Drain Current Saturation in Line Tunneling-Based TFETs: An Analog Design Perspective

This paper highlights the output current saturation in a line tunneling-based tunnel FET (LT-TFET). Thereafter, a novel method to extract the onset of saturation voltage ( ${V}_{\textsf {DSAT}}$ ) for LT-TFET is proposed for the first time. A soft saturation state is attained when the electron density in the epitaxial layer over the source region saturates with the drain bias ( ${V}_{\textsf {DS}}$ ) and the conduction band energy ( ${E}_{\textsf {C}}$ ) gets pinned. In addition, at the onset of deep saturation, the electron density in the epitaxial layer over the channel region drops below its doping level and ${E}_{\textsf {C}}$ becomes invariant for any further increase in ${V}_{\textsf {DS}}$ . The difference between gate–drain bias ( ${V}_{\textsf {GD}}$ ) is found to be a constant at the onset of saturation and remains independent of the gate–source overlap lengths ( ${L}_{\textsf {OV}}$ ). A shift in ${V}_{\textsf {DSAT}}$ and ${V}_{\textsf {GD}}$ is also observed with change in the thickness and doping of the epitaxial layer. The transconductance and output resistance are reasonably good in the soft saturation regime. Furthermore, a nominal change of ~5% in the voltage gain ( ${A}_{\textsf {V}}$ ) of a common source amplifier is observed when the n-device is biased in the either soft or deep saturation regime, without any tradeoff in the bandwidth.