Diamond junction FETs based on δ-doped channels

Diamond junction field effect transistors (FETs) utilizing delta-boron-doped diamond films were fabricated and analyzed. In order to allow full charge modulation by the gate, the total channel sheet charge must not exceed the order of 10(13) cm(-2). However, boron doping shows full activation only for concentrations above ca 10(20) cm(-3) [1]. This yields a thickness for a fully activated channel in the range of ca 1 nm. To approach such narrow doping spikes any parasitic boron doping tails need to be eliminated. One possible way of achieving this is to compensate boron doping with nitrogen doping, an extremely deep donor. This results in the formation of a pn-junction, where the nitrogen doped part is not activated at room temperature and which therefore represents a semi-insulating (lossy) dielectric at low temperature and high frequency. At elevated temperature and low frequency the nitrogen doped layer becomes conducting acting as a series resistor to the interfacial pn-junction. Using this concept of a lossy dielectric pn-junction in the delta-doped channel FET, two gate diode configurations were investigated. In the first the nitrogen doped (Ib) synthetic diamond substrate served as a large area back gate, while in the second the nitrogen doped gate layer was grown on top of the delta-channel. The devices show high drain currents of up to 100 mA mm(-1) and full channel modulation even at moderate operation temperatures of 200-250 degrees C. By extrapolation a current density of 1 A mm(-1) is expected for a 0.25 mu m gate length device. (C) 1999 Elsevier Science S.A. All rights reserved.