Insulated gate field-effect transistor input protection circuit

Abstract

Abstract A circuit for protecting an insulated gate field-effect transistor (IGFET) circuit from damage caused by spurious, high voltage inputs resulting primarily from static charge is made up of a pair of IGFET''s. A first protection circuit IGFET has a drain connected to the gate of at least one main circuit IGFET to be protected, the gate of the main circuit IGFET being connected to an input terminal. The source of the first protection circuit IGFET is connected to a common reference. The gate of the first protection circuit IGFET is connected to the drain of a second protection circuit IGFET whose source is connected to the common reference and whose gate is connected to a discharge terminal for applying a voltage to the gate sufficient to turn on the second protection circuit IGFET. The first protection circuit IGFET goes into an avalanche condition when a spurious signal of a polarity to cause a reverse bias is of sufficient amplitude to start an injection of carriers from the drain to the gate. The avalanche condition is maintained until the drain voltage drops below the avalanche maintenance value. The charge on the gate of the first protection circuit IGFET may or may not leak off by the time the circuit is ready for testing. Whether the charge has leaked off or not is of no consequence because when the circuit is connected to the tester, a voltage is applied to the gate of the second protection circuit IGFET turning it on and causing the charge on the gate, if any, of the first protection circuit IGFET to be conducted to ground.

Inventors

• 👤 L Clark

Bibliographic Details