_T}{I_D}\]
When the AC input signal applied to a diode has a large amplitude, the operating point (Q-point) moves over a large section of the diode’s I-V curve. so we take average AC resistance to measure the section. For this equation will be \[r_{av} = \frac {\Delta V_D}{\Delta I_D}\]
Piecewise Equivalent Circuit of a Zener Diode
When considering the piecewise equivalent circuit of a Zener diode, we must account for its behavior across all regions of the characteristics curve due to the variety of applications. In the forward bias region, the Zener diode behaves like a normal diode. The equivalent circuit is modeled as a voltage source (approximately 0.7 V for silicon) in series with a small forward dynamic resistance. In the reverse bias region before breakdown, the Zener diode offers extremely high resistance, meaning almost no current flows. In this region, the diode is considered an open circuit in the equivalent model. In the reverse bias region after breakdown (Zener region), the voltage across the diode becomes nearly constant, even as current increases significantly. Here, the equivalent circuit is represented simply by a voltage source equal to the Zener breakdown voltage.
How to Read Datasheet of a Zener Diode
You will find the Zener voltage \(V_Z\) listed first in the datasheet. This is the voltage at which the Zener diode enters breakdown and maintains a nearly constant voltage across its terminals. The datasheet usually specifies a nominal value, meaning the average Zener voltage, along with a tolerance range (commonly \(\pm 5\%\), but it could also be \(\pm 10\%\) or tighter for precision diodes).
Next, you will find the Zener test current \(I_Z\), typically labeled as test current. This is the current at which the Zener voltage is accurately measured, and it also defines the dynamic resistance at the breakdown region.
Following that, you will see the reverse leakage current \(I_R\), which is the very small amount of current (usually in the range of micro amperes) that flows when the diode is reverse biased but before breakdown occurs. This parameter is critical for applications needing very low standby currents.
You will also find dynamic resistance (sometimes called Zener impedance, noted as \(Z_Z\). This represents how much the voltage changes with changes in current within the breakdown region. Lower dynamic resistance means better voltage regulation, which is important in precision voltage reference circuits.
Regulator current \(I_{ZM}\), this is max safe current when operating at rated breakdown voltage.
Zener potential very sensitive to temperature. we can find Zener potential using temperature coefficient by using the equation \[T_C=\frac {\Delta V_Z / V_Z}{ T_1 -T_0} \times 100 \% ^\circ \text {C}\]
\(T_1\) is new temperature
\(T_0\) room temperature \(25^\circ \text{C}\).
\(T_C\) temperature coffecient
\(V_C\) Zener potential
Conclusion
In this blog, we have explored the complete behavior of the Zener diode, from its basic structure and reverse bias operation to the concepts of avalanche and Zener breakdowns. We understood how doping levels affect the breakdown voltage, how the depletion region behaves under reverse bias, and how minority carriers contribute to current flow. We discussed the piecewise equivalent circuit models, dynamic and static resistance concepts, and how the Zener diode operates under AC and DC conditions. Finally, we learned how to read a Zener diode specification sheet, understanding critical parameters like Zener voltage, test current, leakage current and dynamic resistance. With all this knowledge, we can now confidently analyze, design, and even solve practical problems involving Zener diodes. By mastering these concepts, you gain a full professional-level understanding of how Zener diodes work and how to use them effectively in real-world electronic applications.