It is reported in this paper that Cd

x

Zn

1-x

has been used to replace ZnS as the host in order to improve SmF

3

red thin film electrolu-minesence.According to the PL excitation spectra shown in Fig.2

SmF

3

exhibits better EL characteristics in Cd

x

Zn

1-x

than in ZnS crystal.The TFEL panels prepared for the experiment were made with the usual MISIM structure. It is demonstrated in Fig. 3 and Fig. 4 that the luminescence of SmF

3

in CdS doped ZnS host shows higher brightness at the same applied voltage than in pure ZnS when the dope concentration is appropriate.Comparison between the simultaneous emission of Er

3+

and SmEr

3+

in the two different hosts has been made to investigate the cause of brightness increase. As Er

3+

is considered having approximately the same exciting cross-section

it can be deduced from Fig. 6 that Sm has larger cross-section in Cd

x

Zn

1-x

than in ZnS. This is probably due to the change of crystal field of Sm3+ luminecence center.However

the Cd concentration in host influences Sm3+ TFEL characteristics not only in increasing the SmEr

3+

exciting cross-section

but also in causing the drop of the high field in thin film. This is because of the narrower forbidden band and higher mobility of holes and electrons of CdS.The drop of high field will destroy the SmEr

3+

TFEL when the CdS dope is too high. The best value of x in Cd

x

Zn

1-x

obtained in our lab is around 0.13.