Avalanche Multiplication of Photo-Generated Carriers in Amorphous Semiconductor, and its Application to Imaging Device
- PDF / 1,030,201 Bytes
- 11 Pages / 420.48 x 639 pts Page_size
- 6 Downloads / 173 Views
AVALANCHE MULTIPLICATION OF PHOTO-GENERATED CARRIERS IN AMORPHOUS SEMICONDUCTOR, AND ITS APPLICATION TO IMAGING DEVICE T. HIRAI*, E. MARUYAMA*, Y. TAKASAKI*, K. TSUJI*, and K. TAKETOSHI** J. YAMAZAKI**, K. SHIDARA**, K. TANIOKA**, *Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan **NHK Science and Technical Research Laboratories, Setagaya, Tokyo 157, Japan ABSTRACT It has been confirmed that avalanche multiplication occurs in amorphous semiconductors, especially in amorphous selenium. The obtained hole and electron ionization rates, fS and cý, have single exponential dependence on the reciprocal electric field. This phenomenon has been successfully applied for the first time to a highly-sensitive image-pickup tube with quantum efficiency as high as 10. This avalanche vidicon has excellent S/N characThis may be due to the large ratio of hole to teristics. electron ionization rates from the shot noise analysis. INTRODUCTION Since a proposal of the electrical switching device by Ovshinsky(1), high electric field effects in amorphous semiconductors have been extensively studied(2). Almost all experimental results have been obtained for a sample structure with carrier injecting contacts. The double injection model have seemed to be the most reliable model explaining the switching operation, although Mott(3) also proposed a multiplication process based on Hindley's(4) theoretical work. We have precisely investigated high-field photoconduction in amorphous semiconductors using blocking-type amorphous selenium(a-Se) photocells. We have taken considerable care to prevent the injection of carriers from electrodes caused by field concentration due to substrate surface roughness. And we have confirmed that avalanche multiplication occurs in reverse -biased a-Se photocells at electric fields larger than 106 V/cm (5, 6, 7). Measured values and electric field dependence of carrier ionization rates in a-Se have been different from those which are expected from the results in typical crystalline semiconductors. We have applied these phenomena to the photoconductive targets of image-pickup tubes. High sensitivity with quantum efficiency larger than 10 has been easily obtained, with no degradation of photoresponse or uniformity. We have also found that avalanche noise in such vidicons is fairly small due to the large ratio of carrier ionization rates. AVALANCHE MULTIPLICATION IN AMORPHOUS SELENIUM Experimental The cell structure used for conduction measurements is Mat. Res. Soc. Symp. Proc. Vol. 118. ý 1988 Materials Research Society
388
glass Sub. blocking layer(Ce0
L' "
2
)
Au
Fig. 1 Schematic illustration of the a-Se photocell.
shown in Fig. 1. The CeO 2 layer acts as a hole-blocking layer which prevent hole injection from the bottom electrode. The a-Se layer thickness L varies from 0. 5 to 6Am. The potential barrier which prevents electron injection from the electrode is formed between the gold top electrode and the a-Se layer. The barrier height measured from the open circuit voltage is about 0.4e
Data Loading...
