Chapter 8
Perspective
8.1 TFET-CMOS Hybrid Cores
The previous chapters have demonstrated the extraordinary potential of implementing processors and digital ICs using TFETs in today’s world of mobile devices
operating on meager power budgets. The promise of TFET devices does not stem
only from their own characteristics but from their compatibility and integration with
CMOS devices on the same chip. The unique characteristics of the TFET led us
to imagine a few extremely power efficient and compact new circuit topologies
described in this book.
In order to envisage TFET or hybrid applications the first consideration is the
type of the TFET device to be used. The majority of recent investigations focus on
the use of heterojunction TFETs (HTFET) since the intent is to match or overtake
the performance of currently used MOS devices while decreasing the leakage. The
reason behind this approach is that TFETs will replace MOSFETs. However, as
mentioned in Chap. 2 and further explored in the following chapters HTFET devices
suffer from a number of problems. First is the difficulty in fabrication, which makes
it a major challenge for foundries to manufacture and classify the device as viable
for large-scale integration and production. Moreover, as detailed in Chap. 2, due to
the introduction of additional process variation sources, the problem is becoming
even worse. Second, even if the HTFET device could be used it has its limitations
in circuit applications due to the turn-on of the pin diode at very low reverse bias,
much lower than in the case of Si-based devices. This poses major challenges to
any design where a device is supposed to be symmetric and conduct current in
both directions, like in the vast majority of CMOS memory circuits, as the circuit
techniques applicable to Si-TFETs to mitigate diode current at reverse bias, are very
limited.
Silicon TFETs, on the other hand, offer lower drive current but also provide
reduced leakage and are less constrained in reverse-bias operation as detailed in
Chap. 2. Compared to HTFETs the fabrication in silicon is straightforward as it is
© Springer Nature Switzerland AG 2021
N. Gupta et al., TFET Integrated Circuits,
https://doi.org/10.1007/978-3-030-55119-3_8
123
Perspective
8.1 TFET-CMOS Hybrid Cores
The previous chapters have demonstrated the extraordinary potential of implementing processors and digital ICs using TFETs in today’s world of mobile devices
operating on meager power budgets. The promise of TFET devices does not stem
only from their own characteristics but from their compatibility and integration with
CMOS devices on the same chip. The unique characteristics of the TFET led us
to imagine a few extremely power efficient and compact new circuit topologies
described in this book.
In order to envisage TFET or hybrid applications the first consideration is the
type of the TFET device to be used. The majority of recent investigations focus on
the use of heterojunction TFETs (HTFET) since the intent is to match or overtake
the performance of currently used MOS devices while decreasing the leakage. The
reason behind this approach is that TFETs will replace MOSFETs. However, as
mentioned in Chap. 2 and further explored in the following chapters HTFET devices
suffer from a number of problems. First is the difficulty in fabrication, which makes
it a major challenge for foundries to manufacture and classify the device as viable
for large-scale integration and production. Moreover, as detailed in Chap. 2, due to
the introduction of additional process variation sources, the problem is becoming
even worse. Second, even if the HTFET device could be used it has its limitations
in circuit applications due to the turn-on of the pin diode at very low reverse bias,
much lower than in the case of Si-based devices. This poses major challenges to
any design where a device is supposed to be symmetric and conduct current in
both directions, like in the vast majority of CMOS memory circuits, as the circuit
techniques applicable to Si-TFETs to mitigate diode current at reverse bias, are very
limited.
Silicon TFETs, on the other hand, offer lower drive current but also provide
reduced leakage and are less constrained in reverse-bias operation as detailed in
Chap. 2. Compared to HTFETs the fabrication in silicon is straightforward as it is
© Springer Nature Switzerland AG 2021
N. Gupta et al., TFET Integrated Circuits,
https://doi.org/10.1007/978-3-030-55119-3_8
123