Abstract: A semiconductor test system includes at least one semiconductor wafer having working dies and at least one test die formed therein. Each of the working dies includes at least one bipolar transistor. A tester selectively supplies a changing direct current (DC) input signal to a selected test die and monitors a DC output signal therefrom. Each test die includes a test oscillator having at least one sample bipolar transistor substantially identical to the bipolar transistors of the working dies. The test oscillator switches between a non-oscillating state and an oscillating state as the DC input signal changes, and generates the DC output signal to the tester indicative of switching between the non-oscillating state and the oscillating state. A threshold level of a bias current that causes the test oscillator to switch between the non-oscillating state and the oscillating state is correlated to the maximum oscillation frequency and the transition frequency of the sample bipolar transistor.