The powder formation in a germane (GeH4)-silane (SiH4) plasma was investigated for a range of deposition parameters used in the processing of amorphous silicon devices, in particular the pressure, p, and the total gas flow, phi. For that purpose the plasma was monitored continuously by measuring the de self-bias, V-sb, as a function Of time. At certain plasma conditions oscillations in the self-bias are observed. In literature these oscillations have been related to the spatial oscillation of the powder-forming region in the plasma and they can be used as an indication of the so-called alpha-gamma' transition of the plasma, i.e., the moment at which powder formation starts to occur in the plasma. Changes in the plasma are monitored by measuring the amplitude of the oscillations of the self-bias, Delta V-sb. It was found that the dependence of Delta(sb) on P can be divided into three ranges: (i) low-pressure range in which Delta V-sb is small; (ii) medium-pressure range in which Delta(sb) varies significantly and powder is formed in the plasma; and (iii) high-pressure range in which the plasma is visually unstable. The pressure at which the transition from range (i) to (ii) occurs is indicative for the a-y' transition and appears to depend on the gas. composition, in particular the [GeH4]/[SiH4] flow ratio. This ratio also determines the rate of powder formation. We argue that the production of GeH2 radicals is responsible for this formation. In addition we conclude that the alpha-gamma' transition is determined by the partial pressure of SiH4 plus GeH4 and is not dependent on the residence time of the gas in the reaction chamber.