Zener Effect:
Occurs in heavily doped diodes with a narrow depletion layer.
Electric field becomes intense, approximately 300,000 V/cm.
Field pulls electrons from valence orbits (known as high-field emission).
Dominates in breakdown voltages less than 4 V.
Avalanche Effect:
Minority carriers are accelerated, dislodging other electrons.
Causes an avalanche of free carriers, resulting in large reverse current.
Dominates in breakdown voltages greater than 6 V.
Between 4-6 V:
Both Zener and Avalanche effects are present.
Naming:
Historically, all breakdown diodes are called Zener diodes, even though both effects may occur.
Impact of Temperature on Zener Voltage:
Zener voltage changes with ambient temperature.
Listed in data sheets as the temperature coefficient:
Change in breakdown voltage per degree Celsius (\(^{\circ}\mathrm{C}\)).
Breakdown Voltage \(<\) 4 V:
Negative temperature coefficient (Zener effect).
Example: A 3.9 V zener diode may have a coefficient of -1.4 mV/\(^{\circ}\mathrm{C}\).
For each \(1^{\circ}~\mathrm{C}\) increase, breakdown voltage decreases by 1.4 mV.
Breakdown Voltage \(>\) 6 V:
Positive temperature coefficient (Avalanche effect).
Example: A 6.2 V zener diode may have a coefficient of +2 mV/\(^{\circ}\mathrm{C}\).
For each \(1^{\circ}\mathrm{C}\) increase, breakdown voltage increases by 2 mV.
Between 4 and 6 V:
Temperature coefficient shifts from negative to positive.
Some diodes have a zero temperature coefficient, offering stability across temperature ranges.
Important for applications needing stable Zener voltage over wide temperature variations.
Is the zener diode operating in the breakdown region?
\[V_{TH}=\frac{1 \mathrm{k}\Omega}{270 \Omega + 1 \mathrm{k}\Omega}(18 \mathrm{V})=14.2 \mathrm{V}\]
The Thevenin voltage (\(V_{th}\)) is greater than the Zener voltage (\(V_Z\)).
Operates in the breakdown region.
What is the zener current?
\[\begin{aligned} I_{S}&=\frac{(18-10)\:\mathrm{V}}{270\:\Omega}=29.6\:\mathrm{mA}\\ I_{L}&=\frac{10\:\mathrm{V}}{1\:\mathrm{k}\Omega}=10\:\mathrm{mA}\\ I_{Z}&=29.6\:\mathrm{mA}-10\:\mathrm{mA}=19.6\:\mathrm{mA}\\ \end{aligned}\]
What is the behaviour of this circuit?
First Zener diode acts as a preregulator.
Second Zener diode serves as a Zener regulator.
Preregulator output: 20 V.
Zener regulator output: 10 V.
Provide the second regulator with a well-regulated input.
Ensure the final output is extremely well regulated.
Zener diodes are commonly used in voltage regulators, remaining in the breakdown region.
Zener diodes can be utilized in circuits for waveshaping, as shown in the configuration with two back-to-back Zener diodes.
Positive Half-Cycle:
Upper diode conducts; lower diode breaks down.
Output is clipped at \(\text{Zener Voltage} + 0.7 \, \text{V}\).
Negative Half-Cycle:
Lower diode conducts; upper diode breaks down.
Output approaches a square wave.
Conclusion:
Larger input sine waves produce a better-shaped output square wave.
Multiple DC Output Voltages:
Combination of zener diodes and ordinary silicon diodes used to generate multiple DC output voltages from a 20-V power supply.
Bottom diode produces 10 V output.
Forward-biased silicon diodes add small voltage steps: 10.7 V and 11.4 V.
Top zener diode with a 2.4 V breakdown voltage gives an output of 13.8 V.
Different combinations of zener and silicon diodes can yield various DC outputs.
Over-voltage Protection:
If a 6-V relay is connected directly to a 12-V supply, it might be damaged.
Solution is to connect a 5.6-V zener diode in series with the relay.
Zener diode drops 5.6 V, leaving 6.4 V across the relay (within tolerance).
Capacitor Over-voltage Protection:
Electrolytic capacitors often have low voltage ratings (e.g., 1000 \(\mu\)F, rated at 6 V).
If used with a 12-V supply, the capacitor could be damaged.
By placing a 6.8-V zener diode in series, the voltage across the capacitor is reduced to 5.2 V, safely within its 6-V rating.