Field effect transistor with asymmetric gate structure and method
Disclosed is a lateral double-diffused metal oxide semiconductor field effect transistor (LDMOSFET) with a replacement metal gate (RMG) structure that includes a first section, which traverses a semiconductor body at a channel region in a first-type well, and a second section, which is adjacent to the first section and which traverses the semiconductor body at a drain drift region in a second-type well. The RMG structure includes, in both sections, a first-type work function layer and a second-type work function layer on the first-type work function layer. However, the thickness of the first-type work function layer in the first section is greater than the thickness in the second section such that the RMG structure is asymmetric. Thus, threshold voltage (Vt) at the first section is greater than Vt at the second section and the LDMOSFET has a relatively high breakdown voltage (BV). Also disclosed are methods for forming the LDMOSFET.
1. A transistor comprising:
a first-type well in a semiconductor body and having a first-type conductivity;
a second-type well in the semiconductor body positioned laterally adjacent to the first-type well and having a second-type conductivity different from the first-type conductivity; and
a gate structure on the semiconductor body at a junction between the first-type well and the second-type well,
wherein the gate structure has a first section traversing the semiconductor body adjacent to the first-type well and a second section traversing the semiconductor body adjacent to the second-type well,
wherein the gate structure comprises a first-type work function layer having a first portion with a first thickness in the first section and further having a second portion with a second thickness that is less than the first thickness in the second section,
wherein the gate structure further comprises a second-type work function layer on the first-type work function layer in the first section and in the second section, and
wherein the first-type work function layer has a first-type work function within a first range and the second-type work function layer has a second-type work function within a second range that is different from the first range.
2. The transistor of claim 1 , wherein the first-type work function layer comprises multiple layers and wherein the first portion has a greater number of the multiple layers than the second portion.
3. The transistor of claim 1 , wherein the first-type work function layer comprises a single layer.
4. The transistor of claim 1 , wherein the semiconductor body comprises a semiconductor fin and wherein the gate structure is adjacent to a top surface and opposing sides of the semiconductor fin.
5. The transistor of claim 1 , further comprising:
a channel region in the first-type well, wherein the first section of the gate structure is adjacent to the channel region;
a drain drift region in the second-type well and positioned laterally immediately adjacent to the channel region at the junction between the first-type well and the second-type well and wherein the second section of the gate structure is adjacent to the drain drift region;
a source region immediately adjacent to the first-type well such that the channel region is positioned laterally between the source region and the drain drift region; and
a drain region immediately adjacent to the second-type well such that the drain drift region is positioned laterally between the channel region and the drain region,
wherein the source region and the drain region comprise second-type source and drain regions with a higher conductivity level than the second-type well.
6. The transistor of claim 5 , further comprising a trench isolation region in the second-type well positioned laterally between the drain drift region and the drain region.
7. An N-type transistor comprising:
a Pwell in a semiconductor body;
an Nwell in the semiconductor body positioned laterally adjacent to the Pwell; and
a gate structure on the semiconductor body at a junction between the Pwell and the Nwell,
wherein the gate structure has a first section traversing the semiconductor body adjacent to the Pwell and a second section traversing the semiconductor body adjacent to the Nwell,
wherein the gate structure comprises a P-type work function layer having a first portion with a first thickness in the first section and further having a second portion with a second thickness that is less than the first thickness in the second section, and
wherein the gate structure further comprises an N-type work function layer on the P-type work function metal layer in the first section and in the second section.
8. The N-type transistor of claim 7 , wherein the P-type work function layer comprises a single layer.
9. The N-type transistor of claim 7 , wherein the semiconductor body comprises a semiconductor fin and wherein the gate structure is adjacent to a top surface and opposing sides of the semiconductor fin.
10. The N-type transistor of claim 7 , further comprising:
a channel region in the Pwell, wherein the first section of the gate structure is adjacent to the channel region;
a drain drift region in the Nwell and positioned laterally immediately adjacent to the channel region at the junction between the Pwell and the Nwell and wherein the second section of the gate structure is adjacent to the drain drift region;
a source region immediately adjacent to the Pwell such that the channel region is positioned laterally between the source region and the drain drift region; and
a drain region immediately adjacent to the Nwell such that the drain drift region is positioned laterally between the channel region and the drain region,
wherein the source region and the drain region comprise N-type source and drain regions at a higher conductivity level than the Nwell.
11. The N-type transistor of claim 10 , further comprising a trench isolation region in the Nwell positioned laterally between the drain drift region and the drain region.
12. The N-type transistor of claim 7 , wherein the P-type work function layer comprises multiple layers and wherein the first portion has a greater number of the multiple layers than the second portion.
13. The N-type transistor of claim 12 , wherein the multiple layers comprise titanium nitride.
14. The N-type transistor of claim 12 , wherein one of the multiple layers that is in both the first section and the second section comprises titanium nitride.