DIode for Alternating Current, is a bidirectional trigger diode that conducts current only after its breakdown voltage has been exceeded momentarily. It stops conducting when the current through it drops below a value characteristic for the device, called the holding current. The behavior is typically the same for both directions of current flow. Most DIACs have a breakdown voltage around 30 V. It is similar to two Shockley or Zener diodes connected in inverse parallel. Since the DIAC is a bidirectional trigger diode, its terminals are not labeled as anode or cathode but mostly labeled as A1 or MT1 and A2 or MT2.
The diagram to the right shows the resulting five-layer device, which is known as a diac. At first glance, it seems unreasonable or even impossible, considering that each connection to the semiconductor crystal overlaps a pn junction. However, the device does work, and indeed works well.
The terms anode and cathode no longer apply, so the connections are simply named terminal 1 (T1) and terminal 2 (T2). Each terminal can serve as either anode or cathode, according to the polarity of the applied voltage.
That same applied polarity also determines which of the end junctions is active, and which one is bypassed. Thus, if T1 is positive with respect to T2, T1's N-type region is ignored (electrons are pulled away from that junction) and its P-type region serves as the anode. At the same time, the relative negative voltage at T2 pulls holes from the P-type region towards the terminal (removing them from the next junction), but tends to push electrons from its N-type region across that junction into the P-type region, thus making them available for conduction.
The diac, like the four-layer diode, remains non-conducting until its breakover voltage is reached, at which point it turns on fully and remains on until the applied voltage or circuit current are reduced below the holding values at which conduction can be maintained. Since the diac is normally used in ac circuits, operating as part of the control circuit for devices powered from a household wall socket or similar source, this is not a problem. In such applications, the diac is triggered each half-cycle of ac power, and then turns off at the end of the half-cycle when the line voltage reverses polarity.
The diagram to the right shows the resulting five-layer device, which is known as a diac. At first glance, it seems unreasonable or even impossible, considering that each connection to the semiconductor crystal overlaps a pn junction. However, the device does work, and indeed works well.
The terms anode and cathode no longer apply, so the connections are simply named terminal 1 (T1) and terminal 2 (T2). Each terminal can serve as either anode or cathode, according to the polarity of the applied voltage.
That same applied polarity also determines which of the end junctions is active, and which one is bypassed. Thus, if T1 is positive with respect to T2, T1's N-type region is ignored (electrons are pulled away from that junction) and its P-type region serves as the anode. At the same time, the relative negative voltage at T2 pulls holes from the P-type region towards the terminal (removing them from the next junction), but tends to push electrons from its N-type region across that junction into the P-type region, thus making them available for conduction.
The diac, like the four-layer diode, remains non-conducting until its breakover voltage is reached, at which point it turns on fully and remains on until the applied voltage or circuit current are reduced below the holding values at which conduction can be maintained. Since the diac is normally used in ac circuits, operating as part of the control circuit for devices powered from a household wall socket or similar source, this is not a problem. In such applications, the diac is triggered each half-cycle of ac power, and then turns off at the end of the half-cycle when the line voltage reverses polarity.
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