Characteristics of Thyristor or Characteristics of SCR

The silicon-controlled rectifier (SCR) described here is a thyristor with four alternating p-type and n-type semiconductor layers. These layers form three p-n junctions. The device has an anode, cathode and gate. It blocks voltage until a valid turn-on condition occurs, then latches into conduction while its anode current remains high enough. The symbol and test circuit below introduce the characteristics of thyristor.

V-I Characteristics of a Thyristor

The anode-cathode path connects to the main supply voltage through a current-limiting load. A separate source, E_s, supplies positive gate current relative to the cathode when switch S closes.
The plotted V-I characteristics of a thyristor relate anode-cathode voltage (V_a) to anode current (I_a). They show reverse blocking, forward blocking and forward conduction. A real design must use the device datasheet limits rather than treating the idealised curve as a rating.

Reverse Blocking Mode of Thyristor

In the reverse blocking mode of the thyristor, the cathode is positive relative to the anode. The gate supply E_s is disconnected by leaving switch S open. This operating region appears in the third quadrant of the conventional anode-current versus anode-voltage plot.

Junctions J₁ and J₃ are reverse biased, while junction J₂ is forward biased. Only reverse leakage current flows. Its value depends on the device, reverse voltage and junction temperature, so it is not always a few microamps.
If reverse voltage reaches breakdown, avalanche current rises at J₁ and J₃. The curve labels this region V_BR. Normal operation must remain within the datasheet’s repetitive peak reverse-voltage rating because excessive current and junction temperature can damage the thyristor. Below V_BR, the device has high reverse impedance. V_BR is a breakdown boundary, not a recommended operating point. Reverse blocking can be modelled as an open circuit only when leakage is negligible for the application.

Forward Blocking Mode

In forward blocking mode, the anode is positive relative to the cathode and the gate circuit remains open. The SCR is forward biased but still off, as shown below.

Junctions J₁ and J₃ are forward biased, while junction J₂ is reverse biased. A small forward off-state leakage current flows. The device continues blocking while anode voltage stays within its rated off-state limit and no valid trigger occurs.

While the SCR is forward blocking, most of the applied voltage appears as its anode-cathode voltage drop; the low on-state drop does not apply yet. A positive gate-current pulse normally triggers conduction at a forward voltage well below breakover. With the gate open, raising anode voltage far enough can make junction J₂ enter avalanche breakdown at the forward breakover voltage labelled V_B0 on some diagrams. Deliberate breakover turn-on is normally avoided. Below V_BO and within the datasheet off-state rating, the SCR retains high impedance apart from leakage current.

Forward Conduction Mode

A gate pulse or forward breakover at V_BO initiates regenerative internal action and moves the SCR into forward conduction. The anode current must exceed the latching current long enough for the device to remain on after the gate pulse ends. Once latched, a standard SCR cannot be turned off through its gate. It returns to blocking only after anode current falls below the holding current long enough for charge to clear. The on-state voltage is low compared with the blocking voltage, but its actual value and permitted current come from the datasheet.