A Zener diode is a type of semiconductor diode that operates in reverse-biased mode. Unlike regular diodes, which are designed to conduct current in the forward direction and have a specific breakdown voltage to protect against reverse voltage, Zener diodes are specifically designed to operate in the reverse breakdown region. This means that once the reverse voltage across the diode reaches a certain value, known as the Zener voltage (V_Z), the diode starts to conduct and maintain a nearly constant voltage drop across its terminals.
When a Zener diode is reverse-biased, the majority carriers (electrons for N-type and holes for P-type) are pushed away from the junction, creating a depletion region. As the reverse voltage increases, eventually, the electric field across the depletion region becomes strong enough to cause a phenomenon called Zener breakdown or avalanche breakdown.
In Zener breakdown, the covalent bonds in the depletion region are broken due to the high electric field, generating electron-hole pairs. These newly formed charge carriers cause the diode to conduct in the reverse direction. As the current increases, the voltage drop across the Zener diode remains relatively constant, equal to the Zener voltage (V_Z).
Voltage Regulation Characteristics:
The key characteristic of a Zener diode used as a voltage regulator is its ability to maintain a nearly constant voltage drop (V_Z) across its terminals, even when the input voltage varies over a certain range. This is what allows it to regulate the output voltage in a circuit.
To use a Zener diode as a voltage regulator, it is typically connected in parallel with the load (i.e., the component or circuitry that requires a stable voltage). Here's how the voltage regulation works:
Zener Voltage (V_Z): Each Zener diode is rated with a specific breakdown voltage (V_Z), which is the voltage at which the Zener breakdown occurs. For example, you might find Zener diodes with ratings like 5.1V, 12V, or 24V.
Load Resistance (R_L): The load resistance (R_L) is the resistance presented by the load component connected to the Zener diode.
Input Voltage (V_in): The input voltage (V_in) is the voltage supplied to the voltage regulator circuit. It can vary, and the voltage regulator's role is to maintain a stable output voltage despite these variations.
Output Voltage (V_out): The voltage across the load, or the output voltage (V_out), is the regulated voltage that should remain nearly constant.
Current (I_Z): The current flowing through the Zener diode, also known as the Zener current (I_Z), determines the voltage drop (V_Z) across the diode.
As long as the input voltage (V_in) remains within a certain range, the Zener diode regulates the output voltage (V_out) by adjusting its current (I_Z) to keep the voltage drop across the diode constant at the Zener voltage (V_Z). If the input voltage increases, the Zener diode will conduct more current to maintain the output voltage. Conversely, if the input voltage decreases, the Zener diode will conduct less current.
However, it's important to note that Zener diodes have some limitations as voltage regulators. They are not suitable for high-power applications and are typically used for low to moderate current applications. For higher power applications, other types of voltage regulators like integrated circuits (ICs) or transistor-based regulators may be more appropriate.