Triggering Mechanisms of PNPN diode
The most common method of triggering a two-terminal p-n-p-n is simply to raise the bias voltage to the peak value Vp. This type of voltage triggering results in a breakdown (or significant leakage) of the reverse-biased junction j2, the accompanying increase in current provides the injection at j1 and j3, as well as the transport required for switching to the conducting state. The breakdown mechanism commonly occurs by the combination of base-width narrowing and avalanche multiplication.
When carrier multiplication occurs in j2, many electrons are swept into n1 and holes into p2. This process provides to these regions the majority carriers needed for increased injection by the emitter junctions. Because of transistor action, the full breakdown voltage of j2 need not be reached. As we see that breakdown occurs in the collector junction of a transistor with iB = 0 when Mα = 1. In the coupled-transistor case of the p-n-p-n diode, breakdown occurs at j2 when
where Mp is the hole multiplication factor and Mn is the multiplication factor for electrons.
As the bias v increases in the forward-blocking state, the depletion region about j2 spreads to accommodate the increased reverse bias on the center junction. This spreading means that the neutral base regions on either side (n1 and p2) become thinner. Since α1 and α2 increase as these base widths decrease, triggering can occur by the effect of base-width narrowing. A true punch-through of the base regions is seldom required, since moderate narrowing of these regions can increase the alphas enough to cause switching.
Furthermore, switching may be the result of a combination of avalanche multiplication and base-width narrowing, along with possible leakage current through j2 at high voltage. From the above equ, it is clear that, with avalanche multiplication present, the sum α1 α2 need not approach unity to initiate breakdown of j2. Once breakdown begins, the increased numbers of carriers in n1 and p2 drive the device to the forward-conducting state by the regenerative process of coupled transistor action. As switching proceeds, the reverse bias is lost across j2 and the junction breakdown mechanisms are no longer active. Therefore, base narrowing and avalanche multiplication serve only to start the switching process.
If a forward-bias voltage is applied rapidly to the device, switching can occur by a mechanism commonly called dv/dt triggering. Basically, this type of triggering occurs as the depletion region of j2 adjusts to accommodate the increasing voltage. As the depletion width of j2 increases, electrons are removed from the n1 side, and holes are removed from the p2 side, of the junction.
For a slow increase in voltage, the resulting flow of electrons toward j1 and holes toward j3 does not constitute a significant current. If dv/dt is large, however, the rate of charge removal from each side of j2 can cause the current to increase significantly. In terms of the junction capacitance (Cj2) of the reverse-biased junction, the transient current is given by
where vj2 is the instantaneous voltage across j2. This type of current flow is often called displacement current. The rate of change of Cj2 must be included
in calculating current, since the capacitance varies with time as the depletion width changes.
The increase in current due to a rapid rise in voltage can cause switching well below the steady state triggering voltage VP. Therefore, a dv/dt rating is usually specified along with VP for p-n-p-n diodes. Obviously, dv/dt triggering can be a disadvantage in circuits subjected to unpredictable voltage transients.
The various triggering mechanisms discussed in this section apply to the two-terminal p-n-p-n diode. As we see, the semiconductor-controlled rectifier is triggered by an external signal applied to a third terminal.