Reduction of UHF power transistor distortion with a nonuniform collector doping profile

The linearity of a class-A, bipolar UHF power transistor is investigated. T he device is intended for transmission of signals with multiple (TV) channe ls. Mixing between the various channels must be suppressed, which makes lin earity an important parameter. This paper presents a novel approach, where process technology is specifically optimized to minimize distortion. It is shown that particularly the nonlinear collector-base capacitance strongly a ffects the linearity of this type of device. With a modified collector-base doping profile, the capacitance is manipulated. A nonuniform profile is in troduced that significantly reduces third-order intermodulation. The profil e consists of an arsenic doped spike that is grown epitaxially into the (ot herwise) lightly doped collector. This reduces the bias dependence of C-cb and improves the tradeoff between linearity and breakdown or ruggedness. Th e reduction in third-order intermodulation is demonstrated both experimenta lly and by mixed-level MDS/MAIDS simulations.