Autohotkey - toggle keys

I want to achieve this

1. press "a" > output "a"


2. press "a" again > output "b"


3. press "a" again > output "a"


4. press "a" again > output "b"


5. keep repeating these...

Below script does what I want, but with little issue.
When I press and hold "a", it output "abababab"......
But I need it to act like keep outputing the current character until I release the key and activate the toggling.

i.e. press and hold "a" > output "aaaaaaaa"... > release "a" > stop outputing and toggle to "b"

Anyone know how can I achieve that? Thanks in advance!

Toggle:=0
$a::
Toggle:=!Toggle
if(Toggle=1) {
    send, a
} else {
    send, b
}
return