The voltage gain of an MOS transistor inverter stage is made independent of the device threshold voltages and of channel lengths by making the length and width of the channel region of the upper load transistor equal to the length and width of the channel region of the lower driver transistor....http://www.google.com/patents/US4714840?utm_source=gb-gplus-sharePatent US4714840 - MOS transistor circuits having matched channel width and length dimensions
