Why Polysilicon is used as Gate Material?

In the early days, Aluminum metal was used as the preferred gate material in MOSFET’s but later it was replaced with polysilicon.

The main reason behind this transition was:-

1. In the early MOS fabrication process, the gate region was created after the source and drain region is formed. The major drawback of such a fabrication process is that if the gate mask is misaligned, it led to the formation of parasitic capacitance Cgd and Cgs.

Where capacitance Cgd is more harmful as it a feedback capacitance. As a result of miller capacitance, there was a reduction in the MOS transistor’s switching speed.

2. “Self-aligned Gate Process” was introduced as a solution to fix the misalignment of the gate mask. This process starts with the formation of the gate region followed by the creation of the source and the drain region.

The thin gate oxide under the gate (polysilicon) acts as a mask for the doping process thus preventing further doping under the gate region. Hence, this process makes the gate self-aligned with respect to the source and the drain i.e. the source and drain do not extend under the gate any further resulting in the reduction of Cgd and Cgs value. 

3. The doping process for the source and drain uses the annealing method which requires very high temperature (>8000 ^oC). If Aluminum is used as gate material then it would melt under such high temperature.

Why? Because the melting point of Al is ~660 ^oC. But in case if polysilicon is used as the gate material, it would not melt. That’s the reason the self-aligned process will work with polysilicon and not with Al as the gate material.

**Undoped polysilicon has very high resistivity ~10⁸ ohm/Cm. Hence, the doping level in polysilicon is modulated to reduce the resistance.

4. Also earlier, metal as gate material was used when the operating voltage was in the range of 3-5V but now as the transistor size has scaled-down, so does the operating voltages. Metal gate results in very high threshold voltage (Vt) compared to polysilicon and it becomes non-operational at such lower operating voltages.

Further, polysilicon has a similar composition as that of the bulk-Si channel. Implies the threshold voltage of MOS is now correlated with the work function difference between the gate and the channel (also its work function can be modulated by adjusting the level of doping).

Strictly speaking, modern technology is not CMOS but rather CPOS (Complementary Polysislicon Oxide Semiconductor).