Chapter 2
State-of-the-Art TFET Devices
2.1 TFET Introduction
TFETs are p-i-n gated junctions that operate in reverse regime. Figure 2.1 shows a
conceptual TFET structure compared to a CMOS transistor. For an n-type (NTFET),
p+ (n+) doping is used for the source (drain), while for a p-type (PTFET) n+ (p+)
doping is used for the source (drain), the doping being reversed between the source
and the drain as opposed to a MOSFET where drain and source have identical
doping.
There are two types of TFETs, homo- and hetero-junction [15, 16, 19, 20],
depending on the type of semiconductor of the p-i-n junctions. The former is
implemented in Silicon, while the latter uses different III-V materials such as InAs
or GaSb-InAs. In this text we are addressing Si TFETs due to their potential of being
integrated with CMOS on the same substrate.
The operation of the TFET is based on band-to-band tunneling (BTBT) consisting in the modification of the position of the band gap of the intrinsic region of the
device relative to the energy levels of the source and drain, see Fig. 2.2 [12, 13].
In the ON state a positive voltage is applied on the gate of an NTFET leading to
sufficient narrowing of the band gap such that tunneling can occur. If the gate bias
is low, close to 0 V, the band gap of the channel blocks the tunneling, corresponding
to the OFF state in Fig. 2.2. The n+ drain is always biased with positive voltage,
V DS > 0 V, to ensure the operation in the reverse regime of the p-i-n diode; for
the TFET this biasing represents the forward operation region. This polarization
guarantees extremely low I OF F currents when the device is OFF with V G = 0 V.
TFET I D = f (V G ) typical characteristics are plotted in Fig. 2.3 along with those
of a FDSOI MOSFET. It can be seen that the TFET has a lower I ON compared to
any MOSFET; on the flip side, however, the TFET operating by quantum-tunneling
does not suffer from the subthreshold slope limitation as does a MOSFET, [12, 13,
20], and therefore, has a very low OFF current, I OF F .
© Springer Nature Switzerland AG 2021
N. Gupta et al., TFET Integrated Circuits,
https://doi.org/10.1007/978-3-030-55119-3_2
5
State-of-the-Art TFET Devices
2.1 TFET Introduction
TFETs are p-i-n gated junctions that operate in reverse regime. Figure 2.1 shows a
conceptual TFET structure compared to a CMOS transistor. For an n-type (NTFET),
p+ (n+) doping is used for the source (drain), while for a p-type (PTFET) n+ (p+)
doping is used for the source (drain), the doping being reversed between the source
and the drain as opposed to a MOSFET where drain and source have identical
doping.
There are two types of TFETs, homo- and hetero-junction [15, 16, 19, 20],
depending on the type of semiconductor of the p-i-n junctions. The former is
implemented in Silicon, while the latter uses different III-V materials such as InAs
or GaSb-InAs. In this text we are addressing Si TFETs due to their potential of being
integrated with CMOS on the same substrate.
The operation of the TFET is based on band-to-band tunneling (BTBT) consisting in the modification of the position of the band gap of the intrinsic region of the
device relative to the energy levels of the source and drain, see Fig. 2.2 [12, 13].
In the ON state a positive voltage is applied on the gate of an NTFET leading to
sufficient narrowing of the band gap such that tunneling can occur. If the gate bias
is low, close to 0 V, the band gap of the channel blocks the tunneling, corresponding
to the OFF state in Fig. 2.2. The n+ drain is always biased with positive voltage,
V DS > 0 V, to ensure the operation in the reverse regime of the p-i-n diode; for
the TFET this biasing represents the forward operation region. This polarization
guarantees extremely low I OF F currents when the device is OFF with V G = 0 V.
TFET I D = f (V G ) typical characteristics are plotted in Fig. 2.3 along with those
of a FDSOI MOSFET. It can be seen that the TFET has a lower I ON compared to
any MOSFET; on the flip side, however, the TFET operating by quantum-tunneling
does not suffer from the subthreshold slope limitation as does a MOSFET, [12, 13,
20], and therefore, has a very low OFF current, I OF F .
© Springer Nature Switzerland AG 2021
N. Gupta et al., TFET Integrated Circuits,
https://doi.org/10.1007/978-3-030-55119-3_2
5