Early transistors were not available with sufficient voltage and current ratings for most inverter applications .
It was the 1957 introduction of the thyristor or silicon - controlled rectifier ( SCR ) that initiated the transition to solid state inverter circuits .
Since early transistors were not available with sufficient voltage and current ratings for most inverter applications , it was the 1957 introduction of the thyristor or silicon - controlled rectifier ( SCR ) that initiated the transition to solid state inverter circuits .