p-Type double barrier resonant tunneling diodes with high-quality strained Si0.6Ge0.4/Si heterostructures have been realized at low temperatures using the ultraclean low-pressure CVD system. In order to suppress a voltage drop due to series resistance, the diode structure has been improved with a B-doping concentration in the p(+) contact layers of 10(19)-10(20) cm(-3) The sharp resonant current peaks have been observed from the I-V characteristics in the temperature range of 10-200 K. The peak-to-valley current ratio is as high as 2.9 at 120 K. The peak voltage decreased with increasing temperature and hovered around extremely low values with a minimum value of 8 mV at 200 K. The decrease of the peak voltage can be explained mainly by the decrease of the resonant state energy of heavy holes in the quantum well due to the increase of the hole effective mass, and by the increase of the peak energy of the hole distribution in the emitter. A sudden drop of the peak voltage was found at 110-120 K, and possible processes affecting the observed tunneling characteristics are discussed.