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What Is a FET Latch-Up?

FET is an abbreviation for Field Effect Transistor, a type of electronic device that controls the flow of electrical current through a circuit. The simplest type of FET is a voltage controlled resistor, in which the resistive element is a bar of silicon. The term FET latch-up refers to a destructive, high-current state that can be triggered by certain electrical conditions acting on the components of the FET. FET latch-up typically prevents normal circuit control.

Semiconductor
- A FET is composed of two types of semiconductor crystal -- materials that conduct electricity, but very poorly -- known as n-type and p-type materials. Two terminals, or electrodes, known as the drain and source, are connected to the n-type material, while a third terminal, known as the gate, is connected to the p-type material. The current flowing between the source and drain is controlled by an electric field created by a voltage applied between the source and the gate.
Cause
- FET latch-up occurs when four alternating n-type and p-type regions are brought close together, such that they effectively form two bipolar transistors -- transistors that use both positive and negative charge carriers -- known as NPN or PNP transistors. The electrical current applied to the base of the first transistor is amplified and passed to the second transistor. If the output current of both transistors is greater than the input current -- or in other words, the current ̶0;gain̶1; is greater than 1 -- the current through both of them increases.
Effects
- FET latch-up leads to excessive dissipation of power and faulty logic in the affected gate, or gates. Excessive power dissipation generates excessive heat, which can destroy the FET altogether in extremes cases. FET latch-up is therefore extremely undesirable and its prevention has become a major design issue, particularly in modern transistors. Modern transistors have shrunk to sizes as small as 59 micro inches, or 59 millionths of an inch, in an effort to increase circuit density and improve overall performance.
Prevention
- A FET is what is known as a majority carrier device. In other words, current is conducted by the majority carrier species -- either negatively charged particles, called electrons, or positively charged carriers, called holes -- depending upon the precise design of the FET. FET latch-up can be avoided by separating the n-type and p-type materials with the FET structure. Separation is often achieved by etching a deep, narrow trench filled with insulating material between the n-type and p-type materials.

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