Importance of source and drain resistance to the maximum fT of millimeter-wave MODFETs

The usual approximate expression for measured f_T =[g_m/2π (C_gs+C _gd)] is inadequate. At low drain voltages just beyond the knee of the DC I-V curves, where intrinsic f _t is a maximum for millimeter-wave MODFETs, the high values of C_gd and G_ds combine with the high g_m to make terms involving the source and drain resistance significant. It is shown that these resistances can degrade the measured f_T of a 0.30-μm GaAs-AlGaAs MODFET from an intrinsic maximum f_T value of 73 GHz to a measured maximum value of 59 GHz. The correct extraction of maximum f_T is essential for determining electron velocity and optimizing low-noise performance     

A new two-mode channel FET (TMT) for super-low-noise and high-powerapplications

A super-low-noise two-mode channel FET (TMT) with high- and plateau-shaped transconductance (g_m) characteristics has been developed. It has two electron transport modes against the applied gate voltage (V_gs). That is, the electrons mainly drift in a highly doped channel region at a shallow V_gs. A plateau g_m region and the maximum g_m were achieved at a V_gs range of -0.25~+0.5 V and 535 mS/mm, respectively. The minimum noise figure and associated gain for the TMT were superior in the low-drain-current (I_ds) region and nearly equal in the middle and high I_ds region to those of an AlGaAs/InGaAs pseudomorphic HEMT fabricated using the same wafer process and device geometry     

Detection of the trapped electron distribution of PMOSFET's afterhot-carrier stress

A method of obtaining the spatial distribution of hot-carrier-induced trapped electrons in the gate oxide (N_0t(x)) of PMOSFETs is introduced with the aid of a two-dimensional simulator. The measured I_ds versus V_ds for various V_gs for low drain bias and I_ds versus V_gs have been compared with data obtained from the simulation concerning the obtained spatial distribution of trapped electrons in the gate oxide. There exists a high degree of agreement between the measured current-voltage characteristics after hot-carrier stress and the simulation results concerning the newly obtained spatial distribution of trapped electrons in the gate oxide     

Nonlinear GaAs MESFET modeling using pulsed gate measurements

The effects of traps in GaAs MESFETs are studied using a pulsed gate measurement system. The devices are pulsed into the active region for a short period (typically 1 μs) and are held in the cutoff region for the rest of a 1-ms period. While the devices are on, the drain current is sampled and a series of pulsed gate I-V curves are obtained. The drain current obtained under the pulsed gate conditions for a given V_GS and V_DS gives a better representation of the instantaneous current for a corresponding V_gs and V_ds in the microwave cycle because of the effects of traps. The static and pulsed gate curves were used in a nonlinear time-domain model to predict harmonic current. The results showed that analysis using pulsed gate curves yielded better predictions of harmonic distortion than analysis based on conventional state I-V curves under large-signal conditions     

Charge control, DC, and RF performance of a 0.35-μmpseudomorphic AlGaAs/InGaAs modulation-doped field-effecttransistor

The authors describe a study of charge control in conjunction with DC and RF performance of 0.35-μm-gate-length pseudomorphic AlGaAs/InGaAs MODFETs. Using C-V measurements, they estimate that a two-dimensional electron gas (2DEG) with density as high as 1.0×10¹² cm^-2 can be accumulated in the InGaAs channel at 77 K before the gate begins to modulate parasitic charges in the AlGaAs. This improvement in charge control of about 10-30% over a typical AlGaAs/GaAs MODFET may partially be responsible for the superior DC and RF performance of the AlGaAs/InGaAs MODFET. At room temperature, the devices give a maximum DC voltage gain g _m/g_d of 32 and a current gain cutoff frequency f_T of 46 GHz. These results are state of the art for MODFETs of similar gate length     

Application of the floating-gate technique to the study of then-MOSFET gate current evolution due to hot-carrier aging

The evolution of the gate current-voltage (I_g- V_gs) characteristics of n-MOSFETs induced by DC stresses at different gate voltage over drain voltage (V_ds ) ratios is studied by the floating-gate (FG) measurement technique. It is shown that the I_g-V_gs curves are always lowered after aging, and that the kinetics are dependent on the aging conditions. A time power law is representative of the V_gs=V_ds case. It is demonstrated that electron traps are created in the oxide by both hot-hole and hot-electron injection stresses. They are not present in the devices before aging. They can be easily charged and discharged by short electron and hole injections, respectively     

Dependence of ionization current on gate bias in GaAs MESFETs

The nonmonotonic behavior of gate current I_g as a function of gate-to-source voltage V_gs is reported for depletion-mode double-implant GaAs MESFETs. Experiments and numerical simulations show that the main contribution to I_g (in the range of drain biases studied) comes from impact-ionization-generated holes collected at the gate electrode, and that the bell shape of the I_g(V_gs) curve is strongly related to the drop of the electric field in the channel of the device as V_gs is moved towards positive values     

Comparisons of microwave performance between single-gate anddual-gate MODFETs

Both a 1.2-μm and a 0.3-μm gate length, n⁺-GaAs/InGa/n⁺-AlGaAs double-heterojunction MODFET have been fabricated with single-gate and dual-gate control electrodes. Extrinsic DC transconductance of 500 mS/mm has been achieved from a 0.3-μm single-gate MODFET. The device also has a current gain cutoff frequency f_T of 43 GHz and 14-dB maximum stable gain at 26 GHz with the stability factor k as low as 0.6 from the microwave S-parameter measurements. At low-frequency dual-gate MODFETs demonstrate higher gain than the single-gate MODFETs. However, the k of dual-gate MODFETs approaches unity at a faster rate. Power gain roll-off slopes of 3-, 6-, and 12-dB/octave have been observed for the dual-gate MODFETs     

DC and microwave performance of a 0.1 μm gate InAs/In0.52Al0.8As MODFET

The authors have measured and analyzed the performance characteristics of 0.1-μm gate InAs/In_0.52Al_0.48 MODFETs grown by molecular beam epitaxy. The transistors are characterized by measured g_m(max)=840 mS/mm, f_T=128 GHz, and a very high current carrying capability, e.g. I_dss=934 mA/mm at V _gs=0.4 V and V_ds=2.7 V. The value of f _T is estimated from extrapolation of the current gain (H ₂₁) at a -6 dB/octave rolloff. This is the first report on the microwave characteristics of an InAs-channel MODFET and establishes the superiority of this heterostructure system     

Experimental and theoretical investigation of the DC andhigh-frequency characteristics of the negative differential resistancein pseudomorphic AlGaAs/InGaAs/GaAs MODFET's

The authors investigated the negative differential resistance (NDR) in the I-V characteristics of pseudomorphic AlGaAs/InGaAs/GaAs modulation doped field-effect transistors (MODFETs) with gate lengths of 0.3 μm. They experimentally verified the existence of abrupt multiple NDR in both the input circuit and the output circuit. The NDR occurs over a short range of drain voltage (less than 200 mV) and gate voltage (less than 5 mV) for NDR induced by thermionic emission. The authors provide a general interpretation of the measured DC results based on tunneling real-space transfer (TRST) which occurs because of the formation of hybrid excited states across the InGaAs channel and AlGaAs donor layer. The existence of stable reflection is verified in both the input and output circuits with stable broadband frequency response in the output circuit to at least 49 GHz. These results show that NDR via TRST in pseudomorphic MODFETs can provide wideband frequency response not limited by the electron transit time from source to drain     

Low-power performance of 0.5-μm JFET for low-cost MMIC's inpersonal communications

The low-power microwave performance of an enhancement-mode ion-implanted GaAs JFET is reported. A 0.5-μm×100-μm E-JFET with a threshold voltage of V_th=0.3 V achieved a maximum DC transconductance of g_m=489 mS/mm at V _ds=1.5 V and I_ds=18 mA. Operating at 0.5 mW of power with V_ds=0.5 V and I_ds =1 mA, the best device on a 3-in wafer achieved a noise figure of 0.8 dB with an associated gain of 9.6 dB measured at 4 GHz. Across a 3-in wafer the average noise figure was F_min=1.2 dB and the average associated gain was G_a=9.8 dB for 15 devices measured. These results demonstrate that the E-JFET is an excellent choice for low-power personal communication applications     

An InAlAs/InAs MODFET

An InAs modulation-doped field-effect transistor (MODFET) using an epitaxial heterostructure based entirely on arsenides is reported. The heterostructure was grown by MBE on InP and contains a 30-Å InAs channel. A 2-μm-gate-length device displays well-behaved characteristics, showing sharp pinch-off (V_th=0.8 V) and small output conductance (5 mS/mm) at 300 K. The maximum transconductance is 170 mS mm with a maximum drain current of 312 mA/mm. Strong channel quantization results in a breakdown voltage of -9.6 V, a severalfold improvement over previous InAs MODFETs based on antimonides. Low-temperature magnetic field measurements show strong Shubnikov-de Haas oscillations which, over a certain range of gate voltage, strongly indicate that the electron channel resides in the InAs layer     

Electron transport in 0.15-μm gate In0.52Al0.48As/In0.53Ga0.47As HEMT

Magneto-transport and cyclotron resonance measurements were made to determine directly the density, mobility, and the effective mass of the charge carriers in a high-performance 0.15-μm gate In_0.52 Al_0.48As/In_0.53Ga_0.47As high-electron-mobility transistor (HEMT) at low temperatures. At the gate voltage V_G=0 V, the carrier density n _g under the gate is 9×10¹¹ cm^-2, while outside of the gate region n_g=2.1×10¹² cm^-2. The mobility under the gate at 4.2 K is as low as 400 cm²/V-s when V_G<0.1 V and rapidly approaches 11000 cm²/V-s when V_G>0.1 V. The existence of this high mobility threshold is crucial to the operation of the device and sets its high-performance region in V_G>0.1 V     

Current-crowding effect in diagonal MOSFET's

Electrical and reliability characteristics of diagonally shaped n-channel MOSFETs have been extensively investigated. Compared with the conventional device structure, diagonal MOSFETs show longer device lifetime under peak I_sub condition (V_g =0.5 V_d). However, in the high-gate-bias region (V_g=V_d), diagonal MOSFETs exhibit a significantly higher degradation rate. From the I_sub versus gate voltage characteristics, this larger degradation rate under high gate bias is concluded to be due mainly to the current-crowding effect at the drain corner. For a cell-transistor operating condition (V_g>V_d), this current-crowding effect in the diagonal transistor can be a serious reliability concern     

1/f noise in MODFETs at low drain bias

The 1/f noise in normally-on MODFETs biased at low drain voltages is investigated. The experimentally observed relative noise in the drain current S_I/I² versus the effective gate voltage V_G=V_GS-V_off shows three regions which are explained. The observed dependencies are S_I/I²∝V_G ^m with the exponents m=-1, -3, 0 with increasing values of V_G. The model explains m =-1 as the region where the resistance and the 1/f noise stem from the 2-D electron gas under the gate electrode; the region with m=0 at large V_G or V_GS≅0 is due to the dominant contribution of the series resistance. In the region at intermediate V_G , m=-3, the 1/f noise stems from the channel under the gate electrode, and the drain-source resistance is already dominated by the series resistance     

An InAs channel heterojunction field-effect transistor with hightransconductance

A report is presented on an InAs channel field-effect transistor (FET) based on AlGaSb/InAs/AlSb/AlGaSb structures grown by molecular-beam epitaxy. Excellent pinch-off characteristics have been obtained. An FET with a gate length of 1.7 μm showed transconductances ranging from 460 mS/mm (at V_ds=0.5 V) to 509 mS/mm (at V_ds=1 V) and a K factor of 1450 mS/Vmm (at V_ds=1 V) at room temperature     

An anisotype GaAs/InxGa1-xAs heterojunctionfield-effect transistor for digital logic applications

An anisotype heterojunction field-effect transistor (A-HJFET) for GaAs digital integrated circuit applications is proposed. A thin, highly doped, strained In_xGa_1-xAs (x⩽0.2) n-channel is employed for improved transconductance while a p⁺-GaAs cap is used to enhance the dynamic gate voltage range of the device. Prototype devices with 5-μm gate lengths show a maximum transconductance of 80 mS/mm at V_ds=2 V and a forward gate bias voltage of up to +2 V without significant leakage current     

8-element linear array monolithic p-i-n MODFET photoreceivers usingmolecular beam epitaxial regrowth

An 8-element linear array of single-stage integrating front-end photoreceivers using molecular beam epitaxial (MBE) regrowth was investigated. Each element consisted of a p-i-n In_0.53Ga_0.47As photodiode integrated with a selectively regrown pseudomorphic In_0.65Ga_0.35As/In_0.52Al_0.48 As MODFET. Cutoff frequencies of 1.0-μm discrete regrown MODFETs were f_t=24 GHz and f_max=50 GHz. Transconductance of the regrown MODFETs was as high as 495 mS/mm with a current density (I_ds) of 250 mA/mm. The 3-dB bandwidth of the photoreceiver was measured to be 1 GHz. The bit rate sensitivity at 1 Gb/s was -31.8 dBm for BER 10^-9 using 1.55 μm excitation for a photoreceiver with an anti-reflection coating. The single-stage amplifier exhibited up to 25 dB flatband gain of the photocurrent, and a two-stage amplifier was up to 31 dB of gain. Good uniformity between each photoreceiver element in the array was achieved. Electrical crosstalk between photoreceiver elements was estimated to be ~-34 dB     

High-transconductance insulating-gate InP/InGaAs buried p-bufferDH-MODFETs grown by MOVPE

Buried p-buffer double heterostructure modulation-doped field-effect transistors (BP DH-MODFETs) with an InGaAs quantum-well channel were fabricated with high transconductance and good breakdown voltage, by placing the metal gate directly on Fe-doped InP insulating layer. Excellent extrinsic DC transconductance of 560 mS/mm and a high gate-to-drain diode breakdown voltage (greater than 20 V) were achieved at room temperature with FETs of 1.2-μm gate length. Unity currently gain cutoff frequency f_T of 24 GHz and maximum oscillation frequency f_max of 60 GHz were demonstrated for a drain to source voltage V_DS=4 V, which corresponds to an average electron velocity of 2.2×10⁷ cm/s in the quantum well     

The influence of gate-feeder/mesa-edge contacting on sidegatingeffects in In0.52Al0.48As/In0.53Ga0.47As heterostructure FET's

Sidegating effects in InAlAs/InGaAs heterostructure field effect transistors (HFETs) were experimentally investigated. Two different configurations of gate feeder across the mesa edges are compared in In _0.52Al_0.48As/In_0.53Ga_0.47As. HFETs. HEMTs and heterostructure insulated-gate FETs (HIGFETs) were fabricated, each with different gate-feeder configurations. HFETs with the gate air bridge over the mesa edge can maintain 99% of the drain-source (I_ds) current level at sidegate voltages (V_sg) extending up to -30 V, while the non-air-bridge configuration of HFETs show a 30% drop of I_ds at the same V _sg. This significant discrepancy of sidegating effect is attributed to depletion region modulation at the mesa edge below the gate feeder. By lifting the gate feeder above the mesa step, sidegating is reduced, which suggests the channel/substrate trap effects are negligibly small. The role of air-bridge structures in determining the sidegating characteristics is discussed