We report the performances of LWIR (λc = 9.0 μm at 80K) HgCdTe electron injected avalanche photodiodes (e-APD). In these devices, the exponential gain curve, up to gains equal to 23 at 100K, and the low excess noise factor close to unity (F ~ 1-1.25) are indicative of a single carrier multiplication process, which is electron impact ionization. The dark current is mainly due to a diffusion current at low reverse bias and tunneling currents at high reverse bias. A Monte Carlo model has been developed for understanding the multiplication process in Hg1-xCdxTe e-APDs. We find a good agreement between first simulation results and experimental measurements of the gain and the excess noise factor in both MWIR (x = 0.3) and LWIR (x = 0.235) e-APDs at 80K. Furthermore, simulations do not show any heavy hole impact ionization. This model which enables to perform phenomenological studies aims at identifying the main physical and technological parameters that influence the gain and the excess noise. In the present work, it is used to study the influence of the thickness of the ndoped region on the gain and the excess noise factor. We found that F still decreases while the thickness of the nlayer decreases. However, an optimum thickness of the nlayer exists around 1μm in terms of gain-voltage characteristic.