P. W. Anderson, Absence of Diffusion in Certain Random Lattices, Physical Review, vol.109, issue.5, p.1492, 1958.
DOI : 10.1103/PhysRev.109.1492

V. Ambegaokar, B. I. Halperin, and J. S. Langer, Hopping Conductivity in Disordered Systems, Physical Review B, vol.4, issue.8, p.2612, 1971.
DOI : 10.1103/PhysRevB.4.2612

B. I. Shklovskii and A. L. Efros, Electronic properties of doped Semiconductors, Springer series in Solid- State Science, 1984.

A. Aharony, Y. Zhang, and P. Sarachik, Universal crossover in variable range hopping with Coulomb interactions, Physical Review Letters, vol.68, issue.26, p.3900, 1992.
DOI : 10.1103/PhysRevLett.68.3900

M. Pollak and I. Riess, A percolation treatment of high-field hopping transport, Journal of Physics C: Solid State Physics, vol.9, issue.12, p.2339, 1976.
DOI : 10.1088/0022-3719/9/12/017

R. Rentzsch, I. S. Shlimak, and H. Berger, Hopping Conductivity of Undoped ZnSe Thin Films, Physica Status Solidi (a), vol.6, issue.2, p.487, 1979.
DOI : 10.1002/pssa.2210540207

M. Van-der-meer, R. Schuchardt, and R. Keiper, Strong-Field Hopping in Disordered Semiconductors A Problem of Directed Percolation, physica status solidi (b), vol.42, issue.2, p.571, 1982.
DOI : 10.1002/pssb.2221100226

B. I. Shklovskii and A. L. Efros, Percolation theory and conductivity of strongly inhomogeneous media, Uspekhi Fizicheskih Nauk, vol.117, issue.11, p.401, 1975.
DOI : 10.3367/UFNr.0117.197511a.0401

H. Böttger and V. V. Bryksin, Effective medium theory for the hopping conductivity in high electrical fields, Physica Status Solidi (b), vol.12, issue.1, p.219, 1979.
DOI : 10.1002/pssb.2220960121

H. Böttger, P. Szyler, and D. Wegener, Numerical Investigation of Non-Ohmic Hopping Conduction at Low Temperatures, physica status solidi (b), vol.22, issue.2, p.179, 1985.
DOI : 10.1002/pssb.2221280267

H. Böttger, P. Szyler, and D. Wegener, Numerical study of non-ohmic R-?? hopping transport, physica status solidi (b), vol.11, issue.2, p.143, 1986.
DOI : 10.1002/pssb.2221330256

H. Böttger and D. Wegener, On the Percolation Approach to the Hopping Conduction in Strong Electrical Fields, physica status solidi (b), vol.28, issue.1, p.413, 1984.
DOI : 10.1002/pssb.2221210144

H. Böttger, V. V. Böttger, and D. Wegener, Bryksin in Hopping Conduction in Solids, Hopping and Related Phenomena, pp.236-259, 1990.

E. We-have, ?. Mob, and B. , 1 (see Sec. III) with the mobility edge E Mob given by the Ioffe-Regel criterion (2mE Mob ) 1/2 ? el = where ? el is the elastic mean free path. Since our sample is amorphous, ? el ? 0.3 nm) yielding E Mob ? 0.35 eV ? E F . With ? = 2 ? 6 nm (see Sec. V) it is found

M. Telnic, L. Vescan, N. Croitoru, and C. Popescu, High-field conduction mechanism in amorphous germanium, Physica Status Solidi (b), vol.24, issue.2, p.699, 1973.
DOI : 10.1002/pssb.2220590239

C. E. Nebel, R. A. Street, N. M. Johnson, and C. C. Tsai, -Si:H. II. Dark conductivity, Physical Review B, vol.46, issue.11, p.6803, 1992.
DOI : 10.1103/PhysRevB.46.6803
URL : https://hal.archives-ouvertes.fr/inria-00510068

B. Popescu, T. Wright, C. J. Adkins, and S. Iovan, Analysis of High Field Hopping Transport in a-Si1?yTay:H Based on the Effective Temperature Model, physica status solidi (b), vol.205, issue.1, p.77, 1998.
DOI : 10.1002/(SICI)1521-3951(199801)205:1<77::AID-PSSB77>3.0.CO;2-R

T. W. Kenny, P. L. Richards, I. S. Park, E. E. Haller, and J. W. Beeman, He temperatures, Physical Review B, vol.39, issue.12, p.8476, 1989.
DOI : 10.1103/PhysRevB.39.8476

F. Tremblay, M. Pepper, R. Newbury, D. Ritchie, D. C. Peacock et al., -type GaAs, Physical Review B, vol.40, issue.5, p.3387, 1989.
DOI : 10.1103/PhysRevB.40.3387
URL : https://hal.archives-ouvertes.fr/hal-01059230

N. Wang, F. C. Wellstood, B. Sadoulet, E. E. Haller, and J. Beeman, Electrical and thermal properties of neutron-transmutation-doped Ge at 20 mK, Physical Review B, vol.41, issue.6, p.3761, 1990.
DOI : 10.1103/PhysRevB.41.3761

S. M. Grannan, A. E. Lange, E. E. Haller, and J. W. Beeman, <1 K, Physical Review B, vol.45, issue.8, p.4516, 1992.
DOI : 10.1103/PhysRevB.45.4516

R. W. Van-der-heijden, G. Chen, A. T. De-waele, H. M. Gijsman, and F. P. Tielen, Nonlinear current-voltage characteristics of ion-implanted Si:As in the hopping transport regime, Philosophical Magazine Part B, vol.165, issue.4, p.849, 1992.
DOI : 10.1080/13642819208204925

P. Stephanyi, C. C. Zammit, P. Fozooni, M. J. Lea, and G. Ensell, A new critical point in the non-linear conductivity due to variable-range hopping in Si, Journal of Physics: Condensed Matter, vol.9, issue.4, p.881, 1997.
DOI : 10.1088/0953-8984/9/4/008

L. Dumoulin, L. Bergé, S. Marnieros, and J. Lesueur, Progress in low temperature thin film thermometers, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, vol.370, issue.1, p.211, 1996.
DOI : 10.1016/0168-9002(95)01120-X
URL : https://hal.archives-ouvertes.fr/in2p3-00000504

N. Perrin, Nonlinearities in the current???voltage characteristics of neutron transmutation doped germanium at millikelvin temperatures, Journal of Applied Physics, vol.82, issue.7, p.3341, 1997.
DOI : 10.1063/1.365644

M. Sanquer, R. Tourbot, and B. Boucher, Correlation between High- and Low-Temperature Conductivities in High Resistive Disordered Metals, Europhysics Letters (EPL), vol.7, issue.7, p.635, 1988.
DOI : 10.1209/0295-5075/7/7/011

J. L. Pichard, M. Sanquer, K. Slevin, and P. Debray, Broken symmetries and localization lengths in Anderson insulators: Theory and experiment, Physical Review Letters, vol.65, issue.14, p.1812, 1990.
DOI : 10.1103/PhysRevLett.65.1812

P. Hernandez and M. Sanquer, Negative magnetoconductance in an Anderson insulator with strong spin-orbit scattering, Physical Review Letters, vol.68, issue.9, p.1402, 1992.
DOI : 10.1103/PhysRevLett.68.1402

M. Specht, L. P. Levy, F. Ladieu, and M. Sanquer, Magnetopolarizability at the Metal-Insulator Transition, Physical Review Letters, vol.75, issue.21, p.3902, 1995.
DOI : 10.1103/PhysRevLett.75.3902
URL : https://hal.archives-ouvertes.fr/cea-01396483

F. Ladieu, M. Sanquer, and J. P. Bouchaud, Depinning transition in Mott-Anderson insulators, Physical Review B, vol.53, issue.3, p.973, 1996.
DOI : 10.1103/PhysRevB.53.973

Y. Imry, Y. Gefen, and D. J. Bergman, Dielectric anomalies near the Anderson metal-insulator transition, Physical Review B, vol.26, issue.6, p.3436, 1982.
DOI : 10.1103/PhysRevB.26.3436

E. T. Schwartz and R. O. , Thermal boundary resistance, Reviews of Modern Physics, vol.61, issue.3, p.605, 1989.
DOI : 10.1103/RevModPhys.61.605

J. R. Cooper, W. P. Beyermann, S. W. Cheong, and G. Gruner, alloys, Physical Review B, vol.36, issue.14, p.7748, 1987.
DOI : 10.1103/PhysRevB.36.7748

P. Sebbah, D. Sornette, and C. Vanneste, Anomalous diffusion in two-dimensional Anderson-localization dynamics, Physical Review B, vol.48, issue.17, p.12506, 1993.
DOI : 10.1103/PhysRevB.48.12506

A. Prez-garrido, M. Ortuno, E. Cuevas, J. Ruiz, and M. Pollak, Conductivity of the two-dimensional Coulomb glass, Physical Review B, vol.55, issue.14, p.8630, 1997.
DOI : 10.1103/PhysRevB.55.R8630

J. Talamantes and A. Moebius, Phononless Hopping in the Coulomb Glass, physica status solidi (b), vol.205, issue.1, p.45, 1998.
DOI : 10.1002/(SICI)1521-3951(199801)205:1<45::AID-PSSB45>3.0.CO;2-8