Help needed with Shell scripting

Hi All,
I need to split a flatfile based on it's first character, I am using the following script
awk '{print > "TEST_substr($0,1,1).txt"}' PROVIDER.txt

It is returning files
TEST_1 and
TEST_2
But I am not getting the .txt file extension.
I need the files like
TEST_1.txt and
TEST_2.txt
how can I achieve this. Thanks in advance.

awk '{print > "TEST_"substr($0,1,1)".txt"}' PROVIDER.txt