B.S.T.J. Brief: A Silicon Diode Microwave Oscillator

Microwave oscillations have been obtained on a pulse basis from silicon diodes. This brief reports fabrication details and performance data. The similarities of this device to that proposed by Read1 are discussed. The diodes were made by diffusing boron to a depth of approximately 25 m into one face of a slice of 0.15 ohm-cm n-type silicon, and then lapping the other face to a final slice thickness of 175 ju. Electroless nickel was then applied to both faces and sintered at 800°C for 5 minutes in N2. Nickel was then replated and followed by a final plate of gold. The slice was then ultrasonically sectioned into squares 125 m X 125 m- To remove cutting damage the "wafers" were etched for about 10 seconds in CP-8 (3 parts H N 0 3 , 1 part HF), and were then incorporated into a microwave encapsulation. A sketch of a wafer before encapsulation is shown in Fig. 1. Oscillations are observed when a critical reverse voltage is applied to the diode. This voltage has been observed on a variety of samples to correspond to that required to produce enough reverse current to create an electric field on the order of 2 kv/cm in the 150-pt n-type region. A typical reverse V-I characteristic obtained 011 a pulse basis using a sampling oscilloscope is shown in Fig. 1. On samples which were lapped to reduce the drift region length from 150 n to 75 n, the required voltage in excess of the avalanche voltage was halved. Voltages in excess of threshold produce more output until a maximum is reached.