Resonance phase operation of heterobipolar transistors beyond their transit frequency

In this paper we explain the resonant enhancement of the current gain at frequencies beyond the transit frequency and we describe the experimental verification of the resonance phase effect. A resonance phase transistor (RPT) is an HBT where transit time effects in the base region and the collector junction are utilized to achieve amplification beyond the transit frequency f_T. In this work a relatively thick graded SiGe base layer of 120nm is used to facilitate accurate experimental characterization. In the setup analyzed, a current gain of 6.5 dB is measured at 40 GHz. The RPT is a potentially unstable device with a small region of stability well suited for oscillator applications. The breakdown voltage (20V in the considered case) is much higher as that of an HBT. With the RPT cascode circuit we propose a circuit and an integration topology that allows the realization of stable RPT amplifiers.