Proceedings Article | 1 July 1991
KEYWORDS: Charge-coupled devices, Interfaces, Silicon, Gallium arsenide, Digital electronics, Field effect transistors, Image sensors, Dielectrics, Signal processing, Multiplexers
The two-dimensional electron gas charge-coupled device (2DEG-CCD) structure is an outgrowth of recent advances in 2DEG-FET structures for digital logic circuitry and microwave devices. The 2DEG-FET structures, which are known by severalacronyms such as HEMT, SDHT, TEGFET, HIGFET, and MODFET, utilize the abrupt heterointerface between two semiconductor materials and consequent conduction band discontinuity to confine electrons [1]. Due to confinement in the direction perpendicular to the interface, the resultant electron distribution is known as a two-dimensional electron gas. Because of the confinement dimensions and the low electron effective mass usually associated with group 111-V materials, quantum mechanics plays an important role in broadening the spatial electron distribution and defining allowed energy states. However, quantum effects typically play a minor role in understanding device behavior at temperatures above 77 K. 2DEG-FET devices have several attributes which include very high mobility of the channel charge (typically in excess of 5,000 cm2/V-sec at room temperature), high transconductance, and low voltage swing requirements. The 2DEG structure is attractive for CCD applications for several reasons. First, the high low-field mobility and use of a semi-insulating substrate suggest very high speed device operation. Second, the charge-handlin capabilty of the 2DEG-CCD structure is large compared with MESFET-type CCDs, and can exceed 1x101 carriers/cm2. Third, the lattice-matched heterointerface has a potentially lower interface trap density than the intrinsically mismatched silicon-silicon dioxide interface, as well as improved radiation hardness. Finally, the useful operating temperature of the 2DEG-CCD is expected to be lower than that of the silicon CCD. Other features of the 2DEG-CCD include fabrication compatibility with high-performance, low noise 2DEG-FET output circuitry and an anti-blooming gate structure.
