微電子製造工藝科普（7）? TFT ? EN

As you have seen, I am using English to compose this manuscript, and I guess I will keep doing that. You may feel weird or even yell "what the hell", but it is what it is....As I stated in the previous release, the reason I use English is because I am starting practicing English Writing for research papers.

Some day, my advisor told me to write a research summary for applying a scholarship which worth $5000. I do know what to write, but I dont know how. So one possible solution came to my mind: Cut, paste, and reorganize what senior students already published!!! Isnt it a good idea?

But you know, this is a temporary solution, but actually you cannot depend on this forever. You have to generate your own words after some day you used up others words, right? So I made up my mind to practice English Writing from day on. Any comment or suggestion to my English writing is highly appreciated!!!

As I promised earlier, here I will briefly introduce the working mechanism of Thin Film Transistor (TFT), in order to establish the foundation for my last article (Measuring).

Actually, there is a TFT related article released earlier (微電子製造工藝科普（1）? 晶體管 - 知乎專欄). You may want to refer to that for your better understanding.

Anyway, lets get started.

PS. Many readers comments suggest me to write a Chinese version as well. Sure, I agree this is a good idea. I will release a Chinese version right after the English one was published.

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Ⅷ. Transistors

First of all, forget all the concepts you may already know about transistors, because the stereotype you already have may distract your mind while reading this paper. I am trying to simplify its physical model and present to you something that makes sense.

8.1 ON & OFF

Basically, a transistor is just an electrical switch, just like any switches you learned in high school. Therefore, a transistor has two states, ON state and OFF state. There is a current flowing through the transistor that is in ON state, and we expect the "switch" has no resistance to the current, i.e. ON current is infinitely large (though it is impossible); whereas in OFF state, no current flowing at all (although this is impossible either).

8.2 Control Signal

On the other hand, how to control the state? We need a specific terminal (gate) which dedicates to control signal. By the way, thats why all transistors are three-terminal devices. A low intensity of effective control signal is expected due to power conservation purposes.

Ⅸ. Thin Film Transistors (TFTs)

That is the universal working mechanism for all transistors. Here we introduce the Thin Film Transistor (TFT). The name is called TFT simply because thin film is the major role here. In other words, no bulk materials at all.

Look at the above device. It contains 4 basic parts: Source/Drain (S/D), Channel, Dielectric layer (or to say insulator), and Gate.

The materials for S/D, Channel, Dielectric, and gate are metal, semiconductor, insulator, and metal, respectively.

ZnO is a natural n-type semiconductor, i.e. the major carrier is electrons (not holes). Current flows through Source and Drain via channel.

The sandwiched structure of channel/dielectric/gate is just a capacitor, where gate is the lower electrode and channel is the upper electrode.Therefore, Si substrate serves as the gate which controls the carrier concentration in the channel layer. The voltage applied on the gate influence the charges in the channel; positive bias leads to electron accumulation (ON mode), and negative bias leads to electron depletion (OFF mode).

As shown in the above figure, a positive voltage is applied to gate, so an electrical field towards to channel is generated and attracts the electrons accumulating at the interface between channel and dielectric layer, which forms a conductive path for current flow in and out. On the other hand, if a negative voltage is applied to gate, electrons are depleted, in other words, no conductive path at all. Thats how the so called switch works.

Fig. 27 A typical Vg-Id curve

Vg: The voltage applied to gate

The above graph shows a typical Id-Vg curve. When negative biasing, drain current is 1E-12, which we claim OFF; when Vg>3V, drain current reaches 1E-2, which we treat as ON. A ON/OFF ratio of 1E10 has been achieved by using this transistor.

In conclusion, TFT is just like a switch which contains two states, ON and OFF. Gate is used to control the state. Positive Biasing results in ON state, whereas negative biasing results in OFF state.

Thanks ~~~