Which device can be viewed as a diac with an added gate terminal?

The device that can be viewed as a diac with an added gate terminal is the triac. A diac is typically used to control alternating current (AC) and can conduct in both directions, while a triac is similar in functionality but includes an additional gate terminal that allows for better control of the device's conduction properties.

The gate terminal in a triac enables it to be turned on at a specific point in the AC cycle, making it more versatile for applications that require precise control over timing and power delivery. This is particularly important in various control systems like light dimmers and motor speed controllers, where being able to trigger the device at different points of the AC waveform is beneficial.

In contrast, a thyristor, while also a semiconductor device used for switching and controlling, does not have the same bidirectional current capabilities and is typically used in DC applications. A transistor is a different type of device altogether, primarily used for amplification in electronic circuits. Phototransistors are transistors that are activated by light rather than an electrical input and thus serve a different purpose than a diac or triac. This distinction underscores why the triac stands out as similar to a diac with its added gate terminal.