Gate capacitance—Voltage characteristic of MODFET's: Its effect on transconductance

Gate capacitance Cgin modulation-doped field-effect transistors (MODFET's) is one of the most important parameters for small- and large-signal applications as well as for understanding device operation at dc and high frequencies. We have modeled the gate capacitance-voltage characteristics of AlGaAs/GaAs MODFET's and obtained good agreement with experiments at 300 and 77 K. Rigorous numerical simulations relating the Fermi level at the GaAs/AlGaAs heterointerface to the two-dimensional electron gas (2DEG) concentrations were fitted with analytical functions to simplify the model. The fit is quite reasonable with an accuracy of 1 percent in a wide range of sheet carrier concentrations. By incorporating AlGaAs charge response to gate voltage near full turn-on, discrepancies between the experiments and earlier models (predicting a nearly constant capacitance-voltage characteristic) were minimized. The model also shows that theC_{g}(V)characteristic below threshold is governed by the doping level in the buffer layer, in the intermediate region by the 2DEG and near complete turn-on by the 2DEG and the AlGaAs layer. Considering the contribution of the AlGaAs layer, a detailed RC network was developed. The model also explains the rise in capacitance and fall in the transconductance for large forward gate voltages (low frequency) due basically to the contribution of AlGaAs to the gate capacitance but almost no contribution to the current conduction. In contrast, the transconductance and capacitance both increase in MESFET's.     

An electrically and optically gate-controlled Schottky/2DEGvaractor

A novel gate-controlled varactor is reported. The three-terminal varactor is a modulation-doped heterostructure of AlGaAs/GaAs with two Schottky contacts directly made to a two-dimensional electron gas (2DEG). The third, gate, contact is formed from highly doped n⁺ GaAs material to allow an open optical window that can be used for optical gating and mixing. Structure capacitance is less than 1 pF and a change of more than 30% from the zero bias capacitance is observed with the applied gate voltage. The capacitance also increases proportionally with applied light and inversely with the terminal voltage     

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     

Metal-(n) AlGaAs-GaAs two-dimensional electron gas FET

Theoretical calculations have been developed for a two-dimensional electron gas FET (TEGFET) constituted by a AlGaAs (n)-GaAs (n^-or p^-) heterostructure in which the Schottky gate is deposited on the AlGaAs(n) top layer. The theory takes into account: i) the subband splitting in the two-dimensional electron gas (2-DEG); and ii) the existence of an undoped AlGaAs spacer layer which has been found to enhance the electron mobility. The sheet carrier concentration of the TEGFET has been calculated, and a simple analytical formula has been established for the charge control in large and small gate FET.     

Gate controlled 2-DEG varactor for VCO applications in microwave circuits

A novel gate controlled Schottky diode varactor is introduced. The three-terminal varactor is a modulation-doped heterostructure of AlGaAs/GaAs with two Schottky contacts, similar to a metal–semiconductor–metal (MSM) diode. Schottky metal contacts are made to a two-dimensional electron gas (2-DEG). The third contact, the gate contact is formed from highly doped n⁺ GaAs material to allow an open optical window that can be used for optical gating and mixing. Structure capacitance is less than 1 PF and a change of more than 30% from the zero bias capacitance is observed with the applied gate voltage. On the basis of our quasi two-dimensional C–V model, the layer structure and device dimensions can be optimized and scaled to cover a wide range of operations in the microwave and millimeter wave regimes.     

A fast numerical algorithm for heterojunction structures

In this paper, the two-dimensional electron gas (2DEG) concentration is numerically calculated for single and double heterostructure band profiles by solving Schroödinger's and Poisson's equations self-consistently. An expected energy level is introduced to enhance the speed of calculation in obtaining quantized energy levels through the iteration process. Conventional AlGaAs/GaAs and AlGaAs/InGaAs/GaAs structures are selected to prove the validity of this calculation. Three different concentrations, namely, positively ionized donors, free electrons in the conduction band and 2DEG, are considered through the band profiles. Both the 2DEGs in the narrow bandgap and free electrons in the wide bandgap have been calculated and compared with the data available in the literature. Furthermore, positively ionized donors are also obtained and correlated with 2DEG and free electrons to predict the gate capacitance characteristics of the devices, and showed good agreement with the experimental data.     

Interface states of modulation-doped AlGaAs/GaAs heterostructures

We have used the admittance spectroscopy to investigate interface states associated with heterojunction of modulation-doped AlGaAs/GaAs FET's. Anomalous frequency dispersion of the capacitance was observed. The results of the measurements were interpreted in terms of an equivalent circuit containing a series resistance of the two-dimensional electron gas in the ungated region between the gate and the source and drain electrodes. The maximum density of the interface states was found to be 1.3 × 10¹²cm^-2. eV^-1around 0.13 eV below the Ecedge of GaAs.     

Comparison for the carrier mobility between the Ⅲ-Ⅴ nitrides and AlGaAs/GaAs heterostructure field-effect transistors

Using the measured capacitance-voltage curves of Ni/Au Schottky contacts with different areas and the current-voltage characteristics for the AlGaAs/GaAs, AlGaN/AlN/GaN and In_0.18Al_0.82N/AlN/GaN heterostructure field-effect transistors (HFETs) at low drain-source voltage, the two-dimensional electron gas (2DEG) electron mobility for the prepared HFETs was calculated and analyzed. It was found that there is an obvious difference for the variation trend of the mobility curves between the Ⅲ-Ⅴ nitride HFETs and the AlGaAs/GaAs HFETs. In the Ⅲ-Ⅴ nitride HFETs, the variation trend for the curves of the 2DEG electron mobility with the gate bias is closely related to the ratio of the gate length to the drain-to-source distance. While the ratio of the gate length to the drain-to-source distance has no effect on the variation trend for the curves of the 2DEG electron mobility with the gate bias in the AlGaAs/GaAs HFETs. The reason is attributed to the polarization Coulomb field scattering in the Ⅲ-Ⅴ nitride HFETs.     

Current-gain cutoff frequency comparison of InGaAs HEMTs

The current-gain cutoff frequency performance of pseudomorphic InGaAs/AlGaAs (20% InAs composition) high-electron-mobility transistors (HEMTs) on GaAs is compared to that of lattice-matched InGaAs/InAlAs HEMTs on InP. The current-gain cutoff frequency (f_t) characteristics as a function of gate length (L_g) indicate that the f_t-L_g product of ~26 GHz-μm in InGaAs/InAlAs HEMTs is 23% higher than that of ~21 GHz-μm in InGaAs/AlGaAs HEMTs. The performance of InGaAs/AlGaAs HEMTs is also 46% higher than that of conventional GaAs/AlGaAs HEMTs (~18 GHz-μm). These data are very useful in improving the device performance of HEMTs for a given gate length     

HEMT器件小信号模型研究

提出一种AlGaAs/GaAs HEMT器件沟道电荷新模型,该模型用一个通用解析函数中系数的不同值来描述二维电子气(2DEG)和AlGaAs层中的电子浓度。在小信号特性上,除考虑了2DEG层外,又在考虑了AlGaAs层、速度饱和、饱和区沟道长度调制效应和源、漏串联电阻RS和RD等效应的基础上,推导出直流特性、跨导、输出电导和栅电容的解析表达式。仿真说明,在较大的栅、漏压范围内,该模型的理论值与实验结果符合良好。     

AlGaN/GaN HEMT器件工艺的研究进展

首先论述了Al GaN/GaN高电子迁移率晶体管(HEMT)在微波大功率领域的应用优势和潜力;其次,介绍并分析了影响Al GaN/GaN HEMT性能的主要参数,分析表明要提高Al-GaN/GaN HEMT的频率和功率性能,需改善寄生电阻、电容、栅长和击穿电压等参数。然后,着重从材料结构和器件工艺的角度阐述了近年来Al GaN/GaN HEMT的研究进展,详细归纳了目前主要的材料生长和器件制作工艺,可以看出基本的工艺思路是尽量提高材料二维电子气的浓度和材料对二维电子气的限制能力的同时减小器件的寄生电容和电阻,增强栅极对沟道的控制能力。另外,根据具体情况调节栅长及沟道电场。最后,简要探讨了Al GaN/GaN HEMT还存在的问题以及面临的挑战。     

A model for the capacitance—Voltage characteristics of MODFET's

Using Fermi-Dirac statistics, a model for the gate capacitance-voltage characteristics of MODFET's is developed that includes not only the capacitance component due to two-dimensional electron gas density but also the capacitance component due to donor neutralization. To analytically solve Poisson's equation with the Fermi-Dirac statistics, we analyzed the theory by using two regions (the complete ionization region and the partial ionization region of doped impurities). Using the obtained C-V relationships, the gate capacitance is calculated with the numerical calculation method. The results are in good agreement with experimental data.     

An analytical model for high electron mobility transistors

In this paper we present a new model for HEMT's which is based on a single analytical function that describes the electron concentrations in the two dimensional electron gas and in the AlGaAs layer. Besides accounting for the AlGaAs conduction, the model includes the effect of mobility degradation, channel length modulation in the saturation region and the series resistances R_S and R_D. The model results in closed form expressions for the current, transconductance, output conductance and gate capacitance. Finally, the theoretical predictions of the model are compared with the experimental data and shown to be in good agreement over a wide range of bias conditions     

AlGaAs/GaAs heterojunction bipolar transistor circuits with improved high-speed performance

Using a self-aligned base contact process and proton implantation to reduce extrinsic base-collector capacitance, we have improved the high-speed performance of AlGaAs/ GaAs heterojunction bipolar transistor digital circuits. The cutoff frequency of the transistor fT with an emitter width of 2.0 ?m is 45 GHz. NTL ring oscillators have operated at 16.5 ps/gate and CML ring oscillators at 27.6 ps/gate. Frequency dividers (1/4) have operated up to 11 GHz with wafer-probe testing These are record speeds for bipolar circuits.     

Resonance detection of terahertz radiation in submicrometer field-effect GaAs/AlGaAs transistors with two-dimensional electron gas

Semiconductors - Resonance detection of terahertz radiation by submicrometer field-effect GaAs/AlGaAs transistors (with the gate length L = 250 nm) with two-dimensional electron gas in the channel...     

Characteristics of AlGaN/GaN/AlGaN double heteroj unction HEMTs with an improved breakdown voltage

正We studied the performance of AlGaN/GaN double heterojunction high electron mobility transistors (DH-HEMTs) with an AlGaN buffer layer,which leads to a higher potential barrier at the backside of the twodimensional electron gas channel and better carrier confinement.This,remarkably,reduces the drain leakage current and improves the device breakdown voltage.The breakdown voltage of AlGaN/GaN double heterojunction HEMTs (~ 100 V) was significantly improved compared to that of conventional AlGaN/GaN HEMTs(~50 V) for the device with gate dimensions of 0.5 x 100μm and a gate-drain distance of 1μm.The DH-HEMTs also demonstrated a maximum output power of 7.78 W/mm,a maximum power-added efficiency of 62.3%and a linear gain of 23 dB at the drain supply voltage of 35 V at 4 GHz.     

Accurate modeling for parasitic source resistance intwo-dimensional electron gas field-effect transistors

An accurate two-layer model has been developed for parasitic source resistance in two-dimensional electron gas field-effect transistors (2DEGFETs). In this model, the 2DEG concentration-voltage and current density-voltage relations at the cap/barrier/2DEG junction are taken into account, based on the self-consistent charge control model and effective mass tunneling theory. Empirical 2DEG velocity field characteristics are also included. To show the feasibility of this method, the source resistance in conventional GaAs/AlGaAs 2DEGFETs was analyzed and the AlGaAs thickness dependence was discussed. For comparison, the lattice-matched GaInAs/AlInAs 2DEGFET and pseudomorphic GaInAs/AlGaAs 2DEGFET were examined. It was shown that introducing a highly doped cap layer leads to a drastic reduction in source resistance for pseudomorphic 2DEGFETs but has a very small effect for GaInAs/AlGaAs 2DEGFETs     

Modeling for an AlGaAs/GaAs heterostructure device using Monte Carlo simulation

Two-dimensional electron gas behavior in an AlGaAs/GaAs heterostructure FET has been analyzed using the Monte Carlo method. In the channel region, it is assumed that the electrons are subjected to a two-dimensional scattering process. In the other regions, three-dimensional scattering rates are assumed. It is predicted that, in an actual device with 1.20-µm gate length, the transconductance of 250 and 450 mS/mm can be attained at 300 and 77 K, respectively. More efficient performance is possible with improvements in the device structure.     

Highly selective reactive ion etching applied to the fabrication of low-noise AlGaAs GaAs FET's

Selective dry etching of GaAs to AlGaAs (x = 0.25) using pure CCl2F2etching gas has been achieved. During reactive ion etching (RIE), the discharge has been analyzed by optical emission and mass spectroscopy. A high-selectivity ratio up to 1000, associated with a clean and anisotropic etching or an undercut of GaAs can be obtained by adjustment of the pressure. Selective RIE has been used to etch the n⁺GaAs cap layer of an n⁺GaAs/AlGaAs/GaAs heterostructure and to define the gate recess of discrete two-dimensional electron-gas FET's (TEGFET's). Results on low-noise TEGFET's fabricated by this technique are reported for the first time. Thanks to the reduction of side etching, very low source resistances have been obtained (less than 1 Ω . mm) for a source-to-gate distance up to 2 µm. Noise figures of 2 dB have been measured at 12 GHz with an associated gain of 8.3 dB for a gate length of 0.7 µm.     

GaAs/AlGaAs and InGaAs/AlGaAs MODFET inverter simulations

Although MODFET's have exhibited the fastest switching speed for any digital circuit technology, there is as yet no clear consensus on optimal inverter design rules. We therefore have developed a comprehensive MODFET device model that accurately accounts for such high gate bias effects as transconductance degradation and increased gate capacitance. The device model, which agrees with experimental devices fabricated in this laboratory, is used in the simulation of direct-coupled FET logic (DCFL) inverters with saturated resistor loads. Based on simulation results, the importance of large driver threshold voltage not only for small propagation delay times but for wide logic swings and noise margins is demonstrated. Furthermore, minimum delay times are found to occur at small supply voltages as seen experimentally. Both of these results are attributed to the reduction of detrimental high gate bias effects. The major effect of reducing the gate length on delay time is to decrease the load capacitance of the gate. Using 0.25-µm gates, delay times of 5 and 3.6 ps at 300 and 77 K, respectively, are predicted. Finally, the recently introduced In-GaAs/AlGaAs MODFET's are shown to have switching speeds superior to those of conventional GaAs/AlGaAs MODFET's.