Database forge ser Unique not null field

Question

On Codeigniter's DBforge I want to create a unique not null varchar(30) field how can I do that? For now I have this Migration script:

defined('BASEPATH') OR exit('No direct script access allowed');

class Migration_CreateUserTable extends CI_Migration
{
    private $table='user';

    public function up()
    {
        $fields=[
                    'id'=>[
                            'type'=>'INT',
                            'unsigned' => TRUE,
                            'auto_increment' => TRUE
                          ],
                    'username'=>[
                                  'type'=>'VARCHAR',
                                  'constraint'=>30,
                                  'null'=>false
                                ],
                    'password'=>[
                                    'type'=>'VARCHAR',
                                    'constraint'=>200,
                                    'null'=>false
                                ]
                ];

        $this->dbforge->add_field($fields);
        $this->dbforge->add_key('id',true);
        $this->dbforge->create_table($this->table);
    }

    public function down()
    {
        $this->dbforge->drop_table($this->table);
    }
}

I want the username field to have a unique index do you have any idea how to do that?