When Can a Diode Break Down Due to High Voltage: Understanding the Limits

When a diode is subjected to a high reverse voltage, it can break down due to two physical mechanisms: avalanche breakdown and tunneling breakdown. The breakdown voltage is the minimum voltage at which this breakdown occurs, and it depends on several factors, including the physical mechanism, doping profile, and geometry and structure of the diode.

Avalanche Breakdown in Diodes

Avalanche breakdown occurs in diodes with low carrier density, and the current increases exponentially with the applied voltage. The relationship between the current and voltage is formulated as a multiplication factor and the saturation current.

The avalanche breakdown voltage (V_br) of a diode can be calculated using the following equation:

V_br = (2 * ε_s * E_c^2) / (q * N_d)

Where:
– ε_s is the permittivity of the semiconductor material
– E_c is the critical electric field strength
– q is the elementary charge
– N_d is the doping concentration of the n-type region

The critical electric field strength (E_c) is the maximum electric field that the semiconductor material can withstand before breakdown occurs. For silicon, the critical electric field strength is approximately 3 × 10^5 V/cm.

The doping concentration (N_d) also plays a significant role in the avalanche breakdown voltage. Diodes with lower doping concentrations generally have higher avalanche breakdown voltages. For example, a silicon diode with a doping concentration of 10^16 cm^-3 has an avalanche breakdown voltage of around 600 V, while a diode with a doping concentration of 10^17 cm^-3 has a breakdown voltage of around 200 V.

Tunneling Breakdown in Diodes

In contrast, tunneling breakdown occurs in diodes with high carrier density and is due to quantum tunneling. The tunneling current is calculated in terms of the applied voltage and the doping profile.

The tunneling breakdown voltage (V_t) of a diode can be approximated using the following equation:

V_t = (E_g^3/2 * ε_s^1/2) / (q * N_d^1/2 * m_e^1/2)

Where:
– E_g is the bandgap energy of the semiconductor material
– ε_s is the permittivity of the semiconductor material
– q is the elementary charge
– N_d is the doping concentration of the n-type region
– m_e is the effective mass of the electron

For a silicon diode with a doping concentration of 10^18 cm^-3, the tunneling breakdown voltage is around 6 V.

Factors Affecting Diode Breakdown Voltage

The breakdown voltage of a diode is influenced by several factors, including:

1. Doping Concentration: As mentioned earlier, the doping concentration of the n-type and p-type regions in the diode affects the breakdown voltage. Higher doping concentrations generally result in lower breakdown voltages.

2. Diode Geometry: The physical structure and dimensions of the diode, such as the junction area and the thickness of the depletion region, can impact the breakdown voltage. Diodes with smaller junction areas and thinner depletion regions tend to have lower breakdown voltages.

3. Temperature: The breakdown voltage of a diode can vary with temperature. As the temperature increases, the breakdown voltage typically decreases due to changes in the semiconductor material properties.

4. Diode Material: The choice of semiconductor material used in the diode can also affect the breakdown voltage. For example, silicon (Si) diodes generally have higher breakdown voltages compared to gallium arsenide (GaAs) diodes.

5. Impurities and Defects: Impurities and defects in the semiconductor material can create localized regions with higher electric fields, leading to premature breakdown at lower voltages.

Practical Considerations for Diode Breakdown Voltage

When designing circuits with diodes, it is essential to consider the breakdown voltage to ensure the diode operates within its safe limits. Here are some practical considerations:

1. Voltage Margin: It is recommended to operate the diode at a voltage well below its breakdown voltage to provide a safety margin. A common rule of thumb is to use a diode with a breakdown voltage at least 2-3 times higher than the maximum expected reverse voltage in the circuit.

2. Transient Voltages: In addition to the steady-state voltage, it is crucial to consider any transient or spike voltages that may occur in the circuit, as these can also exceed the diode’s breakdown voltage and cause damage.

3. Thermal Effects: Elevated temperatures can reduce the breakdown voltage of a diode, so it is essential to consider the operating temperature and provide adequate cooling if necessary.

4. Diode Selection: When selecting a diode for a specific application, it is important to consult the manufacturer’s datasheet and choose a device with a breakdown voltage that is appropriate for the circuit’s requirements.

5. Reverse Bias Protection: In some applications, it may be necessary to use additional components, such as a series resistor or a zener diode, to protect the diode from reverse bias voltages that could exceed the breakdown voltage.

By understanding the factors that influence diode breakdown voltage and following best practices in circuit design, engineers can ensure the reliable and safe operation of diodes in their electronic systems.

Conclusion

The breakdown voltage of a diode is a critical parameter that determines the maximum voltage that can be applied to the diode without causing it to break down. Understanding the physical mechanisms of avalanche breakdown and tunneling breakdown, as well as the factors that influence the breakdown voltage, is essential for designing and applying diodes in electronic circuits.

By considering the doping concentration, diode geometry, temperature, material properties, and potential transient voltages, engineers can select the appropriate diode and implement the necessary protection measures to ensure the reliable operation of their electronic systems.

References:

1. Breakdown Voltage – Wikipedia
2. Understanding the Breakdown Voltage of a Diode – OurPCB
3. Understanding the Breakdown Voltage of a Diode – Cadence
4. Maximum Input Voltage a Diode Can Handle – All About Circuits