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_BE) range (<1.35V). On the contrary, if the ledge is fully depleted, HBT's current gain can be modulated in the whole V_BE range up to 1.6V. 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_L<V_BE). These discoveries not only lead to a simple method of monitoring the effectiveness of HBT's ledge passivation bur also create a four-terminal HBT with an extra ledge electrode biased to control device's current gain and 1/f noise.