DEVICE CHARACTERISTICS

DEVICE CHARACTERISTICS :

                      The device can be in one of the three following states :

1. Forward biased and blocking 
2. Forward biased and conducting 
3. Reverse biased and blocking.

               The transition from the first to the second state is called turn-on, while the transition from the second to the third state is called turn-off. The characteristics of the device refer to the parameters of the device both in steady-state and transient conditions(during the transition of state).

Steady state characteristics :

            off-state: The volt-ampere characteristics of the device are during the off-state(both forward and reverse blocking), only a small magnitude of leakage current flows (of the order of 100 mA). The blocking capability with gate open is specified in terms of limiting repetitive peak forward (VDRM) or reverse (VRRM) voltages.

There is also a non-repetitive peak reverse voltage rating (Vrsm) which is specified. The voltage ratings are specified for power frequency (50 or 60 Hz) half-cycle sinusoidal voltages and rated junction temperature of the thyristor(typically 125 C ). The variation of the voltage ratings with junction temperature.

       The behaviour of thyristors under transient voltages is not well understood. However, according to one particular study [10,11], the following conclusions can be drawn:

1. The transient break over voltage of a thyristor is independent of its voltage rating.
2. The forward breakover voltage of a thyristor under a transient voltage may be lower than its voltage rating and decreases with increasing junction temperature. 
3. The instant of forward breakover of a thyristor under slow transients (30600) can be significantly lower compared to those obtained under fast transient voltages(1.2/50s). This behaviour has been attributed to the statistical and formative time lags of avalanche formulation.

         The thyristor capability in the reverse direction is related to the permissible energy losses which in turn is dependent on the variation of the reverse avalanche current with the voltage magnitude of the reverse voltage. The transient voltage blocking capability of a thyristor is perhaps related to the critical power needed to damage the device. Cumulative effects due to transients with less that critical power may result in degradation of the device.

Onstate : There are a number of electrical and thermal parameters that characterize the on-state behaviour. Some of these are as follows:

1. On-state voltage 
2. Mean (average) on-state current ITAV
3. Root mean square value of the on-state current ITRMS
4. Surge (non-repetitive) on-state current (ITSM)
5. Non-repetitive survival rating 
6. Holding curent (IH)
7. Operating temperature range 
8. Junction to case thermal resistance (RTHJC)
9. Contact thermal resistance 

           The on-state voltage is the anode to cathode voltage of a thyristor in the forward conducting state. It is also referred to as the forward voltage drop. This is an important characteristic affecting the power losses during on-state and the parallel operation of thyristors.

           On-state voltage depends upon a number of factors such as the width of various regions, life time and mobility of minority carriers, the physical mechanisms of recombination, etc. The power loss at6 current densities around 100A/Cm (conditions corresponding to maximum contionuous rating) is due to recombinations, whereas at current densities around 1000A/cm (surge conditions), it is mainly due to ohmic heating.

           Silicon wafers of small thickness and long carrier life times give rise to low on-state voltages. However, increasing the carrier life time also increases turn-off time. Trying to optimize both may result in high reverse recovery current.

          On-state curent ratings are determined by the junction temperature which must be kept below the value necessary to ensure that it can block the recovery voltage after a worst case credible overcurrent. The temperature build-up in a thyristor valve.(The failure of a thyristor following an overcurrent is essentially a high temperature voltage failure produced by intense local heating induced by excessive leakage current at high voltage).

               The surge current capability of a thyristor is based on its filamentation temperature at which mesoplasmas are formed. However, the requirement to have surge suppression capability (voltage blocking following the surge current) will result in the operation of the thyristor at reduced junction temperature. The maximum junction temperature attained due to the cumulative effects caused by the passages of repeated current surges should be below the thyristor filamentation temperature.

              The holding current IHis defined as the minimum current required to maintain the thyristor in the on-state. It is the forward current below which it will cease to conduct . As the forward current is reduced, the turn-off occurs when recombination causes the minority carrier level to fall below the base-region doping level. The holding current reduces with increase in junction temperature.