Unity Current Gain Cutoff Frequency Research Articles

A New InP/InGaAs Double Heterojunction Bipolar Transistor With a Step-Graded InAlGaAs Collector Structure

An interesting InP/InGaAs double heterojunction bipolar transistor with a step-graded InAlGaAs layer at the base-collector (B-C) heterojunction is fabricated and studied. Simulated results reveal that the potential spike at the B-C heterointerface is completely eliminated. Experimentally, the operation regime is wider than 11 decades in magnitude of the collector current (Ic = 10-12 A to Ic = 10-1 A). Furthermore, the studied device exhibits a relatively high common-emitter breakdown voltage and low output conductance even at high temperature. In the microwave characteristics, the unity current gain cutoff frequency fT = 72.7 GHz and the maximum oscillation frequency f max = 50 GHz are achieved for a nonoptimized device (AE = 6 times 6 mum2).

Improved temperature model of AlGaN/GaN HEMT and device characteristics at variant temperature

In this paper, an improved temperature model for AlGaN/GaN high electron mobility transistor (HEMT) is presented. Research is being conducted for a high-performance building block for high frequency applications that combine lower costs with improved performance and manufacturability. The effects of channel conductance in the saturation region and the parasitic resistance due to the undoped GaN buffer layer have been included. The effect of both spontaneous and piezoelectric polarization induced charges at the AlGaN/GaN heterointerface has been incorporated. The proposed model is used to determine the transfer characteristics, output current-voltage characteristics and small-signal microwave parameters of HEMTs. The investigated temperature range is from 100–600 K. The small signal microwave parameters have been evaluated to determine the unity current gain cut-off frequency (f T ). High f T (10–70 GHz) values and high current levels (∼550 mA/mm) are achieved for a 1 μm AlGaN/GaN HEMTs. A custom DC measurement system is used to facilitate the DC characterization of the unpackaged GaN HEMT test device. The calculated critical parameters and the simulation results suggest that the performance of the proposed device degrades at elevated temperatures.

Gate length scaling study of InAlAs/InGaAs/InAsP composite channel HEMTs

The dependence of direct current and microwave performance of InGaAs/InAsP composite channel HEMTs on gate length is presented experimentally. Composite channel HEMTs with gate length from 1.13 μm to 0.15 μm were fabricated. Device characterization results showed the extrinsic transconductance increased from 498 mS/mm for 1.13 μm devices to 889 mS/mm for 0.15 μm gate devices, while the unity current gain cutoff frequency increased from 24 GHz to 190 GHz. A simple delay time analysis is employed to extract the effective carrier velocity ( v eff) of the composite channel. The v eff is determined to be 1.9 × 10 7 cm/s. To our knowledge, this is the first systematic study on gate length scaling effect of composite channel HEMTs.

High-speed InGaAsSb/InP double heterojunction bipolar transistor with composition graded base and InAs emitter contact layers

InAs–InGaAs graded emitter contact layer was incorporated into the InGaAsSb–GaAsSb/InP graded base double heterojunction bipolar transistors (DHBTs). Compared to a uniform In 0.53Ga 0.47As emitter contact layer, the graded InAs–InGaAs emitter contact layer effectively reduced specific contact resistance from 0.378 to 0.091 Ω-μm 2. Measured sheet resistance also decreased from 58.3 to 47.4 Ω/□. A graded base DHBT with a 0.5×8 μm 2 emitter showed a unity current gain cutoff frequency ( f T) and maximum oscillation frequency ( f MAX) of 500 GHz and 260 GHz, respectively. The f MAX improves to 350 GHz when a smaller (0.4×3 μm 2) emitter is utilized.

0.15-$muhboxm$-Gate InAlAs/InGaAs/InP E-HEMTs Utilizing Ir/Ti/Pt/Au Gate Structure

High-current 0.15- <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$muhboxm$</tex> -gate enhancement-mode high-electron mobility transistors utilizing Ir/Ti/Pt/Au gate metallization were fabricated using a new process including a high-temperature gate anneal that is required for Schottky-barrier height enhancement for the Ir-based gate contact. <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$hboxSiN_x$</tex> encapsulation was employed to prevent thermal degradation of device layer during the high-temperature gate anneal. Excellent enhancement-mode operation, with a threshold voltage of 0.1 V and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$I_ DSS$</tex> of 2.1 mA/mm, was realized. Both the annealed and unannealed devices exhibited high <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$g_m,max$</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$I_D,max$</tex> of 800 mS/mm and 430 mA/mm, respectively. A unity current-gain cutoff frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_T$</tex> of 151 GHz and a maximum oscillation frequency <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$f_ MAX$</tex> of 172 GHz were achieved. From the dc and RF characteristics, it can be deduced that there was no degradation of the gate contact and the heterostructure due to gate annealing. Furthermore, it was found that the gate diffusion during gate annealing was negligible since no increase in <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$g_m,max$</tex> was observed.

Comprehensive Characterization of In0.45Al0.55As ∕ In0.5Ga0.5As ∕ InxAl1 − xAs Metamorphic High-Electron-Mobility Transistors on GaAs Substrates

An metamorphic high-electron-mobility transistor (MHEMT) with an inverse-step metamorphic buffer, grown by molecular beam epitaxy on substrate, is presented. A typical gate-length MHEMT has a drain current density of and a peak extrinsic transconductance of . High-frequency scattering parameter measurements yield a unity current gain cutoff frequency of as well as a maximum frequency oscillation . High two-terminal and three-terminal breakdown voltages are achieved mainly because of the high bandgap of the Schottky layer and the large conduction-band discontinuity at the heterojunction. The large-signal power performance and noise characteristics are measured. Experimental results on the temperature-dependence study of the saturation drain current density, the extrinsic transconductance, the leakage current, the off-state breakdown voltage, and the threshold voltage are presented. The shift of the device at elevated temperatures is as small as . All of the experimental results confirm the exceptional potential of the MHEMT in microwave circuit and high-temperature applications.

Fabrication and Characterization of Capless In0.52Al0.48As/In0.53Ga0.47As HEMTs

Capless high electron mobility transistors (HEMTs) were fabricated and their DC and RF performances were characterized. Capless HEMTs did not have highly doped InGaAs cap layer so that gate recess process was not required in the fabrication of capless HEMTs. The electrical performances of the capless HEMTs were compared with those of conventional HEMTs with highly doped InGaAs cap layer. A typical 0.2 μm capless HEMT exhibited a maximum transconductance of 805 mS/mm, a threshold voltage of -0.5 V, and a unity current gain cut-off frequency (f T ) of 137 GHz. Capless HEMTs exhibited improved device uniformity compared with conventional HEMTs fabricated by wet gate recess technology.

A novel InGaP/Al/sub x/Ga/sub 1-x/As/GaAs CEHBT

A new and interesting InGaP/Al/sub x/Ga/sub 1-x/As/GaAs composite-emitter heterojunction bipolar transistor (CEHBT) is fabricated and studied. Based on the insertion of a compositionally linear graded Al/sub x/Ga/sub 1-x/As layer, a near-continuous conduction band structure between the InGaP emitter and the GaAs base is developed. Simulation results reveal that a potential spike at the emitter/base heterointerface is completely eliminated. Experimental results show that the CEHBT exhibits good dc performances with dc current gain of 280 and greater than unity at collector current densities of J/sub C/=21kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and 2.70×10/sup -5/ A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , respectively. A small collector/emitter offset voltage /spl Delta/V/sub CE/ of 80 meV is also obtained. The studied CEHBT exhibits transistor action under an extremely low collector current density (2.7×10/sup -5/ A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and useful current gains over nine decades of magnitude of collector current density. In microwave characteristics, the unity current gain cutoff frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> =43.2GHz and the maximum oscillation frequency f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</sub> =35.1GHz are achieved for a 3×20 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> device. Consequently, the studied device shows promise for low supply voltage and low-power circuit applications.

High performance InP/InAlAs/GaAsSb/InP double heterojunction bipolar transistors

DC and RF characteristics of InP/InAlAs/GaAsSb/InP double heterojunction bipolar transistors (DHBTs) are reported. The device heterostructures include InAlAs spacer layer between InP emitter and GaAsSb base layer. The impact of thin InAlAs spacer layer was investigated by comparing the DC characteristics of large-area devices fabricated on InP/InAlAs/GaAsSb/InP and conventional InP/GaAsSb/InP heterostructures. By suppressing the base tunneling current and surface recombination current, the DHBTs with thin InAlAs spacer layer exhibited 5 decade lower crossover current of 7 × 10 −11 A (collector current at unity current gain) than conventional InP/GaAsSb/InP DHBT that exhibited crossover current of 4 × 10 −6 A. The current gain also improved twice by the impact of thin InAlAs layer. To investigate the high-frequency characteristics of InP/InAlAs/GaAsSb/InP DHBTs, small-area devices employing laterally etched undercut micro-airbridges were fabricated. The unity current gain cut-off frequency, f T, of 100 GHz was obtained from a 1 × 30 μm 2 emitter DHBT.

Enhancement-Mode High Electron Mobility Transistors Lattice-Matched to InP Substrates Utilizing Ti/Pt/Au Metallization

Enhancement-mode high electron mobility transistors (EHEMTs) were fabricated on In0.52Al0.48As/In0.53Ga0.47As heterostructures lattice-matched to InP substrates. Vertical scaling of device heterostructures was carried out to realize a positive threshold voltage with Ti/Pt/Au gates. Submicron EHEMTs utilizing Ti/Pt/Au were fabricated and their performances were compared with those of conventional EHEMTs with buried-Pt gates. DC I–V characteristics of both devices exhibited excellent pinch-off characteristics and very low output conductance. Output conductance measured for both devices showed that EHEMTs exhibit smaller kink effects than normal depletion-mode HEMTs (DHEMTs) due to the operating mode of EHEMTs with forward gate bias. EHEMTs with 0.18 µm Ti/Pt/Au gates exhibited a threshold voltage of 100 mV, peak transconductance of 810 mS/mm, and unity current-gain cut-off frequency ( fT) of 150 GHz, while those with buried-Pt gates exhibited a threshold voltage of 300 mV, peak transconductance of 880 mS/mm, and unity current-gain cut-off frequency ( fT) of 100 GHz. Studies on the thermal stabilities of the two types of gate metallization have been carried out. The Ti/Pt/Au gates exhibited better thermal stability than Pt/Ti/Pt/Au gates in terms of variation of threshold voltages and maximum transconductance values at elevated temperatures.

Anomalous effects of temperature and UV illumination on the operation of AlGaN/GaN MODFET

The impact of high temperature rapid thermal annealing (RTA) on the mode of operation of AlGaN/GaN modulation doped field effect transistors (MODFETs) is reported. It is observed that annealing at high temperatures is capable of turning the normally depletion-mode (D-mode) characteristics of an AlGaN/GaN MODFET, towards that of an enhancement-mode (E-mode). This change is shown to be partly reversible through UV illumination. These results support the arguments on the extensive role of deep surface states on the operation of AlGaN/GaN MODFETs. According to this variation of characteristics, fabrication and characterization of close to E-mode AlGaN/GaN MODFETs are reported, using MBE grown material on sapphire. The devices demonstrate maximum extrinsic gate transconductance of 180 mS/mm. Unity current gain cutoff frequency ( f T) of 5 GHz and maximum oscillation frequency ( f max) of 14 GHz were measured.

Influences of sulfur passivation on temperature-dependent characteristics of an AlGaAs/InGaAs/GaAs PHEMT

The influences of (NH/sub 4/)/sub 2/S/sub x/ treatment on an AlGaAs/InGaAs/GaAs pseudomorphic high electron mobility transistor (PHEMT) are studied and demonstrated. Upon the sulfur passivation, the studied device exhibits better temperature-dependent dc and microwave characteristics. Experimentally, for a 1/spl times/100 /spl mu/m/sup 2/ gate/dimension PHEMT with sulfur passivation, the higher gate/drain breakdown voltage of 36.4 (21.5) V, higher turn-on voltage of 0.994 (0.69) V, lower gate leakage current of 0.6 (571) /spl mu/A/mm at V/sub GD/=-22 V, improved threshold voltage of -1.62 (-1.71) V, higher maximum transconductance of 240 (211) mS/mm with 348 (242) mA/mm broad operating regime (>0.9g/sub m,max/), and lower output conductance of 0.51 (0.53) mS/mm are obtained, respectively, at 300 (510) K. The corresponding unity current gain cutoff frequency f/sub T/ (maximum oscillation frequency f/sub max/) are 22.2 (87.9) and 19.5 (59.3) GHz at 250 and 400 K, respectively, with considerably broad operating regimes (>0.8f/sub T/,f/sub max/) larger than 455 mA/mm. Moreover, the relatively lower variations of device performances over wide temperature range (300/spl sim/510 K) are observed.

AlGaN/GaN self-aligned MODFET with metal oxide gate for millimeter wave application

As promising candidates for future microwave power devices, GaN-based high-electron mobility transistors (HEMTs) have attracted much research interest. An investigation of the operation of AlGaN/GaN n type self-aligned MOSFET with modulation doped GaN channels is presented. Liquid phase deposited (LPD) SiO2 is used as the insulating material. An analytical model based on modified charge control equations is developed. The investigated critical parameters of the proposed device are the maximum drain current (IDmax), the threshold voltage (Vth), the peak DC trans-conductance (gm), break down voltage (Vbr) and unity current gain cut-off frequency (fT). The typical DC characteristics for a gate length of 1μm with 100μm gate width are following: Imax=800mA/mm, Vbreak-down=50V, gm_extrinsic=200mS/mm, Vpinchoff=−10V. The analysis and simulation results on the transport characteristics of the MOS gate MODFET structure is compared with the previously measured experimental data. The calculated values of fT (20–130GHz) suggest that the operation of the proposed device effectively, has sufficiently high current gain cutoff frequencies over a wide range of drain voltage, which is essential for high-power performance at microwave frequencies. The proposed device offers lower on-state resistance. The results so obtained are in close agreement with the experimental data.

Effect of Temperature Variation on the Characteristics of Microwave Power AlGaN/GaN MODFET

The prime motivation for developing the proposed model of AlGaN/GaN microwave power device is to demonstrate its inherent ability to operate at much higher temperature. An investigation of temperature model of a 1 μm gate AlGaN/GaN enhancement mode n-type modulation-doped field effect transistor (MODFET) is presented. An analytical temperature model based on modified charge control equations is developed. The proposed model handles higher voltages and show stable operation at higher temperatures. The investigated temperature range is from 100 °K–600 °K. The critical parameters of the proposed device are the maximum drain current (IDmax), the threshold voltage (Vth), the peak dc trans-conductance (gm), and unity current gain cut-off frequency (fT). The calculated values of fT (10–70 GHz) at elevated temperature suggest that the operation of the proposed device has sufficiently high current handling capacity. The temperature effect on saturation current, cutoff frequency, and trans-conductance behavior predict the device behavior at elevated temperatures. The analysis and simulation results on the transport characteristics of the MODFET structure is compared with the previously measured experimental data at room temperature. The calculated critical parameters suggest that the proposed device could survive in extreme environments.

Field-plated 0.25 µm gate-length AlGaN/GaN HEMTs on 6H-SiC with power density of 9.1 W/mm at 18 GHz

MOCVD-grown field-plated 0.25 µm gate-length AlGaN/GaN high electron mobility transistors (HEMTs) have been fabricated on 6H-SiC substrates. The devices exhibited maximum drain current density as high as 1.42 A/mm, peak extrinsic transconductance of 437 mS/mm, unity current gain cutoff frequency (fT) of 41 GHz, and maximum frequency of oscillation (fmax) of 63 GHz. At 18 GHz, a continuous-wave output power density of 9.1 W/mm with power-added-efficiency (PAE) of 23.7% was obtained, yielding the highest reported power performance of AlGaN/GaN HEMTs at 18 GHz.

Growth and characterization of 0.8-μm gate length AlGaN/GaN HEMTs on sapphire sub-strates

AlGaN/GaN high electron mobility transistor (HEMT) structures were grown on 2 inch sapphire substrates by MOCVD, and 0.8-μm gate length devices were fabricated and measured. It is shown by resistance mapping that the HEMT structures have an average sheet resistance of approximately 380 Ω/sq with a uniformity of more than 96%. The 1-mm gate width devices using the materials yielded a pulsed drain current of 784 mA/mm at Vgs=0.5 V and Vds=7 V with an extrinsic transconductance of 200 mS/mm. A 20-GHz unity current gain cutoff frequency (ƒT ) and a 28-GHz maximum oscillation frequency (ƒmax) were obtained. The device with a 0.6-mm gate width yielded a total output power of 2.0 W/mm (power density of 3.33 W/mm) with 41% power added efficiency (PAE) at 4 GHz.

Recessed-gate enhancement-mode GaN HEMT with high threshold voltage

Fabrication of enhancement-mode high electron mobility transistors (E-HEMTs) on GaN/AlGaN heterostructures grown on SiC substrates is reported. Enhancement-mode operation was achieved with high threshold voltage (V/sub T/) through the combination of low-damage and controllable dry gate-recessing and the annealing of the Ni/Au gates. As-recessed E-HEMTs with 1.0 /spl mu/m gates exhibited a threshold voltage (V/sub T/) of 0.35 V, maximum drain current (I/sub D,max/) of 505 mA/mm, and maximum transconductance (g/sub m,max/) of 345 mS/mm; the corresponding post-gate anneal characteristics were 0.47 V, 455 mA/mm and 310 mS/mm, respectively. The RF performance is unaffected by the post-gate anneal process with a unity current gain cutoff frequency (f/sub T/) of 10 GHz.

IF-sampling fourth-order bandpass /spl Delta//spl Sigma/ modulator for digital receiver applications

Bandpass modulators sampling at high IFs (/spl sim/200 MHz) allow direct sampling of an IF signal, reducing analog hardware, and make it easier to realize completely software-programmable receivers. This paper presents the circuit design of and test results from a continuous-time tunable IF-sampling fourth-order bandpass /spl Delta//spl Sigma/ modulator implemented in InP HBT IC technology for use in a multimode digital receiver application. The bandpass /spl Delta//spl Sigma/ modulator is fabricated in AlInAs-GaInAs heterojunction bipolar technology with a peak unity current gain cutoff frequency (f/sub T/) of 130 GHz and a maximum frequency of oscillation (f/sub MAX/) of 130 GHz. The fourth-order bandpass /spl Delta//spl Sigma/ modulator consists of two bandpass resonators that can be tuned to optimize both wide-band and narrow-band operation. The IF is tunable from 140 to 210 MHz in this /spl Delta//spl Sigma/ modulator for use in multiple platform applications. Operating from /spl plusmn/5-V power supplies, the fabricated fourth-order /spl Delta//spl Sigma/ modulator sampling at 4 GSPS demonstrates stable behavior and achieves a signal-to-(noise + distortion) ratio (SNDR) of 78 dB at 1 MHz BW and 50 dB at 60 MHz BW. The average SNDR performance measured on over 250 parts is 72.5 dB at 1 MHz BW and 47.7 dB at 60 MHz BW.

Postprocessing annealing effects on direct current and microwave performance of AlGaN∕GaN high electron mobility transistors

The effects of postprocessing annealing on direct current, radio frequency small signal, and power performances of AlGaN∕GaN high electron mobility transistors with a gate-length of 0.2μm were investigated. The postannealing technique can improve the device performance, especially, after 10min postannealing at 400°C, the gate-to-drain breakdown voltage of devices exhibits remarkable improvement from 25 to 187V. The maximum extrinsic transconductance increases from 223 to 233mS∕mm at a drain bias of 10V after 10min annealing at 400°C. The maximum drain current at a gate bias of 1V increases from 823 to 956mA∕mm. After annealing, the values of the unity current gain cut-off frequency and the maximum oscillation frequency increases from 24 and 80GHz to 55 and 150GHz, respectively. The output power and gain at 10GHz were improved from 16.4dBm and 11.4dB to 25.9dBm and 19dB, respectively.

Self-aligned AlGaN/GaN high electron mobility transistors

Self-aligned T-gate AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated on a sapphire substrate using a thin Ti/Al/Ti/Au ohmic layer. To suppress the gate leakage current, the ohmic contact annealing was performed in a furnace. The self-aligned HEMTs with 0.25 µm gate length and 100 µm width exhibit good pinch-off characteristics, a transconductance of 146 mS/mm, an extrinsic unity current gain cutoff frequency of 38 GHz and a maximum oscillation frequency of 130 GHz.