Try something like:
CREATE TABLE [dbo].[empWithTrigger](
[sal] [decimal](10, 2) NULL,
[code] [varchar](50) NULL,
[tag] [varchar](50) NULL,
[id] [int] IDENTITY(1,1) NOT NULL,
[alphanumeric] [varchar](50)
)
GO
Create TRIGGER alphanumericColumn
ON empWithTrigger
AFTER INSERT
AS
BEGIN
SET NOCOUNT ON;
UPDATE a
SET alphanumeric = 'ABC' + LEFT('00' + CAST(inserted.id AS VARCHAR(10)), 3)
FROM empWithTrigger a
INNER JOIN inserted
ON a.id = inserted.id
END
GO
Discussion related to it here:
how to auto increment alphanumeric primary key..[
^]