Design and fabrication of double modulation dopedInAlAs/lnGaAs/InAs heterojunction FETs for high-speed andmillimeter-wave applications

We report the design, fabrication, and characterization of InP-based double-sided-doped (DSD) MODFETs with InAs-layer-inserted channels. Devices based on optimized structures show a significant improvement in the effective saturation velocity, from 2.4×10⁷ cm/s for lattice-matched MODFETs to 3.1×10⁷ cm/s for InAs MODFETs. This leads to a maximum extrinsic transconductance of 1.95 S/mm and excellent high-speed performance of f_T=265 GHz for 0.13-μm T-gates. A f_max higher than 300 GHz can be achieved by fabricating a wide lateral recess groove, which simultaneously results in an improved breakdown voltage of 6.7 V. The excellent RF performance is primarily due to the reduction of Coulomb scattering from donor layers, especially under the channel, and to the reduction of scattering caused by the interface roughness. This improvement is achieved by inserting a 4-nm InAs layer, which better confines the two-dimensional electron gas (2DEG) at the center of the channel of MODFET's     

Uniform, high-gain AlGaAs/In0.05Ga0.95As/GaAsP-n-p heterojunction bipolar transistors by dual selective etch process

AlGaAs/InGaAs/GaAs P-n-p heterojunction bipolar transistors (HBTs) have been fabricated using a dual selective etch process. In this process, a thin AlGaAs surface passivation layer surrounding the emitter is defined by selective etching of the GaAs cap layer. The InGaAs base is then exposed by selective etching of the AlGaAs emitter. The resulting devices were very uniform, with current gain varying by less than ±10% for a given device size. Current gain at a given emitter current density was independent of device size, with gains of over 200 obtained at current densities above 5×10⁴ A/cm ²     

Fabrication and characterization of high-performance InP/InGaAsdouble-heterojunction bipolar transistors

This paper is on high-performance InP/InGaAs double-heterojunction bipolar transistors (DHBT's) utilizing compositionally step-graded InGaAsP layers between the InGaAs base and InP collector to suppress the current blocking effect. These DHBT's exhibit current gains of 200 and excellent breakdown behavior. Moreover, the DHBT's permit collector current density levels J_C up to 3×10⁵ A/cm ² at V_CE=1.5 V. A current gain cutoff frequency of 155 GHz and a maximum oscillation frequency of 90 GHz have been successfully obtained at J_C=1.6×10⁵ A/cm². We have also investigated electron transport properties in the InP collector using a set of DHBT's with different injection energies into the InP collector. By increasing the injection energies, electron velocity is found to decrease from 3.5×10⁷ cm/s to 1.6×10⁷ cm/s, due to increased population of upper valleys. This result clearly demonstrates the significant role of nonequilibrium Γ-valley transport in determining the high-speed performance of InP/InGaAs DHBT's     

InGaAs metal-semiconductor field-effect transistor withLangmuir-Blodgett deposited gate structure

A high-transconductance n-channel, depletion-mode InGaAs metal-semiconductor field-effect transistor (MESFET) with a Langmuir-Blodgett deposited gate fabricated on organometallic chemical vapor deposition (OMCVD)-grown InGaAs lattice matched to InP is reported. The fabrication process is similar to epitaxial GaAs FET technology and is suitable for making optoelectronic integrated circuits (OEICs) for long-wavelength fiber-optic communications systems. Devices with 1-μm gate and 6×10¹⁶ channel doping achieved 162-mS/mm extrinsic transconductance and -1.8-V pinch-off voltage. The effective saturation velocity of electrons in the channel was measured to be between 3.5 and 3.9×10⁷ cm/s. The drain current ( I_dss), 300 mA/mm at V_ds=2.5 V, is the highest current capability reported for depletion-mode InGaAs MESFET devices with low pinch-off voltages     

Characterization of ultra-high-speed pseudomorphic AlGaAs/InGaAs(on GaAs) MODFETs

The authors report a detailed characterization of ultrahigh-speed pseudomorphic AlGaAs/InGaAs (on GaAs) modulation-doped field-effect transistors (MODFETs) with emphasis on the device switching characteristics. The nominal 0.1-μm gate-length device exhibit a current gain cutoff frequency (f_t) as high as 152 GHz. This value of f_t corresponds to a total delay of approximately 1.0 ps and is attributed to the optimization of layer structure, device layout, and fabrication process. It is shown that the electron transit time in these very short gate-length devices still accounts for approximately 60% of the total delay, and, as a result, significant improvements in switching speed are possible with further reductions of gate length. The results reported clearly demonstrate the potential of the pseudomorphic AlGaAs/InGaAs MODFET as an ultrahigh-speed device. Its excellent switching characteristics are attributed to the high saturation velocity (~2×10⁷ cm/s), 2DEG sheet density (2.5×10¹² cm^-2), and current drive capability (>200 mA/mm at the peak transconductance)     

New evidence for velocity overshoot in a 200 nm pseudomorphic HEMT

It is believed that significant velocity overshoot effects are responsible for the high performance of pseudomorphic HEMTs (PsHEMTs). The overshoot is associated with the low effective mass in the InGaAs channel and the large Γ-L separation. Average channel electron velocities well in excess of 3.0 × 10⁷ cm/s have been predicted in Monte-Carlo PsHEMT simulations. However, average electron velocities extracted from transconductance measurements of such devices are much lower, typically in the range 1.5–2.0 × 10⁷ cm/s. In this paper we analyse real device measurements by using Monte-Carlo and drift diffusion simulations. We show clear evidence that the average velocity in the channel of a 200 nm PsHEMT fabricated in the Nano-electronics Research Centre of Glasgow University exceeds 3.0 × 10⁷ cm/s.     

A high-performance InGaAs/InAlAs double-heterojunction bipolartransistor with nonalloyed n+-InAs cap layer on InP(n) grownby molecular beam epitaxy

An InGaAs/InAlAs double-heterojunction bipolar transistor (DHBT) on InP(n) grown by molecular-beam epitaxy (MBE) that exhibits high DC performance is discussed. An n⁺-InAs emitter cap layer was used for nonalloyed contacts in the structure and specific contact resistances of 1.8×10^-7 and 6.0×10^-6 Ω-cm² were measured for the nonalloyed emitter and base contacts, respectively. Since no high-temperature annealing is necessary, excellent contact surface morphology on thinner base devices can easily be obtained. In devices with 50×50-μm² emitter area, common-emitter current gains as high as 1500 were achieved at a collector current density of 2.7×10³ A/cm² . The current gain increased up to 2000 for alloyed devices     

Hot-electron InGaAs/InP heterostructure bipolar transistors withfT of 110 GHz

A hot-electron InGaAs/InP heterostructure bipolar transistor (HBT) is discussed. A unity-current-gain cutoff frequency of 110 GHz and a maximum frequency of oscillation of 58 GHz are realized in transistors with 3.2×3.2-μm² emitter size. Nonequilibrium electron transport, with an average electron velocity approaching 4×10⁷ cm/s through the thin (650 Å) heavily doped (p=5×10¹⁹ cm^-3) InGaAs base and 3000-Å-wide collector space-charge region, results in a transit delay of 0.5 ps corresponding to an intrinsic cutoff frequency of 318 GHz     

Experimental observation of velocity overshoot in n-channelAlGaAs/InGaAs/GaAs enhancement mode MODFETs

Velocity overshoot phenomena in n-channel Al-GaAs/InGaAs/GaAs enhancement mode MODFETs have been investigated for gate lengths ranging from 1 to 0.5 μm. The study is based on Motorola's established CGaAs ^TM technology. The observed average electron velocity υ _aυ under the gate is 1.05, 1.34, 1.48, and 1.71×10 ⁷ cm/s for a gate length L_G of 1, 0.7, 0.6, and 0.5 μm, respectively. The presence of velocity overshoot in InGaAs channels is clearly proven with average electron velocities exceeding the steady-state saturation velocity of ≅1×10⁷ cm/s for L_G⩽0.7 μm, and with the significant increase of υ_aυ with shorter gate length     

Numerical analysis of nonequilibrium electron transport inAlGaAs/InGaAs/GaAs pseudomorphic MODFETs

Nonequilibrium electron transport in InGaAs pseudomorphic MODFETs has been analyzed with the moment equations approach. In the model, the momentum and energy balance equations for the two-dimensional electrons in the InGaAs channel are solved with relaxation times generated from a Monte Carlo simulation. The two-dimensional electron wave functions and the quantized state energies in the InGaAs quantum well are calculated exactly from the Schrodinger equation along the direction perpendicular to the quantum well. Also included is a two-dimensional Poisson equation solver. In the calculation, all of the equations are solved iteratively until a self-consistent solution is achieved. The simulation results for a realistic device structure with a 0.5-μm recessed gate show a significant overshoot velocity of 4.5×10⁷ cm/s at a drain bias of 1.0 V. Electron temperature reaches a peak value of around 2500 K under the gate. In energy transport, the diffusive component of the energy flux is found to be dominant in the high-field region     

In0.52Al0.48As/In0.53Ga0.47As double-heterojunction p-n-p bipolar transistors grown bymolecular beam epitaxy

P-n-p In_0.52Al_0.48As/In_0.53Ga_0.47 As double-heterojunction bipolar transistors with a p⁺-InAs emitter cap layer grown by molecular-beam epitaxy have been realized and tested. A five-period 15-Å-thick In_0.53Ga_0.47As/InAs superlattice was incorporated between the In_0.53Ga_0.47As and InAs cap layer to smooth out the valence-band discontinuity. Specific contact resistance of 1×10^-5 and 2×10^-6 Ω-cm² were measured for nonalloyed emitter and base contacts, respectively. A maximum common emitter current gain of 70 has been measured for a 1500-Å-thick base transistor at a collector current density of 1.2×10³ A/cm². Typical current gains of devices with 50×50-μm² emitter areas were around 50 with ideality factors of 1.4     

High-performance, 0.1 μm InAlAs/InGaAs high electron mobilitytransistors on GaAs

This letter describes the material characterization and device test of InAlAs/InGaAs high electron mobility transistors (HEMTs) grown on GaAs substrates with indium compositions and performance comparable to InP-based devices. This technology demonstrates the potential for lowered production cost of very high performance devices. The transistors were fabricated from material with room temperature channel electron mobilities and carrier concentrations of μ=10000 cm² /Vs, n=3.2×10¹² cm^-2 (In=53%) and μ=11800 cm²/Vs, n=2.8×10¹² cm^-2 (In=60%). A series of In=53%, 0.1×100 μm² and 0.1×50 μm² devices demonstrated extrinsic transconductance values greater than 1 S/mm with the best device reaching 1.074 S/mm. High-frequency testing of 0.1×50 μm² discrete HEMT's up to 40 GHz and fitting of a small signal equivalent circuit yielded an intrinsic transconductance (g_m,i) of 1.67 S/mm, with unity current gain frequency (f_T) of 150 GHz and a maximum frequency of oscillation (f_max) of 330 GHz. Transistors with In=60% exhibited an extrinsic g_m of 1.7 S/mm, which is the highest reported value for a GaAs based device     

A p-channel BICFET in the InGaAs/InAlAs material system

The bipolar inversion-channel field-effect transistor (BICFET) relies on a field-effect mechanism to induce and modulate an inversion layer placed at the heterojunction interface. The device was fabricated using molecular beam epitaxy. A current gain of ~15 has been achieved at a current density J of ~8×10³ A/cm² at 300 K. At 77 K, the gain rises to 40 with J=2×10⁴ A/cm². This gain, which follows theoretical predictions, continuously increases with collector current and is limited only by the amount of heat generated in a large-area device. Because of its potential for ballistic transport in the collector, the InGaAs/InAlAs BICFET should be very attractive for high-speed applications     

Degradation of InGaAs/InP double heterojunction bipolar transistorsunder electron irradiation

The dc characteristics of InGaAs/InP double heterojunction bipolar transistors (DHBTs) are studied under high-energy (~1 MeV) electron irradiation up to a fluence of 14.8×10¹⁵ electrons/cm ². The devices show an increase in common-emitter current gain (h_fe) at low levels of dose (<10¹⁵ electrons/cm²) and a gradual decrease in h_fe and an increase in output conductance for higher doses. The decrease in h _fe is as much as ~80% at low base currents (~10 μA) after a cumulative dose of 14.8×10¹⁵ electrons/cm². The observed degradation effects in collector current-voltage (I-V) characteristics are studied quantitatively using a simple SPICE-like device model. The overall decrease in h_fe is attributed to increased recombination in the emitter-base junction region caused by radiation-induced defects. The defects introduced in the collector-base junction region are believed to be responsible for the observed increase in the output conductance     

Low emitter resistance GaAs based HBT's without InGaAs caps

Low emitter resistance is demonstrated for AlGaAs/GaAs heterojunction bipolar transistors using Pd/Ge contacts on a GaAs contact layer. The contact resistivity to 2-10×10¹⁸ cm ^-3 n-type GaAs is 4-1×10^-7 Ω-cm² . These are comparable to contact resistivities obtained with non-alloyed contacts on InGaAs layers. The non-spiking Pd/Ge contact demonstrates thermal stability and area independent resistivity suitable for scaled devices. The substitution of Pd/Ge for AuGe/Ni GaAs emitter and collector contacts reduced by an order of magnitude the emitter-base offset voltage at high current densities and increased f_t by more than 15% with significantly improved uniformity for devices with 2 and 2.6 μm wide emitters having lengths two, four and six times the width     

High-performance InGaAs junction field-effect transistor with P/Beco-implanted gate

InGaAs junction field-effect transistors (JFETs) are fabricated in metalorganic chemical-vapor-deposition (MOCVD)-grown n-InGaAs and semi-insulating Fe:InP layers on n⁺-InP substrate with a P/Be co-implanted p⁺ self-aligned gate. The device exhibits a transconductance of 245 mS/mm (intrinsic transconductance of 275 mS/mm) at zero gate bias and good pinch-off behavior for a gate length of 0.5 μm. The effective electron velocity is deduced to be 2.8×10⁷ cm/s, equal to the theoretical prediction     

4H-SiC MOSFETs utilizing the H2 surface cleaningtechnique

The H₂ cleaning technique was examined as the precleaning of the gate oxidation for 4H-SiC MOSFETs. The device had a channel width and length of 150 and 100 μm, fabricated on the p-type epitaxial layer of 3×10¹⁶ cm^-3. The gate oxidation was performed after the conventional RCA cleaning, and H₂ annealing at 1000°C. The obtained channel mobility depends on the pre-cleaning process strongly, and was achieved 20 cm²/N s in the H₂ annealed sample. The effective interface-state density was also measured by the MOS capacitors fabricated on the same chips, resulting 1.8×10¹² cm^-2 from the photo-induced C-V method     

In0.52Al0.48As/InAs/InxAl1-xAs pseudomorphic HEMT's on InP

The DC and microwave performance of an InAs channel HEMT is reported. Room-temperature electron mobility as high as 20200 cm² /Vs is measured, with a high carrier concentration of 2.7×10 ¹² cm^-2. DC extrinsic transconductance of 714 mS/mm is measured and a unity-current-gain cut-off frequency of 50 GHz is obtained for a 1.1-μm gate length HEMT. The success of achieving superior Hall mobility and device performance is strongly dependent on the In_xAl_1-xAs buffer layer design that changes the lattice constant from lattice-matched In_0.52Al_0.48 As to In_0.75Al_0.25As. The multiple In_0.52Al_0.48As/InAs monolayer superlattices buffer achieves the best performance as compared to the step-graded In_x Al_1-xAs and the uniform In_0.76Al_0.25 As buffer     

Impact ionization and transport in the InAlAs/n+-InPHFET

We have carried out an experimental study exploring both impact ionization and electron transport in InAlAs/n⁺-InP HFET's. Our devices show no signature of impact ionization in the gate current, which remains below 17 μA/mm under typical bias conditions for L_g=0.8 μm devices (60 times lower than for InAlAs/InGaAs HEMT's). The lack of impact ionization results in a drain-source breakdown voltage (BV_DS) that increases as the device is turned on, displaying an off-state value of 10 V. Additionally, we find that the channel electron velocity approaches the InP saturation velocity of about 10⁷ cm/s (in devices with L_g<1.6 μm) rather than reaching the material's peak velocity. We attribute this to the impact of channel doping both on the steady-state peak velocity and on the conditions necessary for velocity overshoot to take place. Our findings suggest that the InP-channel HFET benefits from channel electrons which remain cold even at large V_GS and V_DS making the device well-suited to power applications demanding small I_G, low g_d, and high BV_DS     

Electron velocity overshoot at room and liquid nitrogentemperatures in silicon inversion layers

Effective electron velocities in silicon MOSFETs exceeding the bulk saturation values of 10⁷ cm/s at room temperature and 1.3×10⁷ cm/s at liquid-nitrogen temperature are inferred. This conclusion suggests that electron velocity overshoot occurs over a large portion of the device channel length. To infer this phenomenon, submicrometer-channel-length Si MOSFETs with lightly doped inversion layers were fabricated. These devices have low field mobility of 450 cm²/V-s and showed only slight short-channel effects. Effective carrier velocities are calculated from the saturated transconductance g_m at V_DS=1.5 V after correction for parasitic resistances of source and drain