Sub-90 nm n-MOSFETs have been studied at the sub-threshold and saturation operating regimes using infra-red photon emission intensity and current measurements. The results show a distinctive difference in the photon emission yield profile below and above threshold. A new mechanism for the higher photon emission rates in the sub-threshold regime is proposed. Electrical measurements together with 2-D numerical device simulations were carried out in order to verify this new mechanism. (c) 2006 Elsevier Ltd. All rights reserved.