Input filter on textarea for custom field

I've got a custom field that has a textarea in the form but the html tags are being escaped. How can I set it up so that it either won't do that or that the user can select an input filter that will allow certain html tags?

Update: Looking through the text.module file, I see a type called "text_format" which I'm trying to use on the form:

  $widget['download_content'] = array(
    '#title' => t('Content'),
    '#type' => 'text_format',
    '#default_value' => isset($items[$delta]['download_content']) ? $items[$delta]['download_content'] : NULL,
    '#format' => isset($items[$delta]['format']) ? $items[$delta]['format'] : NULL,
    '#base_type' => 'textarea',
  );

The widget now displays correctly but there are SQL errors when saving:

PDOException: SQLSTATE[21S01]: Insert value list does not match column list: 1136 Column count doesn't match value count at row 1: INSERT INTO {field_data_field_page_premium} (entity_type, entity_id, revision_id, bundle, delta, language, field_page_premium_download_fid, field_page_premium_cover_fid, field_page_premium_download_content, field_page_premium_format) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7, :db_insert_placeholder_8_value, :db_insert_placeholder_8_format, :db_insert_placeholder_9); Array ( [:db_insert_placeholder_0] => node [:db_insert_placeholder_1] => 6 [:db_insert_placeholder_2] => 6 [:db_insert_placeholder_3] => page [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => und [:db_insert_placeholder_6] => 11 [:db_insert_placeholder_7] => 12 [:db_insert_placeholder_9] => [:db_insert_placeholder_8_value] => this<br />is<br/>test<br />content [:db_insert_placeholder_8_format] => plain_text ) in field_sql_storage_field_storage_write()