We have prepared YBa_2Cu_3O_(7-x) grain boundary Josephson junctions on MgO bicrystal substrates. Junctions of 2-6 #mu#m width showed current-voltage characteristics of RSJ-type with product I_CR_N approx approx mV at 4.2 K and with response to 106.5 GHz microwave radiation (Shapiro steps) up to 3.5 mV (approx 1.7 THz). Measured dc and ac characteristics follow ideally dependencies calculated using RSJ model with finite capacitance. The temperature dependence of characteristic voltage I_CR_N was measured and discussed in the frame of both tunnel junction and modified SNS junction model.
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