1. Introduction

Recent developments of III-V nitrides has extended their applications greatly in the areas of electrical devices as well as optical devices. The use of III-V nitrides on electrical devices is beneficial from their high breakdown voltages due to the wide band-gap, and high electron velocities. After the first demonstration of Heterojunction Field Effect Transistors (HFET's) on III-V nitrides Reference Khan, Kuznia, Bhattarai and Olson[1], high power Reference Khan, Shur, Chen, Burm, Schaff, Chu and Eastman[2] and high frequency devices Reference Khan, Kuznia, Olson, Schaff, Burm and Shur[3] have developed. Current state of the art transistors on III-V nitride materials have achieved transconductances of 120 mS/mm Reference Morkoç[4] or more and current densities in excess of 600 mA/mm Reference Khan, Shur, Chen, Burm, Schaff, Chu and Eastman[2]. For microwave devices, current gain cut-off frequency, f_t, of 32.1 GHz Reference Khan, Chen, Shur, Dermott, Higgins, Burm, Schaff and Eastman[5] and maximum oscillation frequency, f_max, of 77.5 GHz Reference Burm, Schaff, Eastman, Amano and Akasaki[6] have been reported utilizing 0.25 μm gate, and even higher f_t and f_max have been achieved Reference Khan, Shur, Chen, Burm, Schaff, Chu and Eastman[2]. Employing III-nitride layers, Metal-Semiconductor Field Effect Transistors Reference Khan, Kuznia, Bhattarai and Olson[1] Reference Binari, Rowl, Kelner, Kruppa, Dietrich, Doverspike and Gaskillin[7], Metal-Insulator Field Effect Transistors Reference Binari, Rowl, Kelner, Kruppa, Dietrich, Doverspike and Gaskillin[7], and Modulation Doped Field Effect Transistors (MODFET's) have been fabricated.

2. Fabrication

The MODFET layers are grown by organometallic vapor phase epitaxy (OMVPE) on a (0001) Sapphire substrate. The grown layers are, from the bottom, an AlN buffer layer, thick (3-5 μm) GaN layer, 200 Å Al_0.16Ga_0.84N layer, 75 Å GaN channel, 50 Å Al_0.16Ga_0.84N spacer, 20 Å Si doped Al_0.16Ga_0.84N charge supply layer, 130 Å Al_0.16Ga_0.84N barrier, and 60 Å Al_0.06Ga_0.94N cap layer. The charge supply layer and cap layer were Si-doped with a doping density of 2×10¹⁸cm^{− 3}(Figure 1) Reference Burm, Schaff, Eastman, Amano and Akasaki[6]. A Hall Measurement from Van der Pauw configuration showed a mobility of 680 cm²/Vs and a sheet charge density of 7.3 × 10¹²cm⁻² at room temperature.

Figure 1. MODFET layer structure

On the wafer, MODFET's were fabricated. Ni/AuSi/Ag/Au (100 Å/1000 Å/1000 Å/1500 Å) was used for ohmic contacts and was annealed at 750°C for 30 s under N₂ atmosphere. The contact resistance was measured from a separate transmission line model (TLM) patterns to be 9.2 Ωmm. For the device isolation, proton (H⁺) bombardment was used Reference Khan, Kuznia, Bhattarai and Olson[1]. The penetration depth of protons was on the order of 1.0 μm. Electron-beam lithography was used to define 0.25 μm gates, and Ti/Pd/Au was employed as the gate metal. Before evaporating gate metals, the sample was cleaned with Buffered Oxide Etch to remove any oxides formed. Ti is known to have a Schottky barrier height of 0.58 eV on GaN from the measurement by Binari et al. Reference Binari, Dietrich, Kelner, Rowland, Doverspike and Gaskill[8]. The gate width of the MODFET's was 100 μm.

3. Measurements and Discussions

On fabricated MODFET's, DC characteristics as well as RF characteristics were measured. A drain current of 150 mA/mm was reached at 0 V gate bias (Figure 2a). The peak transconductance was 40 mS/mm (Figure 2b). The series source and drain resistances determined from the voltage divider measurement technique Reference Reeves and Harrison[9] were 14.6 Ωmm and 16.5 Ωmm respectively. The high contact resistances limited the device performance. The intrinsic transconductance calculated from the measured extrinsic conductances of 40 mS/mm was 96 mS/mm. From the gate length of 0.25 μm, the effective gate length can be calculated by adding twice the distance from gate to 2 dimensional electron gas. The calculated effective gate length is 0.31 μm. For the MODFET's, HP8510 network analyzer was used to measured S-parameters for the frequency range from 45 MHz to 26.5 GHz. The cutoff frequency, f_t, and maximum oscillation frequency, f_max were determined from the current gain (h₂₁) and Mason's Unilateral gain (U), respectively. At a drain bias of 24 V, f_t of 21.4 GHz and f_max of 77.5 GHz were obtained (Figure 3). The increase of the drain bias caused the increase of f_t and f_max to a certain bias. This increase is currently believed to be from the composite effects of the increase of transconductance, according to the gradual channel model, and the decrease in the gate-drain capacitance, C_gd, with the drain bias increase as pointed out by Tasker et al Reference Tasker and Hughes[10].

Figure 2a. Current-Voltage Characteristics of a MODFET.

Figure 2b. Transconductance (g_m) and drain current (I_ds) of a MODFET.

Figure 3. f_t of 21.4 GHz and f_max of 77 GHz was obtained at the drain bias of 24 V.

A simple analytical method can be used to solve for the electron sheet density, N _s , in a full thin quantum well, using delta-doping, an Al_0.4Ga_0.6N barrier with a minimum of 0.12 V above the Fermi energy, and 0.15 V ground state for electrons in the 30 Å In_0.25Ga_0.75N channel. The following equations apply.

where

and all for single sided doping and

In the equation , ΔE _c(2a)=3 nm, the thickness of the quantum well, ε _Q is the dielectric constant of the quantum well (12.77 ε _o ), q is the electronic charge, W _s =5 nm, the thickness of the spacer layer, and ε _s is the dielectric constant in the spacer (11.0 ε _o ). The quantum well state density is 88x10¹²/cm²-V. For this single-sided doping case, solving the equation yields N _s ≅ 7.3 × 10¹² cm⁻². For double-sided doping, ΔE _Q is a quarter of the above, while ΔE _s is one half of the above. In this double-sided doping case, N _s ≅ 1.4 × 10¹³ cm⁻². ΔE _s is the potential change in the quantum well, due to an assumed uniform charge density of (N _s /2a, and ΔE _s is the potential change in the spacer layer due to the associated electron charge of qN _s for single-sided doping, or (qN_s /2) for double-sided doping. This simple solution assumes that there is a nearly constant potential in the quantum well, equal to (2/3)ΔE _Q . The results of these simple calculations agree within ~10%, with the dedicated quantum mechanical solution of N _s .

4. Conclusion

A III-V nitride modulation doped field effect transistor structure was employed to fabricate FET's. The OMVPE grown wafer included a 75 Å GaN channel, a 50 Å Al_0.16Ga_0.84N spacer and a 130 Å Al_0.16Ga_0.84N barrier. The Si doped charge supplying layer was confined in 20 Å. The MODFET's were fabricated using proton bombardment for isolation, with Ni/AuSi/Ag/Au ohmic metal, and Ti/Pd/Au gate metal. The 0.25 μm × 100 μm gate devices yielded a maximum transconductance of 40 mS/mm and a drain current of 150 mA/mm at 0 V gate bias. RF results revealed f_t of 21.4 GHz and f_max of 77.5 GHz at the drain bias of 24 V. A simple analytical method can be useful in designing these structures to yield electron sheet density values.