Ultra-fast electro-optic modulators based on plasma dispersion effect of 2DEG in III-V heterostructures

Silicon electro-optic photonic modulators are limited to 50-60GBps bandwidth due to carrier recombination lifetimes. Modulator bandwidth in electro-optic materials is limited by R-C time constants of the electrical contacts, resulting in high bit error rate beyond 100GBps [Nature 562, pp.101–104,2018]. We propose a design inspired by III-V High Electronic Mobility Transistor (HEMT) used in high speed RF circuits and routinely operated at >100GHz frequencies. We present a modified Drude model to analyze the strength of the plasma dispersion effect of the 2DEG at the III-V interface. The 2DEG density is modulated through gate voltage (V_g) applied to a reverse biased III-V Schottky diode [IEEE TED 61(4), pp.1006-1013,2014]. Plasma dispersion due to 2DEG is dominant as compared to free charge carriers in the AlGaN barrier. We obtain an analytical expression for the carrier induced electro refraction:
Δn= -[eλ²ε_x (V_g-V_th)]/[8π²c²nd(m*_2DEG)²]
V_th is threshold voltage corresponding to pinch-off for the 2DEG and other symbols are per standard notation. As compared to multi-quantum well intersubband modulators, the structure we propose is easier to fabricate and analyze. The simple device structure enables several methods of coupling light, making a waveguide manifestation feasible.