Electroluminescence devices of trivalent rare earth ions(RE

3+

) and divalent manganese ions reported so far are high field electroluminescence devices with the excitation mechanism of impact excitation

[1

2]

. We have studied the excitation spectra of ZnS:RE

3+

around the band gap and recognized the possibility to design devices with forward biased electroluminescence of RE

3+

. The necessary requirement is the injection of both electrons and holes into the region where RE

3+

ions are present. Such electroluminescence has been observed in Er

3+

implanted ZnS:Er

3+

diode. The observed electroluminescence has been experimentally proven to be forward biased injection electroluminescence

not by impact excitation.The electroluminescence spectra of ZnS:Er

3+

diode in the forward bias and the reverse bias are the characteristic spectra of Er

3+

. The ratios of the intensities of the 18800cm

-1

(the transition of

4

S

3/2

to

4

I

15/2

) emission lines to that of the 18050cm

-1

(the transition of

4

F

9/2

to

4

I

15/2

) emission lines were measured. The ratios aaaaaaaaaaa in the reverse bias are found to increase with the reverse bias(Fig.1(a))

which is direct evidence of impact excitation according to Krupka’s analyses

[4]

. In contrast

the ratios a in the forward bias are found to be a constant at different forward bias (Fig.1(b))

which shows that the electroluminescence of Er

3+

in the forward biased ZnS:Er

3+

diode is the forward biased injection electroluminescence

not by impact excitation. Electrons injected from the In contact and holes tunneled from the Au contact give energy to Er

3+

ions through certain mechanisms

Er

3+

ions are excited and then give luminescence.Since the efficiency of electroluminescence by impact excitation are not high

[2

5]

the present work gives new clues to raise the efficiency and the brightness of RE

3+

electroluminescence.Optimized structures of devices will reduce operating voltage and raise efficiency of the forward biased injection electroluminescence of RE

3+

.