The quantum confinement effects of Ge quantum-dots (QDs) grown by molecular beam epitaxy (MBE) are investigated by capacitance-voltage (C-V), admittance spectroscopy, and deep level transient spectroscopy (DLTS). The potential height of hole in the Ge dot obtained by C-V intercept, admittance spectroscopy and DLTS are 0.335, 0.341, and 0.338 eV, respectively. The DLTS method is also used to observe the changes of hole concentration, activation energy (E-a), and capture cross-section with filling time duration (t(p)) and bias voltage. The results show that the hole concentration in the Ge quantum-dots is a function of the pulse duration (t(p)) and reversed bias voltage, the activation energy and capture cross-section decrease with the increasing filling time duration due to the Coulomb charging effect. (C) 2003 Elsevier B.V. All rights reserved.