Cite this Article
Zhang Jing, Lv Hongliang, Ni Haiqiao, Niu Zhichuan, Zhang Yuming. Temperature dependence on the electrical and physical performance of InAs/AlSb heterojunction and high electron mobility transistors*

Project supported by the Advanced Research Foundation of China (Grant No. 914xxx803-051xxx111), the National Defense Advanced Research Project of China (Grant No. 315xxxxx301), and the National Defense Innovation Program of China (Grant No. 48xx4).

. Chinese Physics B, 2018, 27(9): 097201  
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Temperature dependence on the electrical and physical performance of InAs/AlSb heterojunction and high electron mobility transistors*

Project supported by the Advanced Research Foundation of China (Grant No. 914xxx803-051xxx111), the National Defense Advanced Research Project of China (Grant No. 315xxxxx301), and the National Defense Innovation Program of China (Grant No. 48xx4).

Zhang Jing1, 2, Lv Hongliang1, †, Ni Haiqiao2, Niu Zhichuan2, Zhang Yuming1
School of Microelectronics, Xidian University and Key Laboratory of Wide Band-Gap Semiconductor Materials and Devices, Xi’an 710071, China
State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

 

† Corresponding author. E-mail: hllv@mail.xidian.edu.cn

Project supported by the Advanced Research Foundation of China (Grant No. 914xxx803-051xxx111), the National Defense Advanced Research Project of China (Grant No. 315xxxxx301), and the National Defense Innovation Program of China (Grant No. 48xx4).

Abstract

In this report, the effect of temperature on the InAs/AlSb heterojunction and high-electron-mobility transistors (HEMTs) with a gate length of 2 μm are discussed comprehensively. The results indicate that device performance is greatly improved at cryogenic temperatures. It is also observed that the device performance at 90 K is significantly improved with 27% lower gate leakage current, 12% higher maximum drain current, and 22.5% higher peak transconductance compared to 300 K. The temperature dependence of mobility and the two-dimensional electron gas concentration in the InAs/AlSb heterojunction for the temperature range 90 K–300 K is also investigated. The electron mobility at 90 K (42560 cm2/V·s) is 2.5 times higher than its value at 300 K (16911 cm2/V·s) because of the weaker lattice vibration and the impurity ionization at cryogenic temperatures, which corresponds to a reduced scattering rate and higher mobility. We also noted that the two-dimensional electron gas concentration decreases slightly from 1.99 × 1012 cm−2 at 300 K to 1.7 × 1012 cm−2 at 90 K with a decrease in temperature due to the lower ionization at cryogenic temperature and the nearly constant ΔEc.

PACS: 72.10.-d;73.40.Kp;81.05.Ea
Keyword:temperature;mobility;two-dimensional electron gas;InAs/AlSb HEMT
1. Introduction

In recent years, InAs/AlSb HEMTs have become one of the most promising candidates for high-speed, low-power, and low-noise applications.[1,2] A very important application of these transistors is for low noise amplifiers (LNA) that are used in deep space exploration, and are operated primarily at cryogenic temperatures to improve the sensitivity of detection.

During the past twenty years, InP HEMTs have been the dominant force in microwave millimeter wave applications.[3,4] Compared with InP HEMTs, InAs has a higher electron mobility and saturation drift speed. Moreover, InAs/AlSb HEMTs exhibit a two-dimensional electron gas (2-DEG) distribution in the channel due to the small lattice mismatch between the InAs and AlSb and the large conduction band offset (about 1.35 eV) that can form deep quantum wells. Therefore, the application of InP HEMTs for use in cryogenic LNAs has recently been challenged by InAs/AlSb HEMTs. In Ref. [5], the first demonstration of an InAs/AlSb HEMT LNA at cryogenic temperatures was reported. At room temperature, the three-stage amplifier exhibited a transducer gain of 29 dB and a noise temperature of 150 K with 17.6 mW power consumption. At cryogenic temperatures, the optimum drain voltage for best noise performance was reduced from 0.55 V to 0.3 V, demonstrating the very low-power and low-voltage capabilities of InAs/AlSb HEMTs-based low-noise amplifiers. InAs/AlSb HEMTs are typical 2DEG-based devices with electrical characteristics that are very sensitive to the operating temperature. The cryogenic characteristics of InAs/AlSb HEMTs are determined by the temperature dependency of the electron mobility and 2-DEG in the InAs channel. The characterization of electron mobility and 2-DEG at different temperatures will assist in improving the general understanding of the change mechanism of the drain current, gate-leakage currents and transconductance.

In this work, the temperature effect on InAs/AlSb heterojunction and HEMTs are comprehensively discussed. The results indicate that device performance is greatly improved at low temperatures. It has also been observed that the leakage current decreases with a reduction of the to 90 K because of the Schottky leakage current component and the hole leakage current component caused by the impact ionization effect decreased. The decrease of the gate–leakage currents with temperature is a clear advantage for devices operating at cryogenic temperatures. The temperature dependence of the mobility and the two-dimensional electron gas concentration in InAs/AlSb heterojunctions in the temperature range 90 K–300 K is also investigated. The lower lattice vibration and the impurity ionization at cryogenic temperatures correspond to a reduced scattering rate and higher mobility. The two-dimensional electron gas concentration decreases slightly with the decrease of temperature due to the lower ionization at cryogenic and the nearly constant ΔEc.

2. Epitaxial structure and device fabrication

Before device fabrication, the material is grown by molecular beam epitaxy (MBE). The material structure and its energy band diagram are illustrated in Figs. 1(a) and 1(b). The epitaxial materials are grown on 2-in semi-insulating GaAs substrates. A composite AlSb/Al0.7Ga0.3Sb metamorphic buffer layer with a thickness of 1.45 μm is utilized to relax the 8% compressive lattice mismatch between the GaAs substrate and the active layers of the device. The HEMT active layers above the metamorphic buffer layers consist of a 50 nm AlSb HEMT buffer layer, a 15 nm InAs channel, a 5 nm AlSb spacer, a 2 × 1012 cm−2 Si δ-doped plane inserted in four monolayers of InAs (∼ 1.2 nm), and an 8 nm AlSb Schottky layer. On the top, there is a 4 nm In0.5Al0.5As protection layer and a 5 nm InAs contact layer. A root-mean-square roughness of 1.27 nm is measured on the as-grown surface by AFM is shown in Fig. 2.

Fig. 1. (color online) (a) Schematic InAs/AlSb HEMT and (b) the energy band diagram.
Fig. 2. (color online) AFM of InAs cap surface.

An alloyed Ohmic contact is formed by e-beam evaporation of Pd/Ti/Pt/Au (20/30/30/30 nm), following by RTA at 300 °C for 20 s. The contact resistance Rc and contact resistivity ρc are 0.116 Ω·mm and 3.50 × 10−7 Ω·cm2, respectively, for TLM at 300 K. Optical images of TLM pattern, I–V curves of two contacts with different dn, and Rtot as a function of dn are shown in Figs. 3(a)–3(c), respectively.

Fig. 3. (color online) (a) Optical image of TLM pattern, (b) I–V curves of two contacts with different dn at 300 K, and (c) Rtot as a function of dn.

Before Schottky gate metal deposition, gate recess etching is performed using a citric acid/H2O2 solution in order to avoid gate-to-source and gate-to-drain conduction, because of the highly doped InAs cap layer. Then, the Schottky gate metal is deposited by e-beam evaporation of Ti/Pt/Au (20/20/300 nm). Finally, a probing pad is deposited by e-beam evaporation of Ti/Au (20/300 nm). A gate width of 50 μm and a gate length of 2 μm are used, as shown in Fig. 4.

Fig. 4. (color online) Optical image of InAs/AlSb HEMT.
3. Results and discussion
3.1. The cryogenic characteristics of InAs/AlSb heterojunction

The cryogenic characteristics of InAs/AlSb heterojunctions are determined by the cryogenic characteristics of a 2-DEG in an InAs channel, in which the temperature dependency of mobility and the density of electrons is important.

The electron mobility μ refers to the average speed of electrons units under the influence of the electric field, which is determined by scattering mechanism. The value of μ can be obtained from the relaxation time τ, which is inversely proportional to the scattering probability P;

where m* is the effective mass of the electron.

There are three primary scatter mechanisms for mobility in the InAs channel: ionized impurity scattering, lattice vibration scattering, and interface roughness scattering.[6–8]

As the temperature decreases, the lattice vibration and the impurity ionization decrease, which results in reduced scattering rate and higher mobility. The interface roughness scattering is associated with the two-electron gas density.

Figure 5 shows Hall measurement results in the temperature range from 90 K to 300 K after the completion of material growth. It can be seen that the electron mobility increases significantly with a decrease in temperature. The electron mobility at 90 K (42560 cm2/V·s) is 2.5 times higher than its value at 300 K (16911 cm2/V·s).

Fig. 5. (color online) The electron mobility μ and ns versus temperature.

The 2-DEG is generated in the InAs channel on the side of the InAs/AlSb heterostructure It can be seen from Fig. 5 that the two-dimensional electron gas concentration ns decreases from 1.99 × 1012 cm−2 at 300 K to 1.71 × 1012 cm−2 at 90 K with a decrease in temperature. This can be explained as follows.

(i) From 300 K to 175 K, the concentration ns decreased rapidly because the ionization rate of impurities decreased significantly. Below 175 K, the concentration decreased slowly because the ionization rate of impurities cannot be significantly reduced any further.

(ii) The relationship between the InAs bandgap and temperature can be expressed by

The relationship between the AlSb gap width and temperature can be expressed by

The conduction band offset ΔEc[9] at different temperatures are shown in Table 1. It can be seen that ΔEc is almost unchanged with a decrease in temperature. Therefore, the corresponding two-dimensional electron gas concentration does not change significantly with temperature because the limiting effect on electrons is almost constant.

Table 1.

ΔEc at different temperatures.

.

It can be seen from the previous analysis that the 2-DEG concentration decreases slightly with a decrease in temperature because of the diminished ionization at cryogenic temperatures.

3.2. The cryogenic characteristics of InAs/AlSb HEMT

Electrical measurements are performed using a cryogenic probe station Lakeshore CRX-4K to connect an Agilent B1500A at room temperature (300 K) and at a cryogenic temperature (90 K). When the temperature is reduced to 90 K, the device performance is greatly improved.

Figure 6 shows that the gate leakage current is significantly reduced as the temperature decreases to 90 K and increases with the drain voltage up to 0.5 V. At room temperature, the measured gate leakage current is −59.9 mA/mm, while at 90 K the device has a 27% lower gate leakage current (−47.24 mA/mm). Here, the total leakage current Ig can be expressed by[10,11]

where Ige is the Schottky leakage current and Igh is the hole current component generated due to the impact ionization effect.

Fig. 6. (color online) Ig (Vg) with Vd ranging from 0.1 V to 0.5 V in steps of 0.1 V at 300 K (solid lines) and 90 K (dot lines).

i) The Schottky leakage current as a function of temperature is given by

where S is the area of the gate electrode, T is the thermodynamic temperature, e is electron charge, k is Boltzmann’s constant, and ϕBn is the Schottky barrier height. Here the Effective Richardson constant
Thus
It can be seen that Ige decreases with a decrease in temperature.

ii) The impact ionization effect means that the collision between carriers and lattice atoms will generate redundant electron–hole pairs because the carriers in the channel experience an increase in energy as the applied voltage is increased. Thus, it can be seen from Fig. 6 that Ig increases with the drain voltage up to 0.5 V. However, the energy of the carrier decreases with a decrease in temperature, thus Igh caused by the impact ionization effect is reduced.

Figure 7 shows the output characteristic curve Id–Vd of InAs/AlSb HEMTs measured at 90 K and 300 K for different gate voltages from −1 V to 0 V.

Fig. 7. (color online) Id (Vd) with Vg ranging from −1 V to 0 V in steps of 0.1 V at 300 K (solid lines) and 90 K (dot lines).

In the linear region, the drain current is given by

In the saturation region, the drain current is given by
where C is the equivalent capacitance per unit area, Vp is the pinch-off voltage, W is the device width, and L is the channel length. Thus Id ∝ μ.

It can be seen from Fig. 7 that the drain current increases with a decrease in the temperature because Id ∝ μ. At a Vg of 0 V, the maximum drain current of the fabricated devices is 160 mA/mm at room temperature. At 90 K, Id increases up to 181.8 mA/mm. Therefore, the required drain voltage at a cryogenic temperature is lower than that at room temperature for the same output current, and the corresponding power consumption will decrease. As shown in Fig. 7, a large leakage current is present in our device. This result is unexpected. It can be explained by considering that, sidewall etching subsequent to the mesa etching used in our fabrication process is performed using citric acid/H2O2 solution to avoid gate-to-channel conduction at the mesa edge. The large leakage current is therefore due to insufficient sidewall etching. In addition, it should be noted that gate-to-channel conduction may also result in a large leakage current due to the absence of passivation after mesa etching. This could be another possible explanation for the large leakage current in our device. Since leakage is included in the Id of the device, this parameter is also increased with a decrease in temperature because the electron mobility μ of the InAs material will be greatly improved at cryogenic temperatures.

Figure 8(a) shows the relationship between transconductance and the gate voltage for the drain voltages of 0.1 to 0.5 V at two different temperatures. It can be seen from Fig. 8(a) that the transconductance peak requires a Vg of approximately −0.62 V at 90 K and −0.46 V at 300 K.

Fig. 8. (color online) (a) gm (Vg) at 300 K and 90 K for drain voltages of 0.1 to 0.5 V. (b) Peak gm (Vd) at 300 K and 90 K.

The relationship between the transconductance and the drain voltages can be expressed by

At low drain voltages, the contribution of the 2-DEG is greater than the electron mobility, so a larger transconductance value is measured at room temperature. Figure 8(b) shows the relationship between the transconductance peaks and the drain voltage at two different temperatures. It can be seen from Fig. 8(b) that the transconductance peaks are 13.12 mS/mm at 90 K and 19.15 mS/mm at 300 K for Vd = 0.1 V. At 0.2 V, the two transconductance peaks are approximately 60 mS/mm. However, compared with wider bandgap semiconductors, the 2-DEG concentration of narrow bandgap semiconductors decreases slightly with a decrease in temperature, which causes the degradation of the transconductance at low temperatures, although a low drain voltage is not usually a concern since a minimum threshold value is maintained. This is the advantage of the InAs/AlSb HEMTs at low temperatures. The improvement in the peak gm at cryogenic temperatures is mainly at a high drain voltage. This is clearly is observed because of the enhanced electron mobility. When Vd is increased to 0.5 V, the increase in the transconductance peak gm at 90 K is 22.5%. Most applications of InAs/AlSb HEMTs are at less than 0.5 V, and the low-temperature characteristics are more advantageous in this range.

4. Conclusion

We report on studies on the room temperature and cryogenic performance of InAs/AlSb high-electron-mobility transistors (HEMTs) with a gate length of 2 μm, which exhibits significant improvement in device performance at low temperatures. It has been observed that the leakage current decreases when the temperature is reduced to 90 K. This is because of the Schottky leakage current component and the hole leakage current component caused by a decrease of the impact ionization effect. The temperature dependence of the mobility and the two-dimensional electron gas concentration in InAs/AlSb heterojunctions in the temperature range 90 K–300 K is also investigated. The weaken lattice vibration and the impurity ionization at cryogenic temperatures correspond to a reduced scattering rate and higher mobility. The two-dimensional electron gas concentration decreased slightly with a decrease in temperature due to the weakened ionization at cryogenic temperatures and the nearly constant ΔEc. Overall, it is observed that the performance of InAs/AlSb HEMTs is greatly improved at cryogenic temperatures, which is more amenable to the applications of ultra-high speed and low dissipation circuits.

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