6
2 State-of-the-Art TFET Devices
Fig. 2.1 CMOS and TFET
transistors
Tox
n +
n +
i
G
D
S
n-type CMOS
n-type TFET
p +
n +
i
G
D
S
Tox
Fig. 2.2 Schematic
representation of the
energy-band diagrams for
NTFET. OFF state:
V D = 1 V, V G = 0 V-black
curves; ON state:
V D = 1 V, V G = 1.6 V-red
curves
Experimental results have shown slopes less than 50 mV/decade and 10 −14 A
I OF F currents at room temperature [16]. Regardless of these encouraging results,
TFETs suffer from a low I ON , well below the conduction current of CMOS.
The TFET is an ambipolar device, i.e., it conducts both for positive and negative
V GS with the BTBT occurring at the metallurgical source-channel junction or at
the metallurgical drain-channel junction respectively, see Fig. 2.4. When positive or
negative voltage is applied to the gate the bands in the channel region are pushed
down at the source edge or up at the drain edge, respectively. This is highlighted
in Fig. 2.2 for positive V G when electrons can tunnel from the valence band of the
source at the channel interface to the conduction band in the channel (drain). The
ambipolar behavior is not desirable in circuit design.
The device used in this text, Structure 1 in Fig. 2.4, displays reduced ambipolar
behavior having under-lapped gate and drain by 30 nm [1, 14].
Another unique feature of the TFET is its I D characteristic in reverse bias, V DS <
0 V and V DS > 0 V for an NTFET and PTFET, respectively. As shown in Fig. 2.5