Current gain modulation and 1/f noise control of GaAs based HBTs using on-ledge Schottky diodes

The HBT´s current gain and 1/f noise can be effectively modulated and controlled by using on-ledge Schottky diodes, respectively. The behavior of the gain modulation is determined by the extent of the emitter ledge depletion. If the ledge is partially depleted, HBT´s current gain can be modulated only in the low base-emitter bias (V/sub BE/) range (<1.35 V). On the contrary, if the ledge is fully depleted, HBT´s current gain can be modulated in the whole V/sub BE/ range up to 1.6 V. Furthermore, in the case with a fully depleted ledge, the 1/f noise spectra density can be greatly reduced to be independent of the base current by biasing the on-ledge Schottky diode at a high voltage (V/sub L/>V/sub BE/). These discoveries not only lead to a simple method of monitoring the effectiveness of HBT´s ledge passivation but also create a four-terminal HBT with an extra ledge electrode biased to control device´s current gain and 1/f noise.