The basic components of a Thin Film Transistor (TFT) include two layers of metal (commonly Al or Cu), two insulating layers (generally SiN_x:H), an active layer (usually a-Si:H), and an ohmic contact layer (n^+ a-Si:H) positioned between the semiconductor and metal layers. The electrodes formed by the two metal layers are the scanning signal metal electrode and the data signal metal electrode. The two insulating layers refer to the insulating layer below the active layer (often called the gate insulating layer, denoted as g-SiN_x) and the insulating layer above the active layer (commonly called the passivation layer, denoted as PVX). The active layer and the doped layer for ohmic contact typically form an island-like structure in the TFT, often referred to as an active island or silicon island.

Based on the position relationship between the gate electrode and the silicon island, the TFT structure can be divided into bottom-gate structure and top-gate structure. The bottom-gate structure is also known as the Inverted Staggered Structure, and the top-gate structure is referred to as the Staggered Structure.

Bottom-Gate Structure

Currently, bottom-gate structures are widely used in liquid crystal displays (LCDs). Since LCDs are passive light-emitting devices requiring a backlight source behind the display screen, the opaque gate electrode metal layer in the bottom-gate structure effectively blocks light from the backlight source. This prevents light from reaching the silicon island, which could generate photo-induced carriers and affect the off-state current characteristics of the TFT.

Based on manufacturing process differences, the bottom-gate structure is generally divided into the Back-Channel Etchant (BCE) structure and the Etched-Stopper Layer (ESL or ISL) structure. The BCE structure has fewer process steps, making it more cost-effective. However, the ESL structure prevents etching damage to the back-channel of the semiconductor layer, resulting in superior electrical characteristics. Due to advancements in design and process technology, BCE structures are now widely adopted by panel manufacturers because their electrical performance meets display requirements. Generally, unless the BCE structure cannot meet process requirements, panel manufacturers prefer not to use the ESL structure.

Top-Gate Structure

The top-gate structure has a significant drawback for use in LCDs: the silicon island is directly exposed to the backlight. However, in reflective electrophoretic displays, the light source for image display comes from ambient light. The metal gate electrode in the top-gate structure covers the silicon island, effectively blocking ambient light and reducing photo-induced carriers. This characteristic allows electrophoretic displays to retain charges in the pixel capacitors for a long time without repeated refreshing, even if the image on the screen remains static.