Covenant portable shield

Did you mean the Halo 3 Covenant equipment, the deployable cover? A stationary shield generator is a device used by the Covenant during offensive ground operations. The human counterpart for the stationary shield is the combat barrier.

Overview
Essentially larger versions of the point defense gauntlets used by Kig-yar, towering at 2.49 meters, or 8.17 feet, these ovoid-shaped shields are used to provide extra protection for Covenant troops and are normally placed at strategic locations. They are effective at deflecting small-arms projectiles, but struggles against explosives or large amount of plasma fire. At least two variations exist: the first variant has a base with three points of field generation whereas the second variant has only one.

Gameplay
In Halo: Combat Evolved if one of these shields is hit by a vehicle, it will be permanently disabled. In Halo 2, vehicles cannot collapse them on impact. In Halo 3 it reappears, but is remade as a piece of Equipment, the Deployable Cover, where it can be deployed by troops.

Deployable cover
The deployable cover replaced the stationary shield generator in Halo 3. It is portable and curved rather than flat, and is destructible.

Trivia

 * The AI won't see past shield generators since it acts as a solid, opaque object to the AI.
 * If a player has a Shotgun, or other long barreled weapon, and they walk right up against a stationary shield, the barrel goes through the shield and one can shoot through it at opposing enemies. Similarly, it is possible to snipe enemies while never having to expose oneself and take cover again. The length of the weapon which is exposed should be disregarded, since the shot from most weapons come from the camera.
 * In Halo 2 on the level Delta Halo, one can use the stationary shield generators placed at the entrance to one of the Covenant-held temple complexes to propel their vehicle away at a high speed by first disabling the shields, placing the vehicle on the generator and waiting until the device activates. This can also happen in Halo: Combat Evolved, though if the player is in a human vehicle, it is likely to fall out and die from fall damage.
 * In Halo 2, if one whacks a Grunt body into a stationary shield generator with the Sputnik Skull on, the body will hit the shield generator hard enough to disable it.
 * In Halo 2 if one stands on top of a deployed generator and shoots it with a Plasma Rifle, the generator will overload and you will be floating on top of an invisible shield.

List of appearances

 * Halo: Combat Evolved
 * Halo 2
 * Halo: Ghosts of Onyx
 * Halo: Reach
 * Halo: Combat Evolved Anniversary
 * Halo 4
 * Spartan Ops
 * Halo: Spartan Assault
 * Halo 2: Anniversary