A PLANAR TYPE MICROWAVE-OSCILLATING SEMICONDUCTOR DIODE HAVING A PN JUNCTION IN A SEMICONDUCTOR SUBSTRATE. THE PN JUNCTION HAS THE SHAPE OF A SPHERICAL SURFACE EXTENDING INTO THE SUBSTRATE AT ITS CENTER TO A DEPTH D WHICH IS AT LEAST ONE FOURTH OF THE RADIUS R AT THE SURFACE OF SAID SUBTRATE. SUCH A SEMICONDUCTOR DIODE HAS INCREASED OSCILLATION FREQUENCY AND OUTPUT POWER. THE DIODE CAN BE MADE BY FORMING A DIFFUSION PREVENTING MASK ON A GERMANIUM SUBSTRATE OF A CERTAIN TYPE OF ELECTRIC CONDUCTIVITY HAVING AN IMPURITY CONCENTRATION RANGING FROM 1X10**14 TO 1X10**18 ATOMS/CM3, AND BY DEEPLY DIFFUSING THROUGH A SMALL DIFFUSION HOLE IN SAID MASK AN IMPURITY WHICH WILL MAKE THE CONDUCTIVITY OPPOSITE TO THAT OF SAID SUBSTRATE.