The absolute value of the threshold voltage of a P-channel TFT is reduced by making its channel length shorter than that of an N-channel TFT by at least 20%, to thereby approximately equalize the threshold voltage absolute values of those TFTs....http://www.google.com.au/patents/US5616935?utm_source=gb-gplus-sharePatent US5616935 - Semiconductor integrated circuit having N-channel and P-channel transistors
