A novel two-way mode of current switching dependent on activated charge transport

We demonstrate a fully printed transistor with a pl nar triode geometry, using nanoparticulate silicon as the semiconductor materi al, which has a unique mode of operation as an electrically controlled two-way (double throw) s witch. A signal applied to the base changes the direction of the current from between the colle ctor and base to between the base and emitter. We further show that the switching charact eris ic results from the activated charge transport in the semiconductor material, and that i t is independent of the dominant carrier type in the semiconductor and the nature of the junction between the semiconductor and the three contacts. The same equivalent circuit, and hence si milar device characteristics, can be produced using any other material combination with non-linear current-voltage characteristics, such as a suitable combination of semiconducting an d conducting materials, such that a Schottky junction is present at all three contacts. We present performance results for two design variants of the printed transistor and confi rm our interpretation of the device’s operation by constructing a model circuit using individual va ristors. Over the last 65 years the transistor has revolutio nised industrial and consumer electronics [1]. Originally conceived and developed as a signal ampl ifier [2, 3, 4], its main use today is as an electrically driven switch in computer logic [1], m e ory addressing and driving displays [5]. In general terms a transistor is a three terminal elec troni device (triode) which exhibits a transconductance. There are two classes of transist or: field effect transistors [1, 2, 5] and junction transistors [3, 4]. In field effect transistors the current between two contacts (the source and drain ) through a semiconductor material is either restrict ed or enhanced by an internal electric field result ing from the application of a potential to the gate ele ctrode. In junction transistors, injection of charg e by a current through the base modulates the potential ba rriers between the base and the emitter and between the base and collector respectively. Both classes o f transistor can therefore function as simple switc hes in which a signal applied to the base (or gate) con trols the current between the emitter and collector (or source and drain). Here we demonstrate a fully prin ted electronic device with a similar triode configuration, using nanoparticulate silicon as the semiconductor material [6, 7], which has a unique mode of operation as an electrically controlled two ay (double throw) switch. By analogy with the junction transistor and the vacuum tube, we denote the electrodes as emitter, base and collector. A signal applied to the base changes the direction of the current from between the collector and base to between the base and emitter. We further show that the switching characteristic results from the 1 To whom any correspondence should be addressed. activated charge transport in the semiconductor mat erial, and that it is independent of the dominant carrier type in the semiconductor and the nature of the junction between the semiconductor and the three contacts. The devices were produced by screen printing patter ns of conducting and semiconducting inks on plain paper substrates, with the contacts arranged in a coplanar geometry. The printed silicon layers consist of a dense network of nanoparticles [8, 9, 10] produced by high energy milling of bulk material, which typically have a log-normal size di stribution with a median size of approximately 100 nm and a logarithmic standard deviation around 1.45 [8, 10]. In the printed layer the network structur e can be described as a fractal structure of intercon necting clusters of particles [9, 10]. The size of these clusters and their connectivity depends strongly on the substrate material and ink composition as well as the deposition process [9, 10], but their intern al topology is similar. Typically for the material used here, the primary clusters contain 600 ± 200 indivi dual particles, and have a size of approximately 50 0 nm [10]. The individual silicon particles are polyc r stalline, with a typical grain size of 10 nm and have predominantly (111) faceted surfaces [7, 8] wh ich ave no noticeable oxide shell [7]. The combination of a highly interconnected network f particles with many interfaces between particles along any percolation path leads to a the rmally activated hopping transport of charge. It is this property, which forms the basis for the applic ation of this material system in commercially available printed thermistors [11], and, more impor tantly, which enables the current switching function presented here. The current-voltage charac teristics of the printed silicon material between t wo metal contacts exhibit a non-linear response of the current I on the potential difference V, which can be described by the form ( ) ( )             − − −       − = nkT IR V e nkT IR V e I I S S exp exp 0 , (1) where k is Boltzmann’s constant and T is the absolute temperature. These characteristics are indicative of a varistor composed of back-to-back diodes, in w hich each diode has the same reverse saturation current I0 and ideality factor n, with an additional series resistance RS. As discussed in the supplementary information, both the saturation curr ent, which represents the activation energy for hopping, and the ideality factor, which is influenc d by the number of junctions in the percolation path, have non-trivial temperature dependences, ind icating the freezing out of some conducting pathways at low temperatures. If a third contact is added to the silicon layer to form a three terminal device, the equivalent circu it becomes a triangular arrangement of three varistors as hown in figure 1 (a). The behaviour of the circuit can be predicted in terms of the potentials applied to each terminal and the voltage dependent resistance between them. The resistance between any p ir of terminals is high if the potential difference between them is low, and consequently a current only passes for a high potential difference . Hence if the emitter is maintained at zero and a po sitive bias is applied to the collector, there will be a current into the collector and out of the emitter. By analogy with a transistor, this can be convenien tly regarded as the off-current. If the base is then bi ased positively there will be a current from the ba se to the emitter. Conversely, if the base is biased nega tively there will be a current into the collector a nd out of the base. Therefore, not only is the sense o f the current through the base reversed as expected when the potential is changed, but its route throug h the circuit is also diverted. In terms of the mechanically operated analogue shown schematically in figure 1(b), in the conventional mode of operation of a transistor applying a signal to the base corresponds to a simple switch closing the gap between the emitter and collector. In figure 1(c), the operation of this new device is fundamentally different, in that applying a signal to the base co rresponds to rotating the arm of a two-way switch s o that it connects either the emitter and base or the bas and collector. Figure 1: (a) Equivalent circuit of the current switching device as a triangular network of varistors, RCE connecting the collector (C) to the emitter (E), RCB connecting the collector to the base (B), and RBE connecting the base to the emitter (E). Under applied potentials VB and VC the currents into the base and collector are IC and IB respectively. The emitter potential VE is by definition zero, and the current out of the emitter IE is equal to the sum of IB and IE. (b) Mechanical switch analogue of a normal mode of transistor operation in which application of a signal to the base is equivalent to a vertical motion of the plunger to complete the circuit between E and C. (c) Mechanical switch analogue of the current switching mode in which application of a signal to the base is equivalent to a rotation of the leve r to switch the circuit from between E and B to