Current-gain in FETs beyond cut-off frequency

Driven by the fast-growing demand for high-frequency applications, electronics engineers are steadily pushing transistor technologies beyond their high-frequency limits. A key figure of merit to quickly assess the potential of a transistor for high-frequency applications is given by the cut-off frequency (fT) that is defined as the frequency at which the short-circuit current-gain (h21) drops to unity. However, field-effect transistors (FETs) can exhibit a current-gain even at frequencies beyond fT, due to the resonance of the extrinsic reactive contributions leading to a peak in the magnitude of h21. Based on an extensive analysis, this letter aims to demonstrate that, although not all FETs exhibit such a current-gain peak at frequencies beyond fT, this effect is inherent in any FET devices and its appearance simply depends on the specific amount of the extrinsic reactive contributions, besides to the operating frequency.