Schottky barrier gate is one of the most important elements of the HEMTs.
Both the dimension length and placement of the gate are very critical. For high speed and high frequency applications, short gate length is desired. Decreasing gate length (Lg) can increase the electronic field under the gate so as to accelerate the transport property of channel electron.
In this study, the 1.5 μm gate length was defined by AZ 2020 photoresist, and then the remnant photoresist was removed by ICP with Ar and O2 ambient.
Beside, the wafer was dipped into the HCl:H2O (1:4) solution for 15 s to remove the negative oxidation before the gate metal deposition. Here, the multilayer gate metals Ti/Pt/Au were deposited by the e-gum system. Finally, the wafer was immersed into the ACE for 30 min to lift –off the undesired metal, and the ICP was used to clean the wafer as shown in Fig 3.5.
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Fig. 3.1 the whole wafer
Fig. 3.2 Ohmic contact formation
27
Fig. 3.3 Mesa isolation
Fig. 3.4 Atomic layer deposition (ALD) Al2O3
28
Fig. 3.5 Gate formation
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Chapter 4
Fundamentals of Electrical Characteristization
After the device fabrication, DC and RF performances of the AlGaN/GaN HEMT and MOS-HEMT were evaluated by using on-wafer measurement. For the DC measurement, the I-V characteristics were obtained by using an HP4142B Modular DC source/monitor and SUSS PA200 semi-auto probe station. The TLM method was used for determining specific contact resistance was by the 4-wires measurement. The S-parameters were measured by HP8510XF vector network analyzer using on-wafer GSG probes from Cascade MicroTech. However, evaluating the RF behaviors of a device on a wafer was a complicated process. For conventional RF measurement of a packaged device, the wafer needs to be diced and then an individual die should be mounted into a text fixture. Discriminating between the die’s and the fixture’s responses became an issue. Furthermore, fixturing die was a time-consuming process, making it impractical for high-volume screening. On-wafer RF characterization can simplify the process [26].
The method of characterization of the AlGaN/GaN HEMT and MOS-HEMT devices are stated in the following section. In this study, de-embedding which must also be performed to obtain the true RF performance of the device is also performable.
4.1 DC Characteristics Measurment [27]
Before RF performance analysis, DC measurement was performed to
30
evaluate the device characteristics, including the saturation drain current (Idss), threshold voltage (Vth), transconductance (Gm), breakdown voltage (VBK). For IDS-VDS curve, the drain voltage sweeps from 0 to 10V, and the gate voltage is from 1 to pinch-off voltage with a step of -1V. For IDS-VGS curve, the gate voltage sweeps from 6V to pinch-off voltage such as -8V for MOS-HEMT and -6 for Schottky-gate HEMT, and the drain voltage is from 4 to 15V. The measured breakdown voltage in this study is off-state breakdown voltage. The gate bias is pinch-off voltage, and the drain bias sweep from 0 to a specific value.
4.2 TLM Method
The specific contact resistance between contact metal and cap layer can be extracted by the TLM method [28]. The TLM pattern, as illustrated in Fig. 4.1, was designed in the process control monitor (PCM). In this particular approach, a linear array of contacts pad is fabricated with various spacing in between them.
The distances between TLM electrodes are 3, 5, 10, 20, and 36 μm, respectively.
The resistance between the two adjacent electrodes can be plotted as a function of the space between electrodes and is expressed by the following equation
R = 2Rc +Rs L/ W , (4-11) where R is measured resistance, RC is contact resistance, RS is sheet resistance of channel region, W is electrode width, and L is the space between electrodes. As Fig. 4.2 shows, extrapolating the data to L=0, one can calculate a value for the term RC. And the specific contact resistance ρC can be further extracted by the following formula.
RS
R W C
2
= 2
ρ (4-12)
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4.3 Linearity
Linearity of amplifiers is often assessed by the third-order intercept point (IP3). If an amplifier is presented with two signals closely spaced in frequency, and a perfectly linear amplifier would simply amplify the two signals. However, the real amplifier is never with perfectly linearity, and nonlinearity will result in additional output signals. A nonlinear amplifier will have a transfer function that can be approximated as：
Po = a1Pin + a2P2in + a3P3in + … (4-13) where Pin and Po are the input and output power, and ai are coefficients. A linear amplifier would have ai =0 for i >1. Consider an input signal with two closely spaced frequencies, f1 and f2：
Pin = P1sin(2πf1t) + P2sin(2πf2t) (4-14) If Eq. (4-14) were substituted into Eq. (4-13), we can use elementary algebra and trigonometric identities to show that the output power (Po) contains the following components：
t
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Assuming P1 = P2, second-order product power is proportional to the square of the input signal power, third-order product power is proportional to the cube of the input signal power, and so on. But only the odd and greater than third-order terms have greater attribution to the fundamental signal. So we usually consider the fundamental signal and the third-order product signal only.
Fig. 4.3 is the output power diagram of the fundamental and the third-order product signals. From Fig. 4.3, we can identify the third-order intercept point (IP3). The Pin value of IP3 is also called IIP3, which is important for low noise amplifier. From the fundamental diagram of microwave front-end device (Fig.
4.4), the low noise amplifier is used to receive signals. So a higher IIP3 value results in a higher linearity of the amplifier, and the less distortion of the input signals.
4.4 Breakdown Voltage (BV
gd)
Breakdown mechanisms and models have been discussed in many articles.
One of the models showing it is dominated by the thermionic filed emission (TFE) / tunneling current from the Schottky gate. This model predicts that the two-terminal breakdown voltage is lower at higher temperature because tunneling current increases with the temperature. Higher tunneling current occurs at higher temperature because carriers have higher energy to overcome the Schottky barrier. Other model suggests that impact-ionization determines the
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final two-terminal breakdown voltage, because the avalanche current decreases with increasing temperature. Lower avalanche current occurs at higher temperature because phonon vibrations as well as carrier-carrier scattering increase with increasing temperature. Either model is incomplete since coupling exists between TFE and impact ionization mechanisms. In addition, different devices may suffer from different breakdown mechanisms, depending on the details of the device design (insulator thickness, recess, channel composition, and so forth). In this study, the gate-to-drain breakdown voltage BVgd is defined as the gate-to-drain voltage when the gate current is 1mA/mm.
4.5 Extrinsic Transconductance (g
m)
The transconductance of the HEMTs indicates the ability of the gate voltage on the control of the drain current. It can be defined as：
(4-15) where the vsat is the electron velocity of the “two dimensional electron gas”
(2-DEG).
The measurement requires specification of the initial gate voltage, the gate voltage step, and the drain voltage at which the measurement is made. Because of the nonlinear behavior of source-drain current as a function of gate voltage, gm typically will become less as the bias approaches pinch-off approaches. This also means that a smaller voltage step will yield a higher transconductance. The extrinsic transconductance is a function of the total gate width of the device, so the width must also be given. Besides, gm may also be normalized to a unit gate
sat
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width, usually mS/mm.
4.6 Scattering Parameters [3-2]
Generally, the Scattering parameters, which referred to as S-parameters, are
fundamental to microwave measurement. S-parameters are a way of specifying return loss and insertion loss or insertion gain. Fig. 4.5 shows the equivalent two-port network schematic at high frequency. The relation of the microwave signals and s-parameters are defined as follows:
⎥⎦
S signals going into or coming out of the input port are labeled by a subscript 1. Signals going into or coming out of the input port are labeled by a subscript 2. The electric field of the microwave signal going into the component ports is designated a; that leaving the ports is designated b. Therefore,
a1 is the electric field of the microwave signal entering the component input.
b1 is the electric field of the microwave signal leaving the component input.
a2 is the electric field of the microwave signal entering the component output.
b2 is the electric field of the microwave signal leaving the component output.
By definition, then,
0
35
Consequently, s11 is the electric field leaving the input divided by the electric field entering the input, under the condition that no signal enters the output. Because b1 and a1 are electric fields, their ratio is a reflection coefficient.
Similarly, s21 is the electric field leaving the output divided by the electric field entering the input, when no signal enters the output. Therefore, s21 is a transmission coefficient and is related to the insertion loss or the gain of the device. s22 is similar to s11, but looks in the other direction into the device.
4.7 Current-Gain Cutoff Frequency (f
T) and Maximum Oscillation Frequency ( f
max)
The intrinsic device model for the HEMT device is shown in Fig. 4.6. If we only consider the intrinsic part, the current can be expressed as：
'
36
fmax can be obtained by using unilateral gain：
d
If we further consider gate resistance Rg, ohmic contact resistance Rs and Rd, then the small signal equivalent circuit is shown as Fig. 4.7.
assume (ωCgsRi)2 <<1
37 Transfer y parameter into Z parameter：
s
fT and fmax are parameters often used to indicate the high frequency capability of the transistors.
38
Fig. 4.1The Transmission Line Method (TLM ) pattern.
Fig. 4.2 The illustration of utilizing TLM to measure ohmic contact resistance
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-30 -25 -20 -15 -10 -5 0 5 10 15
-120 -100 -80 -60 -40 -20 0 20 40 60
3rd ORDER PRODUCTS FUNDAMENTALS
3rd ORDER INTERCEPT POINT OP1dB
OIP3
Output Power (dBm)
Input Power (dBm)
Fig. 4.3 Output power diagram of fundamental andthird-order product signals.
Duplexer Antenna
fRF
fIF
fLO
LNA Filter Mixer IF amplifier
Signal process
circuit
Modulator PA Pre-amplifier
LO
Fig. 4.4 Fundamental diagram of the microwave front-end device.
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Fig. 4.5 The equivalent two-port network schematic at high frequency.
G ' D '
Fig. 4.6 AlGaN/GaN HEMT intrinsic device model.
G D
Fig. 4.7 AlGaN/GaN HEMT small signal equivalent circuit.
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Chapter 5
Study of AlGaN/GaN MOS-HEMTs on Silicon substrate with Al
2O
3Gate Insulator for Device Linearity Improvement
As shown in chapter 2, device performance of conventional Schottky-gate AlGaN/GaN HEMTs suffers from high gate leakage current. Also, the drain current collapses occurs when operating at high bias voltage resulting in poor long-term reliability of the Schottky gate. To improve the leakage characteristics of the conventional Schottky-gate AlGaN/GaN HEMTs devices, the fabrication and characterization of the 1.5-μm AlGaN/GaN MOS-HEMTs with 10 nm Al2O3 high-k gate oxide grown by ALD on Si substrate was investigated. Compared to regular HEMT devices of similar geometry, little degradation of the drain current and gate control ability was observed. The result indicates that AlGaN/GaN MOS-HEMTs were the gate leakage currents several orders of magnitude lower than those of regular HEMTs, and exhibit better linearity, higher channel saturation current with improved higher power performance.
5.1 Introduction
Recently, with the rapid development of wireless communication system, the transmission speeds of next-generation wireless mobile networks, including Mobile Worldwide Interoperability for Microwave Access (WiMAX) and long term evolution (LTE) networks will be several tens of megabits per second.
Higher speeds will require increased output power, leading to increased power
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consumption by transmission amplifiers, so base stations will require significantly higher power and more physical space. Therefore, there is a need to develop compact base stations that offer easy implementation and low operation costs. To make possible a small base station with lower power consumption, high-efficiency power amplifiers are currently being developed using gallium nitride high electron mobility transistors (GaN-HEMTs). The superior properties of AlGaN/GaN HEMTs promising contenders for high-power, high-temperature, high-breakdown, and high-frequency applications.
However, one of the key problems limiting the performance and reliability of AlGaN/GaN HEMTs for high-power RF applications is the high Schottky-gate leakage current, which results in the degradation of DC/RF parameters. At positive gate bias, high forward gate current can shunt the gate-channel capacitance, thus limiting the maximum drain current. At negative gate bias, high voltage drops between the gate and drain resulting in premature breakdown and the maximum applied drain voltage is restricted. Besides, gate leakage current increase resulting in the device sub-threshold currents, which decrease the achievable amplitude of RF output. All these limitations become the most important key factors to be solved for the development of the advanced wireless communication system.
To overcome this problem, several groups have been trying to integrate the MOS structure into conventional Schottky-gate HEMT by looking for proper gate insulators for AlGaN/GaN based HEMT. Al2O3 has been used as the gate insulator to reduce the gate leakage, which allows the application of high positive gate voltage to further increase the sheet electron density in 2D channel.
It also offers additional benefits of a wide band gap (about 8.7eV), high breakdown electric field (5~20 MV/cm), high thermal stability (amorphous up to
43
at least 1000ºC) and chemical stability compared to AlGaN. With well-controlled thickness and uniform Al2O3 layer deposited by ALD technology which employs surface saturation reaction technique, ALD Al2O3 is the leading candidate for the gate insulators in MOS-HEMT device.
On the other hand, in the advanced wireless communication system, multichannel transmissions are extensively used to transmit signals. As transiting signals, there are many operating frequencies with the neighboring frequencies located closely to each other, so it is important to consider that the device used in the communication system would not induce signal distortions.
However, among all intermodulation distortions, third-order intermodulation distortion (IM3) can’t be filtered out by the filter; therefore, IM3 dominates the linearity performance of the device and is the most important linearity criteria for wireless communication system [29]. Therefore, in this research, we study the linearity characteristics of the Al2O3 AlGaN/GaN MOS-HEMTs on Si substrates, and compare it with the regular AlGaN/GaN HEMTs devices for device linearity improvement in this study.
5.2 Device Fabrication
The AlGaN/GaN HEMTs structure was grown on Si substrate using MOCVD technology. Electron mobility of 1600 cm2V-1s-1 and sheet carrier density of 1 1013 cm-2 were measured by hall measurement. The device processing started with ohmic contact formation. Ohmic metal Ti/Al/Ni/Au was evaporated by e-gun system, and then annealed at 800 for 1min in N℃ 2. The spacing between source and drain is 5μm. After Ohmic contact fabrication, mesa
44
isolation was attained through dry etch process was controlled by inductive couple plasma (ICP) with Cl2 in Ar ambient. A 10nm amorphous Al2O3 oxide layer was deposited onto the wafer by atomic layer deposition (ALD) at 300ºC prior to the gate formation. The ALD technique allows high-quality ultra-thin material deposition with atomic layer accuracy. After gate photolithography, a Ti/Pt/Au electrode was evaporated. A schematic comparison between HEMT and MOS-HEMT are fabrication as shown in Fig. 5.1.
5.3 Results and Discussion
Ohmic contact with contact resistance of 2.8×10-6 (Ohm-cm2) was evaluated by TLM method. Fig. 5.2 shows the typical output current-voltage (I-V) characteristics of the 1.5μm gate length AlGaN/GaN HEMT and Al2O3 MOS-HEMT. The Schottky-gate device has a maximum drain current of 404 mA/mm at VGS = 0, while the MOS-HEMT devices have 544.2 mA/mm drain currents, respectively. Besides, the HEMTs and MOS-HEMTs were completely pinch-off at a gate voltage of -5 and -6.7V, respectively. The negative shift in the Vth was attributed to the decrease gate barrier capacitance. The experimental Vth for both HEMTs and MOS-HEMTs were in good agreement with the values obtained from Eq. (5-1), neglecting the residual doping in the AlGaN barrier layer [6]:
= ⋅
Cb s th
V en (5-1) Where e is the electronic charge, ns is the sheet charge density and Cb is the total unit area capacitance of the barrier layer and dielectric.
Fig. 5.3 shows the IDS versus VGS curves of HEMT and MOS-HEMT devices. From a comparison of these device performances, it can be seen that the
45
HEMTs have lower IDS of 747 mA/mm at VGS = 3.6 V, but for MOS-HEMTs, its reaches 880 mA/mm at 6 V gate bias. In this sense, the good quality of both Al2O3 insulator and Al2O3/HEMT interface has rendered a higher applicable gate bias, which result in a higher driving current capacity of MOS-HEMTs compared to HEMTs. Moreover, the drain current at the same gate bias is also higher for MOS-HEMT. This difference arises, thereby making the MOS-HEMT channel depletion for the same gate voltage smaller than that for the HEMT. In Fig. 5.4, a slight transconductance decrease in MOS-HEMTs compared to HEMTs from 171 to 132 mS/mm was observed, which is consistent with a further separation between the control gate and the 2-DEG channel with the presence of an additional Al2O3 layer in MOS-HEMTs. However, due to the high dielectric constant of Al2O3, the degradation in gm,max of MOS-HEMT is only 22.8% relative to that of HEMT, much better than the serve transconductance deterioration in MOS-HEMTs using low-k gate dielectrics such as SiO2 (27.2%), Si3N4 (35.7%). This is in agreement with an estimated reduction of 20% by (5-1), assuming drift velocity saturation (at Lg = 1.5μm) with Vsat = 5x106 cm/s. In additional, the gate voltage swing (GVS), defined as the 10% drop from the gm,max increase from 0.3V for HEMTs to 3.1 V for MOS-HEMTs. The larger GVS suggests a better linear behavior for MOS-HEMTs compared to Schottky-gate HEMTs, from which a smaller intermodulation distortion, a smaller phase noise and a larger dynamic range could be expected, thus desirable for practical amplifier application.
Fig. 5.5 shows the gate leakage performance of the both HEMTs and MOS-HEMTs with the same device dimensions, from which the leakage current of MOS-HEMTs is found to be significantly lower than that of the Schottky-gate HEMTs. The gate leakage current density of MOS-HEMTs is almost 3 orders of
46
magnitude lower than that of the HEMTs. Such a low gate leakage current should be attributed to the large band offsets in the Al2O3/HEMT and a good quality of both the reactive-sputtered Al2O3 dielectric and the Al2O3/HEMT interface. This leads to an increase of the two terminal reverse breakdown voltage (about 25%) and of the forward breakdown voltage (about 30%). This confirms that the Al2O3 dielectric thin film acts as an efficient gate insulator. To investigate the breakdown behavior of Al2O3-insulated gate device, the off-state three-terminal drain-source breakdown characteristics of the HEMT and Al2O3 MOS-HEMT were measured, the results are as shown in Fig. 5.6; the devices were measured at gate voltage Vgs of -8V. The breakdown voltage BVDS is defined as the drain voltage at a gate current of 1ma/mm, which is consistent with the rapidly increased currents caused by avalanche breakdown. The Al2O3 MOS-HEMT with 1.5μm gate length shows a higher breakdown voltage, while the conventional HEMT. The high breakdown voltage is related to the utilization of the Al2O3 gate insulator to reduce the leakage current.
Fig 5.7 shows IDS vs. VGS transfer curves for Al2O3-insulated gate and Schottky-gate AlGaN/GaN HEMTs with the different drain voltages from 4 to 7 V. With increasing the drain voltage, both of the HEMT and MOS-HEMT devices have higher maximum drain current, except the HEMT at VDS is 7 V. In addition, at forward gate bias beyond +2V, high drain current drops for the Schottky-gate HEMT was observed as compare to the MOS-HEMT. This is
Fig 5.7 shows IDS vs. VGS transfer curves for Al2O3-insulated gate and Schottky-gate AlGaN/GaN HEMTs with the different drain voltages from 4 to 7 V. With increasing the drain voltage, both of the HEMT and MOS-HEMT devices have higher maximum drain current, except the HEMT at VDS is 7 V. In addition, at forward gate bias beyond +2V, high drain current drops for the Schottky-gate HEMT was observed as compare to the MOS-HEMT. This is