Alloy-diffused variable capacitance diode with large figure-of-merit

The product of the cutoff frequency and the capacitance-voltage sensitivity is proposed as a figure-of-merit of the variable capacitance diode. It is expected from the theoretical results that the high-voltage sensitivity (dC/dV)/C of the capacitance is obtained by the "hyperabrupt" junction in which the impurity concentration decreases with the distance from the p-n boundary. Assuming the exponential decrease of the impurity concentration, the theoretical expression is derived for the capacitance-voltage characteristic of the "hyperabrupt" junction diode, and is compared with the experiment. "Hyper-abrupt" structure is easily obtained by the alloy-diffusion technique. The experimental capacitance-voltage characteristics of the germanium alloy-diffused diode agree fairly well with the theoretical results. Diodes changing the capacitance in proportion to the -3 power of the applied voltage have been made, with the tuning ratio to about 100. To date, a diode with the maximum cut-off frequency of 30 kMc and with the figure-of-merit Fγof 5.5 kMc/v, has been made. Further increase of these quantities, however, will be obtained by the reduction of the thickness of the base layers which is about 15 microns in the above diode.