Superfast fronts of impact ionization in initially unbiased layered semiconductor structures

We present results of numerical simulations of superfast impact ionization fronts in initially unbiased layered semiconductor structures.We demonstrate that when a sufficiently sharp voltage ramp $A > 10^{12} ; { m V/s}$ is applied in the reverse directionto an initially unbiased Si $p^{+}-n-n^{+}$-structure connected in series with a load $R$, then after some delay the system will reach the high conductivity state via the propagation of a superfast impact ionization frontwhich leaves a dense electron-hole plasma behind.The front travels towards the anode with a velocity $v_f$ several times largerthan the saturated drift velocity of electrons $v_s$.The excitation of the superfast front corresponds to the transitionfrom the common avalanche breakdown of a semiconductor structure toa collective mode of streamer-like breakdown.For a structure with typical thickness of$W sim 100 ; { m mu m}$, first there is a delay of about$1 ; { m ns}$ during which the voltage reaches a value ofseveral kilovolts. Then, as the front is triggered, the voltageabruptly breaks down to several hundreds of voltswithin $sim 100 ; { m ps}$. This provides a voltage rampof up to $sim 2 cdot 10^{13}; { m V/s}$, hence up to 10times sharper than the externally applied ramp.We unravel the source of initial carriers which trigger the frontand explain the origin of the time delay in triggering the front.Further we identify the mechanism of front propagation and discussthe possibility to excite superfast ionizationfronts not in layered structures but in bulk semiconductors.