Display a post by ID attribute with shortcode

I’m trying to create a shortcode that displays a banner containing a post content (thumbnail, title and excerpt), using ID attribute. My goal is to be able to use a shortcode like (postbanner id=123) to display any post by its ID.

Here is my code (in function.php child theme):

function post_banner_shortcode($atts) {
    $atts = shortcode_atts( array(
        'id' => ''
    ), $atts );
    $post_id = $atts('id');
    $HTML  = '<div class="postbanner">';
    $HTML .= '<div class="pb-thumb">' . get_the_post_thumbnail($post_id, 'medium') . '</div>';
    $HTML .= '<div class="pb-ttl-txt">';
    $HTML .= '<h4>' . get_the_title($post_id) . '</h4>';
    $HTML .= '<p>' . get_the_excerpt($post_id) . '</p>';
    $HTML .= '</div>';
    $HTML .= '</div>';
    return $html; 
add_shortcode( 'postbanner', 'post_banner_shortcode' );

Actually this code return nothing and I don’t understant why. I’m missing something here. I’m still a rookie with PHP so that’s why I need some help guys 🙂