GaN tunnel switch diodes

Tunnel Switch Diodes (TSDs) exhibit a S-shaped IV curve with negative differential resistance. Because of tunneling, they are able to switch between a low-current, high-resistance state (HRS) and a high-current, low-resistance state (LRS) fast, making them promising for high-speed memory. The TSD consists of a thin tunnel barrier on top of a pn junction. In the HRS state, the barrier allows only a small tunneling current through. When biased beyond a switching voltage the p-layer of the pn-junction depletes from the surface field-effect, and the surface depletion edge reaches the depletion edge of the buried pn diode. This turns the buried diode on, which floods the p-layer with electrons. Much of these electrons become trapped in the top triangular quantum well barrier, self-biasing the device. Since this state is unstable, the drop in electric field moves from p-layer to barrer, with a sudden onset of tunneling current when the TSD reaches the LRS [Fig. 1(b)]. Reducing the applied bias reverses this process and the TSD switches back to the HRS state. The switching voltage can be found with V_s = qN_A(t_p - x_dep.)²)/2ϵ_semi ϵ_o + t_barrier√2qϵ_semiN_Aφ_s/ϵ_barrierϵ_o, where N_A is the acceptor doping concentration, x_dep is the pn junction depletion width in the p-layer, t_p and t_barrier are the thicknesses of the p-layer and tunneling barrier, and ϵ_semi and ϵ_barrier are the relative dielectric constants of p-layer and the barrier and φ_s is the surface potential needed to deplete the p-layer. TSDs were studied in SiO₂/Si and in AlSb/GaSb heterostructures recently. Advances in GaN pn-diodes and polarization physics present an exciting opportunity to realize TSDs with new functionality. This work demonstrates GaN homojunction and heterojunction TSDs for the first time.