Guard Ring

Definition: A guard ring is a protective structure used in CMOS integrated circuits to prevent latchup and improve the reliability of the device. It is a continuous ring of heavily doped material (usually N+ or P+) that surrounds a sensitive area of the circuit, such as a transistor or a group of transistors. The guard ring serves as a barrier to prevent the formation of parasitic bipolar transistors and the flow of unwanted currents that can cause latchup.

Key Characteristics:

  • Protective structure used in CMOS circuits
  • Surrounds sensitive areas of the circuit, such as transistors or groups of transistors
  • Helps to prevent latchup and improve device reliability
  • Formed using heavily doped material (N+ or P+)
  • Acts as a barrier to prevent the formation of parasitic bipolar transistors
  • Collects and diverts unwanted currents away from sensitive areas

Latchup in CMOS Circuits: Latchup is a phenomenon that can occur in CMOS circuits when parasitic bipolar transistors are unintentionally formed between the N-well and P-substrate regions. These parasitic transistors can create a low-resistance path between the power supply (VDD) and ground (VSS), resulting in excessive current flow and potential damage to the device. Latchup can be triggered by various factors, such as:

  • Voltage overshoots or undershoots on the input/output pins
  • Improper sequencing of power supply voltages
  • Exposure to ionizing radiation or high-energy particles

Guard Ring Placement: Guard rings are strategically placed around sensitive areas of the CMOS circuit to prevent latchup. The placement of guard rings depends on the specific layout and design of the circuit. Common placement strategies include:

  1. Around individual transistors:
    • Guard rings are placed around each transistor to isolate them from neighboring devices
    • Helps to prevent the formation of parasitic bipolar transistors between adjacent transistors
  2. Around groups of transistors:
    • Guard rings are placed around a group of transistors that form a functional block
    • Helps to isolate the block from other parts of the circuit and prevent latchup
  3. Around input/output pads:
    • Guard rings are placed around the input/output pads of the circuit
    • Helps to protect the internal circuitry from voltage overshoots or undershoots on the pads

Advantages:

  • Prevents latchup and improves the reliability of CMOS circuits
  • Helps to protect sensitive areas of the circuit from unwanted currents and parasitic effects
  • Reduces the risk of device failure due to latchup-induced damage
  • Enables the use of CMOS technology in applications that require high reliability
  • Can be easily integrated into the standard CMOS fabrication process

Design Considerations:

  • Proper sizing and spacing of the guard ring to ensure effective latchup prevention
  • Minimizing the impact of the guard ring on the overall circuit area and performance
  • Ensuring proper connection and biasing of the guard ring to the appropriate power supply or ground
  • Considering the effect of the guard ring on the circuit’s electromagnetic compatibility (EMC) and signal integrity

Guard rings are an essential design technique in CMOS integrated circuits to prevent latchup and improve device reliability. By strategically placing heavily doped rings around sensitive areas of the circuit, guard rings help to suppress the formation of parasitic bipolar transistors and divert unwanted currents away from critical regions. The use of guard rings enables the reliable operation of CMOS circuits in various applications, including those that require high levels of robustness and immunity to latchup-induced failures.