Dark current fluctuations in three types of multiquantum well (MQW) GaAs/AlGaAs avalanche photodiodes have been investigated. This work investigated both forward and reverse biases and furthered the understanding of the physics of noise in MQW structures used in photodetection (avalanche photodiodes) and emitters (semiconductor lasers). Under reverse bias, the increase of dark current and flicker noise compared with a classical PIN junction or Schottky junction is caused by thermionic field effect (TFE) conduction at the MQWs barriers. A method is developed as a function of temperature to detect the TFE at each barrier of the MQW before the onset of multiplication. Decreasing temperature is known to produce an increased resistance limit across an MQW emitter photodiode. Under forward bias, white and flicker noises are governed by the increase of resistance limit.