UNIPHASE OPERATION OF A GAAS RESISTIVE GATE CHARGE-COUPLED DEVICE

The operation of a GaAs charge-coupled device (CCD) within the UHF ban d is a technically challenging problem. The GaAs CCDs that were report ed previously in the literature were typically operated with multiphas e clocks, with the majority operated using clocks in phase quadrature. To provide the multiple UHF clock wave forms with phase correlation t o a GaAs CCD demands very stable picosecond timing accuracy between th e adjacent clock phases, otherwise unpredictable variations will occur in the device performance. This requirement is difficult to achieve w hen attempting to incorporate a GaAs CCD in a high-speed analog applic ation, such as the 500 MHz multichannel transient digitizer applicatio n being developed at TRIUMF, because of the nonlinear behaviour of the device as a load impedance. In addition to the difficult timing requi rement, there are also the electronic circuit disadvantages associated with providing a multiphase clock to a GaAs CCD. The general circuit approach used to provide a multiphase UHF clock consists of splitting or dividing a master clock signal into a set of clock signals with the desired phase properties. The disadvantages of this approach are (i) the circuits are generally narrowband, restricting the range of clock frequencies that can be used; (ii) the circuits usually consume consid erable power, producing substantial heat; and (iii) the circuits are t ypically complex, making them costly and difficult to incorporate with a GaAs CCD. A novel uniphase clock scheme was recently developed at T RIUMF that overcomes the above technical limitations for the UHF opera tion of a GaAs resistive gate CCD (RGCCD). The method is unique becaus e the static transverse electric field within the GaAs RGCCD channel r equired to direct the motion of charge is established using the surfac e potential control offered by the resistive gates, under dc-biassed c onditions. This permits a simple planar device comprising four electro des per pixel to be used instead of a more elaborate castellated or io n-implanted device. Application of a single clock signal to the GaAs R GCCD provides the required temporal transverse electric field variatio n to cause charge motion to occur. Charge transfer efficiencies exceed ing 0.999 have been achieved with a 128 pixel GaAs resistive gate CCD using (1) a fixed frequency uniphase clock operating below 100 MHz and (2) using a triggered dual frequency uniphase clock operating at 500 MHz during the signal acquisition period and at 15.6 MHz during the si gnal expenditure period.