(Invited) Achieving Lower Power Logic Using P-Type Metal Oxide Thin Film Transistors
Authors: van Fraassen Niels C A , Han Sanggil , Niang Kham , Flewitt Andrew J. John 
Published: November 2022

Thin film transistors (TFTs) have enabled the active matrix displays that are a ubiquitous part of everyday life, from mobile phones and tablets through to desktop monitors and home televisions. They are used in the circuit at each pixel over the display as they can be fabricated for relatively low cost with excellent uniformity at low temperatures over large areas on glass substrates. These pixel circuits only require either n-channel or p-channel enhancement mode TFTs. However, there has long been a desire to integrate the display driver electronics onto the display panel as an alternative to connecting silicon-based integrated circuits using complementary metal oxide semiconductor (CMOS) field effect transistors (FETs) to displays. More recently, there has been an increasing interest in producing other logic circuits using TFTs on glass and plastic substrates as this could enable a new generation of products with embedded electronics. However, both of these applications need low power logic circuits. CMOS is an inherently low power technology, but it requires both n-channel and p-channel transistors. Whilst this can be realised in silicon CMOS FETs, it has not yet been achieved using TFTs because it has proved difficult to realise both n-channel and p-channel devices using one material system at low cost. For example, hydrogenated amorphous silicon can be used to make reasonable n-type material but not p-type.

Thin film oxide semiconductors such as amorphous indium gallium zinc oxide have been successfully commercialised in the pixel circuits of active matrix displays as they possess high field effect mobility, low threshold voltage, low off-state current and a high switching ratio. There is therefore a lot of interest in finding a suitable complementary p-type metal oxide thin film material to allow CMOS-type logic to be realised using this material system.

Cuprous oxide is one such promising candidate material as the top of the valence band is dominated by states which form as a result of hybridisation of the completely filled Cu 3d orbitals and the O 2p orbitals. These hybridised states are less localised and have a higher dispersion with a resulting decrease in the effective mass of holes in these states and a higher carrier mobility. The results of an extensive study of this material shows that the grain structure is of critical importance with [100] oriented films resulting in a higher mobility. Furthermore, the impact of grain boundaries on conduction must be controlled. Although this allows TFTs with reasonable on-state current to be fabricated, it is found that there is a significant residual off-state current which is a result of electron accumulation in the channel. This is consistent with the high off-state current that has been observed widely in TFTs using p-type metal oxide thin film materials.

The low off-state current in silicon MOSFETs means that the geometric transistor design in low power CMOS logic circuits only needs to focus on the effect of the on-state current in the p-channel and n-channel transistors. However, in TFTs, the much larger off-state current also becomes important as it can end up being a significant parameter when calculating the power consumption of CMOS logic circuits. An alternative geometric design parameter for TFT logic circuits is presented. This compares the maximum switching current with the static currents allowing optimisation based on both the noise margin and static power consumption. The result is improved performance of TFT-based CMOS logic circuits using the n- and p-type thin film oxide semiconductors that are currently available.