Unstrained, modulation-doped, In0.3Ga0.7As/In0.29Al0.71As field-effect transistor grown on GaAssubstrate

The fabrication, structure, and properties of unstrained modulation-doped, 1-μm-long and 10-μm-wide gate, field effect transistors made of In_0.3Ga_0.7As/In_0.29As_0.71As heterojunctions grown on GaAs substrates using compositionally step-graded buffer layers are described. These devices have a transconductance of 335 mS/mm, f_max of 56 GHz, and a gate breakdown voltage of 23.5 V     

Logic Suitability of 50-nm In0.7 Ga0.3As HEMTs for Beyond-CMOS Applications

We have experimentally studied the suitability of nanometer-scale In_0.7Ga_0.3As high-electron mobility transistors (HEMTs) as an n-channel device for a future high-speed and low-power logic technology for beyond-CMOS applications. To this end, we have fabricated 50- to 150-nm gate-length In_0.7Ga_0.3As HEMTs with different gate stack designs. This has allowed us to investigate the role of Schottky barrier height (Phi_B) and insulator thickness (t_ins) on the logic characteristics of In_0.7Ga_0.3As HEMTs. The best 50-nm HEMTs with the highest Phi_B and the smallest t_ins exhibit an I_ON/I_OFF ratio in excess of 10⁴ and a subthreshold slope (S) below 86 mV/dec. These nonoptimized 50-nm In_0.7Ga_0.3As HEMTs also show a logic gate delay (CV/I) of around 1 ps at a supply voltage of 0.5 V, while maintaining an I_ON/I_OFF ratio above 10⁴, which is comparable to state-of-the-art Si MOSFETs. As one of the alternatives for beyond-CMOS technologies, we believe that InAs-rich InGaAs HEMTs hold a considerable promise.     

Strained-insulatorInxAl1-xAs/n+-In0.53Ga0.47As heterostructure field-effect transistors

In an effort to enhance the conduction band discontinuity between channel and insulator, In_xAl_1-xAs/n⁺-In _0.53Ga_0.47As heterostructure field-effect transistors (HFETs) were fabricated with InAs mole fractions in the In _xAl_1-xAs gate insulator of x=0.52 (lattice matching), 0.48, 0.40, and 0.30. Decreasing the InAs mole fraction in the insulator results in reduced forward- and reverse-bias gate currents, increased reverse gate breakdown voltage, and reduced real-space transfer of hot electrons from channel to gate. Down to x =0.40, these improvements trade off with a slightly reduced transconductance, but the gain in gate bias swing results in an increase in maximum current drivability. From x=0.40 to x=0.30, there is a drastic decrease in transconductance, coincident with a high density of misfit dislocations     

Metamorphic In0.4Al0.6As/In0.4Ga0.6As HEMTs on GaAs substrate

New In_0.4Al_0.6As/In_0.4Ga_0.6 As metamorphic (MM) high electron mobility transistors (HEMTs) have been successfully fabricated on GaAs substrate with T-shaped gate lengths varying from 0.1 to 0.25 μm. The Schottky characteristics are a forward turn-on voltage of 0.7 V and a gate breakdown voltage of -10.5 V. These new MM-HEMTs exhibit typical drain currents of 600 mA/mm and extrinsic transconductance superior to 720 mS/mm. An extrinsic current cutoff frequency f_T of 195 GHz is achieved with the 0.1-μm gate length device. These results are the first reported for In_0.4 Al_0.6As/In_0.4Ga_0.6As MM-HEMTs on GaAs substrate     

High-performance self-aligned (Al,Ga)As/(In,Ga)As pseudomorphicHIGFETs

Transconductance as high as 676 mS/mm at 300 K was observed to 0.7×10-μm² n-channel devices (HIGFETs) made on epilayers with Al_0.3Ga_0.7As insulator thickness of 200 Å and In_0.15Ga_0.85As channel thickness of 150 Å. An FET K value (K=W_g Uε/2aL_g) as large as 10.6 mA/V ² was also measured from another device with transconductance of 411 mS/mm. The high K values are achieved under normal FET operation without hole-injection or drain-avalanche breakdown effects. These results demonstrate the promise of pseudomorphic (Al,Ga)As/(In,Ga)As HIGFETs for high-performance circuit applications     

Ga0.51In0.49P/In0.15Ga0.85As/GaAs pseudomorphic doped-channel FET with high-current densityand high-breakdown voltage

Ga_0.51In_0.49P/In_0.15Ga_0.85 As/GaAs pseudomorphic doped-channel FETs exhibiting excellent DC and microwave characteristics were successfully fabricated. A high peak transconductance of 350 mS/mm, a high gate-drain breakdown voltage of 31 V and a high maximum current density (575 mA/mm) were achieved. These results demonstrate that high transconductance and high breakdown voltage could be attained by using In_0.15Ga_0.85As and Ga_0.51In_0.49P as the channel and insulator materials, respectively. We also measured a high-current gain cut-off frequency f_t of 23.3 GHz and a high maximum oscillation frequency f_max of 50.8 GHz for a 1-μm gate length device at 300 K. RF values where higher than those of other works of InGaAs channel pseudomorphic doped-channel FETs (DCFETs), high electron mobility transistors (HEMTs), and heterostructure FETs (HFETs) with the same gate length and were mainly attributed to higher transconductance due to higher mobility, while the DC values were comparable with the other works. The above results suggested that Ga_0.51In_0.49P/In_0.15Ga_0.85 As/GaAs doped channel FET's were were very suitable for microwave high power device application     

Back-gated field effect in a double heterostructuremodulation-doped field-effect transistor

An Al_0.3Ga_0.7As/GaAs/Al_0.3Ga_0.7 As double heterojunction field-effect transistor has been fabricated, the novel feature being a pn junction back gate. A device with 2 μm channel length has yielded a change in transconductance by a factor of 2 for a change in back gate voltage of 1 V. The performance of this device shows that this approach could be used in realising novel devices, such as velocity modulation transistors. Also, the change in threshold voltage with back gate bias could be useful in implementing digital circuits     

Single- and double-heterojunction pseudomorphic In0.5(Al0.3Ga0.7)0.5P/In0.2Ga0.8As high electron mobility transistors grown by gas sourcemolecular beam epitaxy

In_0.5(Al_0.3Ga_0.7)_0.5 P/In_0.2Ga_0.8As single- and double-heterojunction pseudomorphic high electron mobility transistors (SH-PHEMTs and DH-PHEMTs) on GaAs grown by gas-source molecular beam epitaxy (GSMBE) were demonstrated for the first time. SH-PHEMTs with a 1-μm gate-length showed a peak extrinsic transconductance g_m of 293 mS/mm and a full channel current density I_max of 350 mA/mm. The corresponding values of g_m and I_max were 320 mS/mm and 550 mA/mm, respectively, for the DH-PHEMTs. A short-circuit current gain (H₂₁) cutoff frequency f_T of 21 GHz and a maximum oscillation frequency f_max of 64 GHz were obtained from a 1 μm DH device. The improved device performance is attributed to the large ΔE_c provided by the In_0.5(Al_0.3Ga_0.7)_0.5P/In _0.2Ga_0.8As heterojunctions. These results demonstrated that In_0.5(Al_0.3Ga_0.7)_0.5P/In _0.2Ga_0.8As PHEMT's are promising candidates for microwave power applications     

DC and RF performance of LP-MOCVD grownAl0.25Ga0.75As/InxGa1-xAs(x=0.15-0.28) P-HEMT's with Si-delta doped GaAs layer

Si-delta-doped Al_0.25Ga_0.75As/In_xGa_1-xAs (x=0.15-0.28) P-HEMT's, prepared by LP-MOCVD, are investigated. The large conduction band discontinuity leads to 2-DEG density as high as 2.1×10¹²/cm² with an electron mobility of 7300 cm²/V·s at 300 K. The P-HEMT's with 0.7×60 μm gate have a maximum extrinsic transconductance of 380 mS/mm, and a maximum current density of 300 mA/mm. The S-parameter measurements indicate that the current gain and power gain cutoff frequencies are 30 and 61 GHz, respectively, The RF noise characteristics exhibit a minimum noise figure of 1.2 dB with an associated gain of 10 dB at 10 GHz. Due to the efficient doping technique, the electron mobility and transconductance obtained are among the best reported for MOCVD grown P-HEMT's with the similar structure     

High-performance In0.08Ga0.92As MESFETs onGaAs(100) substrates

In_0.08Ga_0.92As MESFETs were grown in GaAs (100) substrates by molecular beam epitaxy (MBE). The structure comprised an undoped compositionally graded In_xGa_1-x As buffer layer, an In_0.08Ga_0.92As active layer, and an n⁺-In_0.08Ga_0.92As cap layer. FETs with 50-μm width and 0.4-μm gate length were fabricated using the standard processing technique. The best device showed a maximum current density of 700 mA/mm and a transconductance of 400 mS/mm. The transconductance is extremely high for the doping level used and is comparable to that of a 0.25-μm gate GaAs MESFET with an active layer doped to 10¹⁸ cm^-3. The current-gain cutoff frequency was 36 GHz and the power-gain cutoff frequency was 65 GHz. The current gain cutoff frequency is comparable to that of a 0.25-μm gate GaAs MESFET     

On the improvement of gate voltage swings in δ-doped GaAs/InxGa1-xAs/GaAs pseudomorphic heterostructures

Significant improvements in gate voltage swings in heterostructures prepared by low-pressure metalorganic chemical vapor deposition are discussed. Structures utilizing a compositionally graded In_xGa_1-xAs channel exhibited a very flat transconductance region of 2 V. The gate voltage swings of single and double δ-doped GaAs/In_0.25Ga_0.75As/GaAs structures were 2.5 and 2.8 V, respectively. All structures also exhibited high extrinsic transconductance as well as high saturation current densities     

An In0.52Al0.48As/n+-In0.53Ga0.47As MISFET with a modulation-doped channel

A heterostructure metal-insulator-semiconductor field-effect transistor (MISFET) with a modulation-doped channel is proposed. In this device, a very thin undoped subchannel is located between the undoped wide-bandgap insulator and a thin heavily doped channel. In the depletion mode of operation, electron transport takes place along the heavily doped channel. When the device enters the accumulation mode of operation, electrons pile up against the heterointerface in the high-mobility undoped subchannel. This results in markedly improved transport characteristics at the onset of accumulation. The concept is demonstrated in the In_0.52Al_0.48As/In_0.53 Ga_0.47As system on InP. A 1.5-μm-gate-length MISFET shows a unity current-gain cutoff frequency of 37 GHz     

0.12 μm transferred-substrateIn0.52Al0.48As/In0.53Ga0.47As HEMTs on silicon wafer

New In_0.52Al_0.48As/In_0.53Ga_0.47 As transferred-substrate high electron mobility transistors (TS-HEMTs) have been successfully fabricated on 2-in Silicon substrate with 0.12 μm T-shaped gate length. These new TS-HEMTs exhibit typical drain currents of 450 mA/mm and extrinsic transconductance up to 770 mS/mm. An extrinsic current gain cutoff frequency f_T of 185 GHz is obtained. That result is the first reported for In_0.52Al_0.48As/In_0.53Ga_0.47 As TS-HEMTs on Silicon substrate     

Characteristics of AlGaAs/GaAs quantum-well delta-doped channel FET(QUADFET)

The transport properties of a two-dimensional electron gas in Al _0.3Ga_0.7As/GaAs quantum-well delta-doped heterostructures are studied. Electron energy subbands in the quantum well were calculated by a self-consistent method. The FETs having a gate length of 1.3 μm showed a transconductance as high as 340 mS/mm. The FETs also showed a broad plateau of transconductance around its peak, which is not typical in MODFETs     

Short pulse transfer characteristics of AlxGa1-xAs/GaAs andAlxGa1-xAs/InyGa1-yAsmodulation-doped heterojunction FET's

Short-pulse drain current versus gate voltage transfer characteristics measured for modulation-doped HFETs (MODFETs) with four donor-layer-channel-layer combinations-(1) Al_0.3Ga_0.7 As-GaAs, (2) Al_0.2Ga_0.8As-GaAs, (3) Al_0.3Ga_0.7As-In_0.2Ga_0.8As, and (4) Al_0.2Ga_0.8As-In_0.2 a_0.8 As-are compared with the DC transfer characteristics. The measurements are relevant to high-speed switching in HFET circuits. Significant shifts in threshold voltage are observed between the DC and short-pulse characteristics for the structures with n⁺-Al_0.3Ga_0.7As donor layers, while the corresponding shifts for structures with n⁺-Al_0.2Ga_0.8As donor layers are relatively small or virtually nonexistent     

Electrical characteristics of an optically controlled N-channelAlGaAs/GaAs/InGaAs pseudomorphic HEMT

Electrical characteristics of an n-channel Al_0.3Ga_0.7As/GaAs/In_0.13Ga_0.87 As pseudomorphic HEMT (PHEMT) with L_g=1 μm on GaAs are characterized under optical input (P_opt). Gate leakage and drain current have been analyzed as a function of V_GS, V _DS, and P_opt. We observed monotonically increasing gate leakage current due to the energy barrier lowering by the optically induced photovoltage, which means that gate input characteristics are significantly limited by the photovoltaic effect. However, we obtained a strong nonlinear photoresponsivity of the drain current, which is limited by the photoconductive effect. We also proposed a device model with an optically induced parasitic Al_0.3Ga_0.7As MESFET parallel to the In_0.13Ga_0.87As channel PHEMT for the physical mechanism in the drain current saturation under high optical input power     

0.2-μm gate-length atomic-planar doped pseudomorphic Al0.3Ga0.7As/In0.25Ga0.75As MODFETswith fT over 120 GHz

The authors report the DC and RF performance of nominally 0.2-μm-gate length atomic-planar doped pseudomorphic Al_0.3Ga_0.7As/In_0.25Ga_0.75As modulation-doped field-effect transistors (MODFETs) with f_T over 120 GHz. The devices exhibit a maximum two-dimensional electron gas (2 DEG) sheet density of 2.4×10¹² cm^-2, peak transconductance g _m of 530-570 mS/mm. maximum current density of 500-550 mA/mm, and peak current-gain cutoff frequency f_T of 110-122 GHz. These results are claimed to be among the best ever reported for pseudomorphic AlGaAs/InGaAs MODFETs and are attributed to the high 2 DEG sheet density, rather than an enhanced saturation velocity, in the In_0.25Ga_0.75As channel     

Low-temperature polysilicon TFT with two-layer gate insulator usingphoto-CVD and APCVD SiO2

The performance of polysilicon thin-film transistors (TFTs) formed by a 600°C process was improved using a two-layer gate insulator of photochemical-assisted vapor deposition (photo-CVD) SiO₂ and atmospheric-pressure chemical vapor deposition (APCVD) SiO₂. The photo-CVD SiO₂, 100 Å thick, was deposited on polysilicon and followed by APCVD SiO₂ of 1000 Å thickness. The TFT had a threshold voltage of 8.3 V and a field-effect mobility of 35 cm²/V-s, which were higher than those of the conventional TFT with a single-layer gate SiO₂ of APCVD. Hydrogenation by hydrogen plasma was more effective for the new TFT than for the conventional device     

A GaAs/AlGaAs asymmetric Fabry-Perot reflection modulator with veryhigh contrast ratio

Performance results for a normally on, electroabsorptive, surface-normal Fabry-Perot reflection modulator are presented. The device employs a cavity with a 100 Å GaAs/100 Å Al_0.3 Ga_0.7As multiple quantum well and top and bottom quarter-wave mirrors with 4 and 19.5 periods, respectively. Very low values of off-state reflectance were measured, giving a maximum contrast ratio >1000 (30 dB) and a maximum reflectance difference of 64.3%. The contrast ratio is, to the authors' knowledge, the largest reported to date     

Temperature dependence of gate forward turn-on voltage (Vf) of i-Al0.3Ga0.7As/n-GaAs HIGFET's

Forward gate current-voltage characteristics and their temperature dependence are investigated for i-Al_0.3Ga_0.7As/n-GaAs doped-channel HIGFET's (DC-HIGFET's) with the gate length of 0.3 μm. The temperature coefficient of the gate forward turn-on voltage (Vf) varies with the thickness (t_U) of an i-AlGaAs layer, and shows a minimum value of -0.8~-0.9 mV/deg at t_u=10 nm