Ammonia Sensing Results

In this section, results of NH₃ sensing are demonstrated. The response sensitivity to NH₃ is defined as follows:

Sensitivity= ∆J

J₀ =J_CE-J_CE0

J_CE0 Eq.4.1-1

where J_CE and J_CE0 are the collector current density exposed to NH₃ for 200 second and the one before NH₃ exposure respectively from transfer characteristics (J_CE-V_BE curve).

Effect of Collector Voltage (VCE) on Sensitivity

The relationship between sensitivity and V_CE is demonstrated at first. Fig. 4-4 shows a plot of sensitivity to NH₃ concentration with three different V_CE bias conditions. The NH₃ concentrations were ranged from 30 ppb to 1000 ppb; the VCE bias conditions are -1.2 V, -1.8 V and -2.4 V. With collector bias (VCE) as -1.2 V, porous SCLT exhibits the maximum sensitivity to NH3.

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Carrier Injection Effect on Sensitivity

In this section, two structure of porous SCLT shown in were utilized to study the effect of carrier injection. Because injection of holes is from emitter to collector, we can change the injecting direction of holes, i.e. bottom-top or top-bottom, by using different metal as emitter and collector. For bottom injection of holes, ITO and Al were used as emitter and collector respectively. For top injection of holes, MoO₃/Al and ITO were used as emitter and collector respectively. Fig 4-5 shows a plot of sensitivity as a function of NH₃ concentration with different injection of holes and the NH₃ concentrations were ranged from 10 ppb to 1000 ppb.

We can find that the response of bottom injection is higher than top injection. The mechanism is still unknown to explain the better sensitivity of bottom injection structure. However, as shown in Fig. 4-6, it is supposed that hole concentration in the bulk area (the exposing area) of P3HT is higher for top injection than that for bottom injection, indicating the proportion of holes reacted with NH₃ is relatively smaller than bottom injection which has less hole concentration in bulk region. As a result, the response of bottom injection is higher.

Effect of Base Voltage (VBE) on Sensitivity

Fig. 4-7(a) shows a plot of JCE-VBE, representing the porous SCLT’s sensing response to NH3. VCE was fixed as -1.2 V and the NH3 concentrations were ranged from 30 ppb to 1000 ppb. The response of the switching region (VBE = -0.4 V to 0 V) of JCE-VBE plot is shown in Fig. 4-7(b). Increasing NH3 concentration makes JCE-VBE curves shift to the left. Because

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NH₃ acts as electron doping (e-doping) agents, increasing NH₃ concentration indicates increasing e-doping concentration in P3HT which turns out to be the increase of the potential barrier. Therefore, a more negative base potential is required to lower down the channel potential barrier and to turn on the transistor. The sensing sensitivities △J/J₀ as a function of V_BE for various NH₃ concentrations are plotted in Fig. 4-8(a). We found that the sensitivity was strongly dependent on V_BE, and that the maximum sensitivity occurred in the switching zone (-0.5 V < V_BE < 0 V). For NH₃ concentrations of 30 ppb, 100 ppb, and 1000 ppb, the maximum sensitivities measured at V_BE = -0.2 V and V_CE = -1.2 V are -0.09, -0.23, and -0.56, respectively.

As shown in Fig. 4-8(b), a power law relationship is found between the maximum sensitivity and NH₃ concentration, indicating that the proposed NH₃ sensor is particularly sensitive in low-concentration regime (i.e. 30 ppb to 1000 ppb). Besides, we also compare sensitivity of a SCLT sensor with that of a diode sensor (previous results by M. Z. Dai) in Fig. 4-8(b). For diode sensor, ITO and porous Al were used as electrodes, and 300 nm P3HT acted as active layer and sensing layer. It shows that SCLT sensor exhibits a better sensitivity to NH₃. However, the mechanism to explain the difference between SCLT and diode sensors is still unclear.

SCLT Bias Stress on Sensing Response

As aforementioned, effect of NH3 on porous SCLT makes JCE-VBE curves shift to the left and the maximum sensitivities were measured at VBE = -0.2 V and VCE = -1.2 V. However, the

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effect of bias stress on SCLT makes J_CE-V_BE curves shift to the right. As shown in Fig. 4-9(a), J_CE-V_BE curves of porous SCLT, undergoing bias stress (V_BE = -0.2 V and V_CE = -1.2 V) for 4200 seconds, shifted to the right; after bias stress, porous SCLT was treated with NH₃ (100 ppb for 200 seconds) and J_CE-V_BE curves then shifted to the left. We found that the bias stress seems to affect sensing response. Fig. 4-9(b) shows the maximum sensitivity as function of the time after finishing of bias stress and NH₃ concentration is 100 ppb for 200 seconds. The sensitivity, right after bias stress, is higher than the others. It is supposed that J_CE-V_BE shifts to the right during bias stress, while J_CE-V_BE recovers back to the left after bias stress. In addition, the effect of NH₃ makes J_CE-V_BE shift to the left. Therefore, two effects, recovery and NH₃ together, make the shift of J_CE-V_BE much more dramatically indicating the increase of sensitivity. However, with the passage of time, the effect of recovery decreases. In the meanwhile, J_CE-V_BE shift is mainly dominant by the effect of NH₃ resulting in decrease of sensitivity.

Switching properties of the transistor under NH3 sensing

Since the proposed sensor is embedded in a vertical transistor, it is therefore important to evaluate the switching properties of the transistor under NH3 sensing. The switching function of the porous SCLT under NH3 sensing is shown in Fig. 4-10. For NH3 sensing, we want to emphasize the ability of switching between on and off states during NH3 sensing or not.

However, on/off ratio at Vce = -1.2V is not obvious to show the switching ability. Therefore,

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we chose Vce = -1.7V at which have obvious on/off ratio as well as the good enough response to NH₃. The porous SCLT exhibits a significant current drop under NH₃ sensing (the shaded areas). During NH₃ sensing, a good on/off switching property is still obtained when switching V_BE between -0.9 V (on state) and 0.9 V (off state). This result suggests that the proposed porous SCLT integrates a NH₃ sensor and a switching transistor in one single device. In sensor array technology, to save the operational power and to improve the signal-to-noise ratio, the pixel circuit is composed of one sensor and one switching transistor [44]. Our proposed device can serve as a pixel circuitry by itself, facilitating the development of low-power sensor array technology.

Note that the poor recovering current after removing NH₃ gas attributed to the electron are recombined by large amount of holes at the on-state SCLT. Thus the NH₃ molecular is hardly desorpted from P3HT after removing the NH₃ gas result in the current doesn’t back to the original current. Our other experiment presented the good recovering response at the off-state SCLT as shown in Fig. 4-11.