Floating body germanium phototransistor having a photo absorption threshold bias region
A floating body germanium (Ge) phototransistor with a photo absorption threshold bias region, and an associated fabrication process are presented. The method includes: providing a p-doped Silicon (Si) substrate; selectively forming an insulator layer overlying a first surface of the Si substrate; forming an epitaxial Ge layer overlying the insulator layer; forming a channel region in the Ge layer; forming a gate dielectric, gate electrode, and gate spacers; forming source/drain (S/D) regions in the Ge layer; and, forming a photo absorption threshold bias region in the Ge layer, adjacent the channel region. In one aspect, the second S/D region has a length, longer than the first S/D length. The photo absorption threshold bias region underlies the second S/D region. Alternately, the second S/D region is separated from the channel by an offset, and the photo absorption threshold bias region is the offset in the Ge layer, after a light p-doping.
1. A floating body Germanium (Ge) phototransistor with a photo absorption threshold bias region, the phototransistor comprising:
a p-doped Silicon (Si) substrate;
an insulator layer overlying a first surface of the Si substrate;
an epitaxial Ge layer overlying the insulator layer;
a channel region in the Ge layer;
a gate dielectric, gate electrode, and gate spacers overlying the channel region;
first and second source/drain (S/D) regions in the Ge layer; and,
a photo absorption threshold bias region in the Ge layer, adjacent the channel region.
2. The phototransistor of claim 1 wherein the epitaxial Ge layer has a thickness in the range of about 300 to 1000 nanometers (nm).
3. The phototransistor of claim 1 wherein the insulator layer is a silicon nitride layer with a top surface;
wherein the Si substrate includes a second surface adjacent the first surface; and,
the phototransistor further comprising:
silicon oxide overlying the Si substrate second surface.
4. The phototransistor of claim 3 further comprising:
a p-doped region in the Ge layer, immediately overlying the silicon nitride top surface.
5. The phototransistor of claim 3 wherein the first S/D region approximately overlies the Si substrate second surface and has a first length;
wherein the second S/D region has a second length, longer than the first length; and,
wherein the photo absorption threshold bias region in the Ge layer underlies the second S/D region.
6. The phototransistor of claim 3 wherein the first S/D region approximately overlies the Si substrate second surface; and,
wherein the photo absorption threshold bias region in the Ge layer includes a p-doped offset separating the second S/D region from the channel.
7. The phototransistor of claim 6 wherein the second S/D region has a channel-side edge offset from the channel region by a distance no larger than d, where d is calculated as follows:
d
=
2
ɛ
(
V
D
-
V
DSAT
)
q
N
where ε is the dielectric constant of the germanium;
where N is the channel doping density; and,
where V D and V DSAT are the second S/D bias voltage and saturation voltage at the operational gate bias voltage, respectively.
8. The phototransistor of claim 1 wherein the insulator layer overlying the Si substrate has a thickness in the range of about 10 to 500 nm.
9. The phototransistor of claim 1 wherein the gate dielectric and gate spacers are formed from wide bandgap materials.
10. The phototransistor of claim 9 wherein the gate dielectric is a material selected from the group consisting of SiO 2 , GeO 2 , Al 2 O 3 , HfO 2 , ZrO 2 , TiO 2 , Ta 2 O 5 , and combinations of the above-mentioned materials;
wherein the gate electrode is a material selected from the group consisting of polycrystalline Ge, polycrystalline SiGe, and polysilicon; and,
wherein the gate spacers are a material selected from the group consisting of SiO 2 and Si 3 N 4 .