2022 OPTICS LETTERS / Vol. 32, No. 14 / July 15, 2007

SiGe optoelectronic metal-oxide semiconductorfield-effect transistor

Ali K. Okyay,* Abhijit J. Pethe, Duygu Kuzum, Salman Latif, David A. B. Miller, and Krishna C. SaraswatCenter for Integrated Systems, Stanford University, Stanford, California 94305, USA

*Corresponding author: aokyay@standford.edu

Received March 28, 2007; revised May 6, 2007; accepted May 11, 2007;posted May 21, 2007 (Doc. ID 81470); published July 5, 2007

We propose a novel semiconductor optoelectronic switch that is a fusion of a Ge optical detector and a Simetal-oxide semiconductor field-effect transistor (MOSFET). The device operation is investigated with simu-lations and experiments. The switch can be fabricated at the nanoscale with extremely low capacitance. Thisdevice operates in telecommunication standard wavelengths, hence providing the surrounding Si circuitrywith noise immunity from signaling. The Ge gate absorbs light, and the gate photocurrent is amplified at thedrain terminal. Experimental current gain of up to 1000� is demonstrated. The device exhibits increasedresponsivity ��3.5� � and lower off-state current ��4� � compared with traditional detector schemes.© 2007 Optical Society of America

OCIS codes: 040.5160, 130.0250, 250.3140, 130.3120.

As device dimensions shrink and clock frequenciesincrease, the volume of chip-to-chip and on-chip com-munication rockets up. Although still efficient atshort distances, copper wires suffer excessive powerdissipation and delay in global lines and cannot copewith the growing bandwidth demand [1]. As the chiparchitecture evolves towards a modular design, thebandwidth density requirements further strain thetraditional interconnects [2]. Chip-to-chip and on-chip optical interconnects are promising to alleviateproblems faced by copper wires [3]. Compound semi-conductors have been the forerunner in optoelec-tronic applications, but their integration with Si isexpensive and hindered by parasitics. Recently, it hasbeen demonstrated that incorporation of Ge into Si isa promising approach that can enable the design oflow-cost modulators on Si with potentially higher ef-ficiencies than their hybrid counterparts [4]. Thistechnology is fast approaching to replace the conven-tional building blocks of the transmitter end in an op-tical link operating at telecommunication standardwavelengths. Si-based classical optical receivers suf-fer from large photodiode capacitance; hence theydissipate huge amounts of power besides occupying alarge footprint area. Increasingly stringent power re-quirements on a chip limit the total number of receiv-ers, and hence the number of links.

Optical interconnects can have a great impact ininterchip links and on-chip clocking applications. Weintroduce a novel optoelectronic metal-oxide semicon-ductor field-effect transistor (OE-MOSFET) for suchhigh-performance applications. The device scaleswith technology and can be made extremely compactwith very low capacitance and small footprint area.By such a design, light can be introduced at the latchlevel eliminating the ever so power-hungry electricalinterconnection hierarchy of clock distribution net-works as well as the high-capacitance optical detec-tors. In this Letter, we demonstrate, with both simu-lations and experiments, the operation of such a

device.

0146-9592/07/142022-3/$15.00 ©

The proposed device is a Si MOSFET with a Gegate, as depicted in Fig. 1. Source and drain regionsare self-aligned to the gate. The signal is generatedremotely and is transmitted in the form of optical en-ergy in the 1.3–1.55 �m window, where Ge is astrong absorber. Light can be coupled into the devicefrom the top surface as well as a through waveguidescheme. Incoming photons are absorbed by the Gegate, and the optically generated carriers movewithin the gate due to band bending and the appliedgate bias, as shown in Fig. 2. This constitutes a netcurrent �IGATE� resulting in accumulation of electronsand holes at either side of the gate insulator. An am-plified version of IGATE flows at the drain terminalowing to increased carrier density in the channel. Si,however, is transparent at these wavelengths, so noabsorption takes place in the channel; hence the sur-rounding Si circuitry is noise free, providing noiseimmunity from clocking and signaling. As light isturned off, the gate oxide capacitance dischargesthrough both increased recombination and diffusiondue to the gradient in the carrier concentration. The

Fig. 1. (Color online) Schematic of the OE-MOSFET.Source/drain and channel regions are formed in Si. The Gegate is deposited on thermally grown SiO2. Absorptiontakes place in the gate. Si is transparent, and hence is

noise immune.

2007 Optical Society of America

July 15, 2007 / Vol. 32, No. 14 / OPTICS LETTERS 2023

overall speed of the device is limited by turn-off,which can be controlled by carrier lifetimes andthickness of the gate region as well as the appliedgate bias.

The Ge–SiO2–Si capacitor governs the operationof the device. Three capacitors are in series: gatedepletion, CGe, insulator, Cox, and channel depletion,CSi. Ge–SiO2–Si structures are fabricated on (100)-oriented p-type ��1015 cm−3� Si wafers. A 3.5 nmthick SiO2 layer is thermally grown at 900°C, fol-lowed by 240 nm polycrystalline Ge. Gate regions arepatterned by photolithography and dry etching. Fi-nally, samples are annealed at 400°C in forming gasambient for 45 min. Figure 3 plots the measured ca-pacitance showing the change with light as predictedby simulations. Due to experimental difficulties, vis-ible light is used in capacitance measurements.When VGATE�−1 V, the Si channel is accumulatedwhile the Ge gate is inverted. In this region, total ca-pacitance, CTOTAL, is roughly equal to CGe becauseCox is considerably high. Light absorbed in the spacecharge region modifies the depletion width in thegate, changing CTOTAL and hence the stored charge.

Fig. 2. (Color online) Energy band diagram of theGe–SiO2–Si stack. Band bending under equilibrium andsteady state illumination are shown by solid and dottedcurves, respectively. In the case illustrated here, opticallygenerated holes accumulate at the Ge–SiO2 interface,while the electrons are swept out of the gate terminal andflow to the SiO2–Si interface inducing a channel.

Fig. 3. (Color online) Measured high-frequency �100 kHz�CTOTAL−VGATE for 3.5 nm-thick SiO2. Due to experimentaldifficulties, visible light is used in measurements. Si alsoabsorbs at this wavelength, and hence Si depletion capaci-

tance, dominant for VGATE�0, is also modulated.

The accumulation and inversion conditions for Si andGe are exchanged when VGATE�0 V. Similarly,equivalent capacitance is now dominated by CSi andmodified by the light absorbed in the channel.

MEDICI software is used for 2-D electrostatic andtransient device simulations. Typical IDRAIN-VDRAINresults obtained by MOSFET simulations are shownin Fig. 4(a). The channel length �LG� and doping are1 �m and p-type 1018 cm−3, respectively. The inputlight, 1 �W/�m2, acts like an additional gate voltagebias modifying the drain current. It should be notedthat the transistor gain is not a linear function of thegate voltage owing to the nonlinear relation ofIDRAIN-VGATE as shown in Fig. 4(b). Therefore, IDRAINdoes not increase linearly with optical power.

Si n-channel MOSFETs are fabricated with a240 nm polycrystalline Ge gate deposited on 6 nmthick thermally grown SiO2. Light is coupled into thegate from the top surface. A waveguide couplingscheme can be used in an integrated system applica-tion. There is no dc gate current (within measure-ment noise) due to the insulating SiO2 layer. An in-ternally modulated 1.55 �m laser with asynchronized lock-in amplifier is used to precisely ex-tract the photocurrent component of the measuredsignal. The measured gate and drain photocurrents

Fig. 4. (Color online) (a) Simulated IDRAIN−VDRAIN resultsof a 1 �m gate length transistor with n-doped Ge�1016 cm−3� and p-doped Si �1018 cm−3�. Incident light(1 �W/�m2 in this case) constitutes a gate signal. (b)

IDRAIN−VGATE characteristics of the same OE-MOSFET.

2024 OPTICS LETTERS / Vol. 32, No. 14 / July 15, 2007

versus VGATE for a 100 �m gate length and widthtransistor are plotted in Fig. 5 for VGATE=VDRAIN.The observed drain current is up to 3 orders of mag-nitude larger than gate current, which can be attrib-uted to the current gain of the transistor. The modu-lation of channel conductance can further beincreased by scaling the gate length, hence increas-ing the gain. The initial drop in the gate photocur-rent is due to transition from gate depletion to accu-mulation region, similar to that shown in Fig. 3.

Preliminary high-speed characteristics are ob-tained by transient simulations. Ge is assumed to bea single crystal in the simulations because there areno existing models for the polycrystalline grainboundaries. The grain boundary influence is pre-dicted to reduce the carrier lifetime, thus further in-creasing the device speed, trading off with sensitivity.The intrinsic speed increases with shrinking gatelength. The calculated full width at half-maximumfrom impulse response simulations are �1 ns and�100 ps for LG=1 �m and LG=100 nm, respectively.The high-speed performance of the OE-MOSFET isbenchmarked against that of a traditional photode-tector. The response of a classical photodetector to atrain of optical pulses is plotted in Fig. 6. The simu-lated metal–semiconductor–metal has 100 nm elec-trode spacing on 100-nm-thick Ge. In comparison,the transient response obtained from theOE-MOSFET with an identical 100�100 nm slab ofGe in the gate is also plotted in Fig. 6. This deviceprovides significantly higher photocurrent ��3.5� �for identical optical pulse energy in addition to loweroff-state leakage ��4� �. With proper scaling andband engineering, the device is expected to operateeasily in excess of 10 GHz. The output capacitance ofthe device, Cswitch, is dominated by the gate-to-drainoverlap, and it is �0.02 fF for an OE-MOSFET with100 nm gate length and width. Therefore, the RClimited bandwidth is very large.

Fig. 5. (Color online) Measured photocurrent at the gateand drain terminals for VGATE=VDRAIN. The absorbed lightin the Ge gate modulate the conductivity of the Si channel.The gate current is amplified by the transistor at the drain

terminal, IDRAIN.

The proposed device essentially is an optical detec-tor with a built-in gain mechanism. In comparison,avalanche photodiodes, for instance, provide gain byimpact ionization. However, avalanche photodiodesrequire high bias voltages �20 V/�m� to achieve de-sired ionization rates, an increasingly difficult chal-lenge to meet at the operating temperatures of to-day’s processors. In contrast, the OE switch allowslow-voltage operation suitable for on-chip applica-tions and potentially better noise performance due tothe separation of absorbing and conduction regions.

We have introduced an optoelectronic switch thatcan be fabricated at the nanoscale along with deeplyscaled conventional MOSFETs using advanced Sitechnology. The device exhibits gain owing to a sec-ondary photoconductive effect based on the currentamplification of the field-effect transistor. Such a de-vice has extremely small capacitance and hence canpotentially be very fast. The intrinsic speed of the de-vice can be increased by tailoring the carrier life-times and built-in band bending using a graded SiGegate region. Such a switch may be used as an opticallatch to distribute an optical clock on chip, eliminat-ing the power dissipation and area due to an H-treeclock distribution network. Plasmonic coupling couldbe used to focus light on such nanoscale dimensionsby nanometallic structures to further enhance deviceperformance [5].

References

1. H. Cho, P. Kapur, and K. C. Saraswat, J. LightwaveTechnol. 22, 2021 (2004).

2. H. Cho, K. Koo, P. Kapur, and K. C. Saraswat, inProceedings of IEEE International InterconnectTechnology Conference (IEEE, 2007), pp. 135–137.

3. D. A. B. Miller, Proc. IEEE 88, 728 (2000).4. Y.-H. Kuo, Y. Lee, Y. Ge, S. Ren, J. E. Roth, T. I.

Kamins, D. A. B. Miller, and J. S. Harris, Nature 437,1334 (2005).

5. L. Tang, A. K. Okyay, J. A. Matteo, Y. Yuen, K. C.Saraswat, L. Hesselink, and D. A. B. Miller, Opt. Lett.

Fig. 6. (Color online) Simulated transient response com-paring a classical detector and the proposed device. The in-put is a pulse train, each pulse delivering 1 fJ optical en-ergy. The OE-MOSFET has significantly enhancedresponsivity and lower off-state leakage.

31, 1519 (2006).