(Invited) Thin Film Transistors as Driving Devices for Attached Devices

TFTs have been the most popular devices for the large area active matrix flat panel displays for the past 20 years. The a-Si:H TFT is suitable for driving the liquid crystal in the pixel level because of its characteristics of low power consumption, gray scale capability, low leakage current, etc. [1]. The a-Si:H TFT array can be easily fabricated on the large area glass substrate with a high throughput at a low temperature. However, the low mobility limits its applications in many areas. For example, the OLED has been widely studied as a potential replacement of the LCD for its low power consumption, contact size, and good color quality. However, for the high quality displays, TFTs are required to drive the OLED pixels [2]. It is difficult or impossible to drive each OLED pixel with a single a-Si:H TFT. The high mobility poly-Si or oxide TFTs are proper for this application. In addition, there are many studies on using the TFT as a driving or sensing device for purposes of chemical or biological sensing, detection, or imaging [2,3]. For some new applications, the high mobility is often required. In this paper, authors investigated limitations on applying a single TFT to driven a device.

Figure 1 shows a simple system of a TFT attached with a resistor to the drain electrode. Since a large number of the attached devices change their  conductivities upon exposure to selected chemicals, biological agents, optical intensity, or mechanical stress,  the resistance is used as a reference of the change.      

Figure 2 shows the drain current change with the resistance (r) of the resistor attached to the TFT. The poly-Si TFT (W/L = 12m/6m) has a field effect mobility (m_eff) of ~ 235 cm²/V.s, an on current (I_on) of 1.8x10^-4 A , a threshold voltage (V_t) of 0.45V, and a subthreshold slope (S) of 0.26 V/dec. The TFT gate voltage (V_g) was fixed at 9 V and the complete system was driven at V_D’ = 10V. Both the current passing the resistor, i.e., I_D, and the drain voltage (V_D) decrease monotonically with the increase of r. Since the I_D’s in Fig. 1 are below that of the TFT without the attached resistor, the performance of the system is controlled by the resistor.  This kind of system can be used to detect the change of the environment, such as the light intensity or gas composition, when the resistor responds to the variation of a parameter. 

For the Fig. 1 system, the power applicable to the resistor is limited. For example, Figure 3 shows that the maximum power is obtained when the resistance is about 100 ohm. TFTs are usually used as switching devices. However, there are limits on the physical characteristics of the attached device. If the power required for the attached device is beyond the peak power in Fig. 3, it cannot be fully turned on although the complete system can be built.  For certain applications, such as MEMS or LEDs, the peak power can be a good guideline on selecting the TFT or designing the attached device.  The peak power can be increased with the improvement of the TFT characteristics, such as the m_eff  or W/L ratio, however, with limits. 

The reliability of the TFT is an important issue in the Fig. 1 system. In addition to the environmental factors, e.g., humidity, light, and temperature, the lifetime has to be kept long. For example, the TFT can deteriorate, e.g., increasing the off current, the threshold voltage, or the subthreshold slope, during the driving of the attached device with a high current or for an extended period of time. Possible solutions and limitations will be discussed.  

Geng-Wei Chang thanks the National Science Council Graduate Students Study Abroad Program for supporting his study in Prof. Yue Kuo’s Thin Film Nano & Microelectronics Research Laboratory.  

[1] Y. Kuo, ECS Interface, 22(1), 55  (2013).

[2] Y. Kuo, Chap. 13, Thin Film Transistors, Vol. 2, p. 464, Kluwer Academic Publishers (2004).

[3]  Y. Kuo, Chap. 11, Thin Film Transistors, Vol. 1, p. 485, Kluwer Academic Publishers (2004).