Re: variable Vth
Variable threshold voltage is one of the many low-power CMOS design techniques used. The aim is to reduce leakage current by increasing Vth, however at the expense of reducing switching frequency or performance in speed.
There is one problem with this technique is that increasing Vth has an adversary or backfire effect. Subthreshold leakage occurs when gate voltage is lower than Vth, increasing Vth increases this leakage.
For more information, there are some renowned experts in this field.
Kaushik Roy (Professor, Purdue USA) - He has written many books.
Yeo Kiat Seng (Professor, NTU Singapore) - Written books with Kaushik Roy
Sharat C. Prasad (CISCO USA) - Written some joint articles and books with Kaushik Roy
Gary Yeap (Motorola Hong Kong and USA) - Written a book
Chandrakasan (Professor MIT USA) - Written numerous articles.