The ‘electrostatic’ doping is a fascinating feature of FD-SOI technology. In an ultrathin SOI device, the electron population induced by a positive gate bias spreads into the entire body (volume inversion or accumulation). The body behaves as an N-doped region albeit it was originally undoped. Reciprocally, negative gate bias turns the body into a P-type region. Such doping metamorphosis has been used for conceiving novel devices containing reconfigurable P-N junctions.

We investigate the virtual P-N junction constructed with electrostatic doping. The primary question is whether and to what extent it behaves as a regular diode with physical doping. Our test devices are PIN diodes with large gate underlap. The doping in the gated and ungated regions is induced by the bias on the front gate and on the back gate (ground plane), such as to emulate a fictive junction at mid-distance from anode to cathode. The devices have been processed with 28nm FDSOI technology including HKMG gate stack, 7 nm SOI film and 25 nm thick BOX.

The experimental I‒V curves look familiar to a diode but without complying with the conventional theory. First, the doping level is adjustable by the gates in a wide range, which offers the possibility to modulate the leakage current and the breakdown voltage. Second, the junction depletion region is no longer defined by the lateral field because the gate action leads to a 2D effect documented by simulations. Third, the virtual doping varies dynamically with the anode/cathode voltage. Finally, the junction position can be selected by appropriate gate biasing from the mid-body to the physical junctions. We show that this intriguing diode is very versatile and enables various applications. In particular, a unique characterization technique for carrier lifetime extraction in ultrathin films will be presented.