diode string
**Diode string** is a **series-connected chain of PN junction diodes used in ESD protection to set a precise trigger voltage and provide a controlled discharge path** — offering predictable turn-on behavior at N × 0.7V (where N is the number of diodes), making it ideal for applications requiring specific clamping voltages without the complexity of snapback-based devices.
**What Is a Diode String?**
- **Definition**: Multiple PN junction diodes connected in series (anode of one to cathode of the next) that collectively provide a forward-bias trigger voltage equal to the sum of individual diode turn-on voltages.
- **Predictable Trigger**: Each silicon diode turns on at approximately 0.7V at room temperature, so a string of N diodes triggers at N × 0.7V (e.g., 5 diodes = 3.5V).
- **No Snapback**: Unlike GGNMOS or SCR, diode strings operate in forward conduction without snapback — voltage increases monotonically with current, eliminating latchup risk entirely.
- **Temperature Sensitivity**: Forward voltage decreases approximately 2 mV/°C per diode, so a 5-diode string's trigger voltage drops by ~10 mV/°C — significant for wide temperature range applications.
**Why Diode Strings Matter**
- **Latchup Immunity**: Zero snapback means zero latchup risk — diode strings are the safest ESD clamp type for latchup-sensitive applications.
- **Precision Trigger Voltage**: The designer can set exactly the trigger voltage needed by choosing the number of diodes — no process variation in snapback behavior to worry about.
- **Fast Turn-On**: Diodes turn on in less than 100 ps — faster than any other ESD clamp type — providing excellent CDM protection.
- **Bidirectional Use**: Back-to-back diode strings protect against both positive and negative ESD events at an I/O pad.
- **SCR Trigger Assist**: Diode strings are commonly used to provide a fast, controlled trigger for SCR-based ESD clamps that would otherwise have unacceptably high native trigger voltages.
**Diode String Applications**
**Power Supply Clamping**:
- Connect a diode string from VDD to VSS (or between power domains) to provide a controlled voltage clamp.
- Example: 5 diodes set a 3.5V trigger for a 3.3V VDD domain.
**I/O Pad Protection**:
- Primary diodes from pad to VDD and pad to VSS steer ESD current to the power rails.
- These are typically single diodes (not strings) for minimum parasitic capacitance.
**SCR Trigger Chain**:
- A diode string triggers an SCR at a controlled voltage, combining the diode's precise triggering with the SCR's high current capacity.
**Cross-Domain Clamping**:
- Diode strings between different power domains provide ESD paths for cross-domain events.
**Design Considerations**
| Parameter | Design Impact | Typical Value |
|-----------|--------------|---------------|
| Number of Diodes (N) | Sets trigger voltage (N × 0.7V) | 3-8 diodes |
| Diode Width | Sets current capacity | 100-500 µm per diode |
| Temperature Coefficient | -2 mV/°C per diode | -10 to -16 mV/°C total |
| Parasitic Capacitance | Affects signal bandwidth | 0.2-0.5 pF per diode |
| Leakage Current | Increases exponentially with temperature | pA at 25°C, nA at 125°C |
**Darlington Leakage Effect**
- **Problem**: In substrate-based diode strings, the parasitic vertical PNP transistor at each stage amplifies the leakage current of subsequent stages.
- **Mechanism**: Each diode's substrate current acts as base current for the next stage's parasitic PNP, creating a Darlington-like multiplication of leakage.
- **Impact**: A 5-diode string may have 100× higher leakage than a single diode at elevated temperature.
- **Mitigation**: Use isolated diodes (deep N-well) to break the parasitic PNP chain, or limit the string length to 3-4 diodes.
Diode strings are **the most predictable and latchup-safe ESD protection element** — their simplicity, speed, and precise voltage control make them indispensable building blocks in every ESD protection scheme, from simple I/O steering to sophisticated SCR trigger assist circuits.