2.8.4 Prototype CNT-FET Circuits

The promising characteristics of individual CNT-FETs have lead to initial attempts at integration of several CNT-FETs into useful circuits that can perform a logic operation, or function as memories [85] or sensors [86]. In the following, we limit our discussion to advances in logic circuitry. The CNT logic gates have been, in most cases, based on a complementary technology analogous to silicon CMOS, which is important as it may ease integration of CNTs onto this well established technology.

The first complementary (CMOS-like) logic gates were reported by DERYCKE and co-workers [67]. In that work, two different techniques were used to produce n-type devices. An inverter gate was created by combining two CNT-FETs: a p-type device in the ambient and a vacuum annealed n-type device. A more compact and integrated approach uses potassium doping to convert one of two CNT-FETs built on the same CNT to n-type. The masking of the other transistor which remained p-type was accomplished by photo-resist. The circuit had a voltage gain of about two, suggesting that integration, without signal degradation, of many devices along a single CNT can be accomplished. Shortly thereafter, BACHTOLD and co-workers [87] used p-type CNT-FETs along with resistors to build prototype logic gates based on an older transistor-resistor scheme. They went a step further in complexity and wired three such inverter gates to form a ring oscillator. The large parasitic capacitances severely degraded the performance of the circuit which oscillated at only about 5 Hz. Later JAVEY and co-workers [70] used another scheme for converting p-type into n-type CNT-FETs and to wire up CMOS inverters with gains in excess of ten and CMOS ring oscillators with frequencies in the 100 Hz range. Very recently CHEN and co-workers [88] reported CMOS ring oscillators operating at frequencies up to 52 MHz (see Fig. 2.20). However, these reported frequencies are well below the expected AC response of CNT-FETs, which is difficult to assess because of the relatively small current signals in these devices. Most recently, the non-linear current-voltage characteristics of CNT-FETs were used to demonstrate that DC characteristics of CNT-FETs are not affected by AC fields at least up to 500 MHz [89]. However, operation up to 10 GHz [90] and later 50 GHz [91] have been reported, albeit with considerable signal attenuation.

Figure 2.20: Image of CNT-FET circuit reported by CHEN et al. [88].

M. Pourfath: Numerical Study of Quantum Transport in Carbon Nanotube-Based Transistors