Surge protection device

A wide range of devices referred to as surge protection devices have been devised. Even the number of such devices that fall in the category of two-terminal surge protectors is considerable. The better of these are not limited to the function of clamping the voltage across the device terminals at a fixed breakdown voltage at the time of breakdown caused by occurrence of a surge (i.e. do not function simply as constant-voltage diodes). Instead they further exhibit negative characteristics when the device current that begins to flow at the time of the device breakdown increases to above the breakover current value. As a result, the voltage across the terminals after breakdown is shifted to a clamp voltage that is lower than the breakdown voltage. It therefore becomes possible to absorb large currents.

A wide range of devices referred to as surge protection devices have been devised. Even the number of such devices that fall in the category of two-terminal surge protectors is considerable. The better of these are not limited to the function of clamping the voltage across the device terminals at a fixed breakdown voltage at the time of breakdown caused by occurrence of a surge (i.e. do not function simply as constant-voltage diodes). Instead they further exhibit negative characteristics when the device current that begins to flow at the time of the device breakdown increases to above the breakover current value. As a result, the voltage across the terminals after breakdown is shifted to a clamp voltage that is lower than the breakdown voltage. It therefore becomes possible to absorb large currents.

Structurally speaking the surge protection device 10 shown in FIG. 9(a) is formed with the regions 1, 2, 3 and 4 stacked vertically in the thickness direction of the region 1. Moreover, as will be clear from the explanation of the device's operation given later, the device current resulting from surge absorption flows mainly in the thickness direction of the first region, between the third and fourth regions. The device can therefore be said to be of the vertical type. In contrast, in the surge protection device shown in FIG. 9(b), the fourth region 4 is situated on the front surface at a position offset laterally from the second and third regions 2, 3. Since the device current during operation also flows laterally, this device can be said to be of the lateral type.

In either type of device, the second region 2 and fourth region 4 have to be of opposite conductivity type from the first region 1 so that each makes a pn junction therewith. Therefore, as shown in the figures, in the case where the first region 1 is of n conductivity type, the second region 2 and the fourth region 4 are of p conductivity type. In a case where punch-through utilized as the initial breakdown phenomenon in the manner to be explained later, however, it is preferable to constitute the second region 2 to be of somewhat low concentration p conductivity type, namely to be of p type.

On the other hand, the third region 3 and the first region 1 have to be of the same conductivity type as each other and of opposite polarity type from the second region 2 so as to form carrier injection junctions for injecting minority carriers into the second region. Since, as will be explained later, the third region 3 constitutes one end of the main device current path after breakdown, it preferably has high conductivity (i.e. is preferably of n.sup. type).