This paper discusses recent advances in state-of-the-art optoelectronic techniques applied to areas pertinent to transistor small-signal and large-signal characterization. First, the aspects of optoelectronic techniques for electrical signal measurement, generation, and transmission are studied. Second, based on these results, a large-signal digital-switching transistor characterization methodology is developed. Specifically, this methodology is used to estimate the parameters of a high electron mobility transistor and to obtain the large-signal characteristics of this device on a picosecond time scale. The measurements are then compared to a SPICE-based time-domain model. Third, the time-domain measurement technique is then extended to obtain two-port frequency-domain characteristics of another, similar transistor with 100 GHz bandwidth. These results compare favorably to conventional HP8510 network analyzer measurements over a common frequency span of 40 GHz.