Chuck is correct in that the derogatory "door knocker" was applied to the 37mm Pak 35/36 which even had difficulties w/heavy French armor in 1940. These were the main AT weapon going into Barbarossa and was moderately adequate for the array of lighter Soviet armor and armored cars. However when the T34/76's and the KV1 and KV2 arrived, the Wehrmacht was sent scrambling.
The appearance of the 5cm Pak 38 helped give the Germans some punch and its lethality was shown against Soviet armor and in N Africa. With the appearance of the 75mm Pak 40, the Germans had a real dominant artillery piece that could kill most anything the United Nations could throw at them for the rest of the war.
And this doesn't even take into account the use of the multi-role 88mm calibre Flak weapons (18, 36, 37, 41) or the dedicated 88mm AT guns (Pak 43, Pak 43/41). I've even built the Azimut Pak 12.8cm K81 super heavy AT gun.