Trying to understand in depth about metal work function used in modern semiconductor devices e.g. gate, Metal gate has better performance compared to poly si gate. Here key role is gate work function made of different metal instead of poly si or combination of both. Metal has higher work function than poly. How does it control flow of electron in the channel in off and on state?
Let me explain a little about metal gate.
Work function of the gate depends on what metal is used for.
Metal can be lower or higher than highly-doped poly (either n+ or p+).
work function modulates energy band of semiconductor devices.
For example, higher work function bends energy band upwards.
Conceptually, it is equivalent that transfer curves (Id-Vg) shift toward right.
The flow of electron is not changed by metal work function, but it is operated at different gate bias.
I hope this can help you to understand, and If you have any questions, please feel free to ask.
Sincerely,
Jun-Sik Yoon
Hi Jun-Sik, Thank you so much for the scholarly answer. If Id-Vg shifts towards right; then Vt is getting higher. But in modern low power devices, Vt is set for low voltage. Channel is intrinsic without doping; metal work function is used for modulating the Vt. How sub-threshold current is better controlled by metal gate? I am interested to understand work function and Vt mechanism. Thank you for your answer.
There is a limit to use poly-Si as a gate because even highly-doped poly-Si is not same as metal gate.
One example is poly-depletion; at moderate and high gate voltage, there is an additional depletion capacitance at poly-Si gate, which reduces gate-to-channel controllability and thus degrades subthreshold characteristics.
To set lower Vt, lower work function is required for NFET, or higher work function is for PFET. It would be better if you draw energy band diagram of MOS (metal-oxide-semiconductor) structure.
Best regards,
Jun-Sik Yoon
Hi Mia,
As explained before, the effective WF (EWF) is the key control parameter of VT, which is an important parameter for the speed and overall performance of devices and circuits. There are several types of devices (e.g., intrinsic vs doped channels) that require different EWF values in order to engineer VT.
Regarding your comment from the original post: Metals don't necessarily have higher WFs than poly, and you don't always need a high WF for devices.
The comment regarding better performance of metal gates compared to poly gates: this has several causes, mostly relating to the limitations and parasitics of poly Si at highly-scaled devices; to name a few: poly-depletion, series resistance and dopant penetration (reliability).
Also note that the introduction of high-k dielectrics into modern devices has pushed out the poly Si gates since they react with the gate oxide under the necessary process conditions. What I'm saying is that despite the advantages of metal gates - another reason for their introduction is that the poly gates had to be retired.
One modern approach for engineering the EWF is to use a so called 'mid-gap metal', often the thermally-stable TiN or TaN (EWF ~~ 4.5 eV) and then shift the EWF to the desired values using ultrathin (
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