A High Signal-to-Noise Oscillator for Contactless Measurement of Photoinduced Carrier Lifetimes

A wide-band apparatus for contactless measurement of small changes in photoinduced carrier population is described. Since for some phenomena the signals of interest have very low amplitude, and since the generator supplying energy for the measurement is an oscillator loosely coupled electromagnetically to the semiconductor, very-low-noise circuitry is required. Further, sensitivity to minute conductivity changes is enhanced when the oscillation amplitude is made directly proportional to the figure of merit, Q, of the coupled oscillator resonant parallel inductance-capacitance (LC) tank. The circuit to approximate the desired dependence, amplitude = kQ, contains only two field-effect transistors and is basically an oscillator supplied by a constant current rather than by a constant voltage source. Its operation is analyzed via modeling that specifies the output signal as a function of device and circuit parameters; excellent correspondence with empirical data is shown. The oscillator also operates as an infinite impedance detector of its own amplitude; this amplitude is shown to be linearly dependent on carrier concentration over significant ranges. Thus, the oscillator steady-state and transient responses may be directly related to wafer conductivity, induced carrier amplitude, and lifetime. The described apparatus is sensitive to carrier concentration changes of less than 1011 electrons or holes per cubic centimetre of silicon and will provide measurement of lifetimes from a microsecond to several seconds.