Electron-hole plasmas in semiconductors

Electron-hole (e-h) plasmas are found in high speed, high power semiconductor switches and oscillators. Their properties are used to describe the operation of high-gain photoconductive semiconductor switches (PCSS), resonant tunneling diodes, Read diodes, impact ionization avalanche transit time (IMPATT) devices, and Gunn oscillators. Models for electronic polarizability, plasma resonances, transit time effects, and plasma cooling in diodes, transistors, 2-dimensional field effect transistors (FET), and quantum wires must reach beyond single particle interactions and include collective, many-body effects and hydrodynamic equations to obtain reasonable agreement with measurements. Despite the radical differences between the physics of semiconductors and gases, the current filaments in high gain PCSS are strikingly similar to those found in gas breakdown switches. Gas plasmas constitute the active region in gas discharge lasers. In a very similar way, we have produced a new type of semiconductor laser that is based on the e-h plasma in semiconductor current filaments. Comparisons and contrasts will be made of the properties and fundamental phenomena of these switches and lasers. This paper will focus on the plasma properties of current filaments in PCSS that we have measured, such as carrier densities, plasma frequencies, transit times, and rates of formation. The understanding and development of 2 and 3 dimensional plasmas in semiconductors is important for the production of higher bandwidth (0.1-10 THz) devices, because plasma wave frequencies and transit times are not limited by saturation velocities and device sizes. Potential uses for semiconductor plasmas include: THz sources, sensors, and lasers for high bandwidth communication, semiconductor and plasma diagnostics, and bio-medical applications.