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Graded p-n junctions that can be fabricated by solidstate diffusion are low-loss nonlinear capacitors at microwave frequencies. These diodes can be used to make low-noise amplifiers, amplifying frequency converters, harmonic and subharmonic generators, switches, limiters, and voltage-tuned passive circuits. Single junctions can control many watts of microwave power. Point-contact diodes are nonlinear resistors and as yet are unchallenged as microwave rectifiers. At lower frequencies, nonlinearresistance action can be obtained in p-n junctions by introducing recombination centers. A p-i-n diode is resistive at high frequencies. The value of the resistance depends upon the dc current. This variable resistance can be used as a broad-band microwave switch or attenuator. At low current densities, the p-i-n structure functions as a transmission line and so can serve as a support, protection, and connection for small-area p-n junctions made in the same single crystal of silicon.