High Q variable capacitance diodes with extraordinary capacitance — Voltage dependence

摘要

If the capacitance-voltage relationship of a semiconductor junction is expressed byfrac{d log C}{d log V} = nwhereCis the capacitance per unit area,Vis the sum of the applied reverse bias and the built-in reverse bias of the junction, and n is a number which in general is a function of V; then it is usually found that-1/2 leq n leq -1/3. The lower limit of -1/2 is obtained fer an abrupt junction and is approximated very well by many alloy diodes. The upper limit of -1/3 is obtained for a linearly graded junction and is approached by junctions made with deep diffusion. The "hyper-abrupt" structure, in which on one or both sides of the p-n junction the impurity density at the depletion layer edge decreases with increasing depletion layer width yields values ofn < -1/2. "Hyper-abrupt" structures withfrac{d log C}{d log V} = -5have been obtained by diffusion techniques. An alloy-diffusion method has been reported to yield diodes withn = -3. This paper discusses "hyper-abrupt'" devices fabricated by an epitaxial technique. The main feature of the devices is an extremely narrow active region which yield a large-frac{d log C}{d log V}ratio in conjunction with cut-off frequencies greater than 300 KMC. The values for -n can be tailored to match circuit parameters. Variable capacitance diodes with -n greater than 12 have been fabricated. The very high cut-off frequency has been attained by a reduction of series resistance with the extremely narrow active region in the device. Thus very high frequency operation is obtained along with the extraordinary voltage-capacitance non-linearity of the "hyper-abrupt" junction.