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Electric Field Modulation of Light Emission in Silicon Gated Diodes


A silicon gated diode, being evolved from MOSFET technology and utilizing a half of MOSFET, has been developed as LED) for the first time to demonstrate unique electric field modulation of light emission characteristics. In contrast to a reverse biased traditional two-terminal Si-diode possessing light emission modulation by current signal, this three-terminal Si-gated diode is capable of modulating optical emission by applied gate voltage signal, which is directly compatible with a standard CMOSFET circuit operation easing monolithic integration of logic circuit implementation and a light emission diode on silicon technology. The gate applied voltage can produce two effects on the gated diode. Firstly, it induces carrier concentration modulation at both channel and source/drain region underneath the gate, thus modulating electric field strength and distribution. Secondly, an inversion layer of surface carriers underneath the gate in the source or drain overlap region can be formed at a certain gate applied voltage. Since the inversion layer of carriers is formed at heavily doped semiconductor, a tunneling current can be observed in such a field induced junction. These two combined effects of electric field modulation and tunneling current injection in the high electric field region lead to unique light emission characteristics. Fabricated in a standard CMOS process technology, silicon p-n junction diodes (source/substrate or drain/substrate) and gated p-n junction diodes (gate/source/substrate or gate/drain/substrate) are used in this study to compare their light emission characteristics and device performance. The pn junction diode operating at avalanche breakdown conditions has visible light emission originating from the depletion region as confirmed by 2D device simulation and experimental measurement results. It is believed that this optical radiation comes from carriers generated by impact ionization losing their kinetic energy by colliding with immobile charged centers in the avalanche region. A theoretical model is presented to show the correlation of photonic emission with the electric field strength, also known as the hot carrier effect with the related high electric field. The gated pn junction diode operating at electric field assisted tunneling current conditions has visible light emission originating from the gate/drain or gate/source overlap region as confirmed by 2D device simulation and experimental measurement results. Both pn junction diode and gated pn junction diode exhibits a linear dependence of light emission intensity on the device terminal current. The only difference between them is that the pn junction diode is controlled by the avalanche current initiated by the junction leakage current while the gated diode by the tunneling current. It has been discovered that, at the same terminal current, the optical output power in gated diode is higher than that in diode, indicating higher quantum efficiency in the gated diode. To compare LED device performance, the electro-optic modulation schemes and speed in the pn diode and the gated diode are analyzed in detail, showing a modulation speed of a few tens of gigahertz achievable in the gated LED. The gated LED promises its potential in realizing silicon optoelectronic integration.

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