A new signal-amplification mechanism discovered in semiconductors

Abstract

Preamplifiers (i.e., electronic devices that amplify signals) are required in optical imaging and detection systems to increase weak current signals.1 If the detector itself can produce sufficient gain, however, the sensitivity of such devices may be able to overcome the limitations that are imposed by the thermal noise of electronics. An internal amplification mechanism (i.e., impact ionization) has been used in photodetection for decades. In an avalanche photodiode—a reverse-biased p-n junction device that is operated at a voltage close to breakdown voltage,2, 3 APD—an ionization collision with the lattice—occurs when the photogenerated primary carriers acquire enough energy: see Figure 1(a). Secondary electron-hole (e-h) pairs are produced from this collision, which in turn cause additional ionization collisions as the pairs cross the depletion region (i.e., the ‘avalanche’ process). APD-based photoreceivers achieve sufficient sensitivity for fiber-optic communications. However, they require a high operation voltage (over 20V) and suffer from high excess noise with increasing gain. In devices with internal gain, interference originates mainly from shot noise that is amplified with the signal.4 The noise of these systems is best characterized by the excess noise factor (ENF), which is calculated from the fluctuation of the amplification gain. In our work, we are proposing a new internal amplification mechanism called the cycling excitation process (CEP). This process relies on the transitions involving localized states, which are formed via dopant compensation within a p-n junction diode. The Coulomb interactions that occur between energetic carriers and these localized states have stronger efficiency Figure 1. Schematic illustration of (a) the avalanche process and (b) the cycling excitation process (CEP). The former is based on impact ionization between the hot and the bound electron in the valance band. In contrast, CEP occurs as a result of the Auger process between a hot electron and an electron in the localized state in the dopant within the n-type region. Eg : Energy bandgap. 0: Primary carrier from direct photo absorption. 1: Carrier produced by Auger excitation.