Space-charge-limited Current Instabilities in n+-π-n+ Silicon Diodes

If the strength of a pulsed electric field of about 10
³
volts/cm is exceeded in a nickel-doped 25,000 ohm-cm π-type silicon sample with n
⁺
contacts, a transient charge distribution is established that leads to a current instability. When the critical applied field is reached a current-controlled negative resistance is observed. The sample impedance decreases by several orders of magnitude, with a switching time in the nanosecond range, and microwave oscillations are produced. Below the threshold a space-charge-limited current flows, and voltage profile measurements show the presence of the expected negative space charge near the cathode. As the field is increased this space charge extends further into the sample. At the threshold the nickel centers become ionized, and a positive space charge is created in the center of the sample. This non-equilibrium distribution, which persists for a period of 200 to 300 microseconds, has properties similar to those of a gaseous plasma.