Components of the RF setup

All the components of the RF setup except the NA and the probe station are in two bottom shelves of the big black cabinet. Tested LED samples for this dissertation are also

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in the same place. Those two shelves are dedicated to our group. The adaptor case is in the top shelf of the same cabinet. The adaptors belong to the lab.

A.5.1 RF micro-probe

GSG RF micro-probe from Cascade Microtech is used to probe the LEDs on wafer. The product number is ACP50-GSG and its -3dB bandwidth is 50 GHz. Therefore, at the range of interest for these measurements the response of the RF micro-probe is flat. However, the response of the probe is measured by the company and it is included in the product package in case if subtraction of the probe response was necessary. Figure 52 shows the configuration of the probe. The pitch size of the probe which is considered as the distance between center of one tip to the center of the adjacent tip is 150 µm. These tips are co-planar; therefore, the metal contacts on the devices are required to be co- planar.

Figure 52: GSG RF micro-probe. The pitch size is 150 µm.

A.5.2 Calibration wafer

To calibrate the port 1 of NA for measurement of S11, all the elements connected to this port including bias-tee, cables, and RF micro-probe are required to be calibrated together. S11 measurement leads to the extraction of the input impedance of the LED and

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it is extremely sensitive to calibration. The calibration wafer map shown in Figure 53 accommodates probes with pitch sizes from 50-150 µm.

Figure 53: Calibration wafer map.

There are short, open/through, and load elements on the map which can be used for calibration using the Wincal software. The information regarding the calibration wafer and the probe pitch size are required to be set in the setup menu in the Wincal software. The map is included along with the calibration wafer in the purchased package.

A.5.3 Cables

SMA cables are mainly in 3.5 mm or 2.4 mm standards. The NA can accommodate the 2.4 mm standard. However, most of the purchased elements have 3.5 mm standard which can be connected to 2.4 mm cables using adaptors. The number in the standard refers to the spacing between the core and the ground of the SMA cables which determines its frequency. 2.4 mm cable for example can go up to 67 GHz while 3.5 mm cable can go up to 18 GHz. Both types of cables are available in the lab and can be connected to each other using adaptors.

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There are two photodetectors (PDs) that were used for this dissertation. A Thorlabs Si fiber-coupled detector (DET025AFC). The fiber coupling should be multimode to ensure the high extraction of the light from the LED. The -3dB bandwidth of this PD was measured using a high-speed laser and it is around 1.65 GHz. Figure 54 shows the PD response. DET025AFC PD has some RF design issues that causes the high-frequency dip around 1 GHz. It also has a impedance mismatch problem at its output port which requires a few dB attenuator to be connected to its output SMA port to reduce its impedance, otherwise the detector bandwidth would reduce drastically.

The other PD is 4.5 GHz GaAs-based PD New Focus high-speed receiver 1591. This receiver has a built-in RF amplifier which it is an optimum design for detection- amplification of a signal due to being shielded against noise, and lack of mismatch issue at its ports. However, the responsivity of this PD at wavelength of 450 nm is one third of the Si PD. Therefore, for high-frequency and high-power measurements of InGaN devices this PD is ideal. Its frequency response is included in the purchased package.

500 1000 1500 2000 -100 -98 -96 -94 -92 -90 -88 -86 Frequency (MHz) RF Power (dB)

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A.5.5 Amplifier

The amplifier that was used in these measurements is a 3GHz, 34 dB gain, low-noise amplifier from Pasternack PE15A1009. The combination of the low-noise feature of this amplifier with the high-responsivity of the Si PD provides smooth measurements compared to the GaAs receiver for the low-level signals like in the pulsed-RF measurement. The amplifier has good DC blocking and low insertion loss features.

A.5.6 Bias-tee

There are three bias-tees available in the RF lab which two of them belong to our group. One is 4.2 GHz, Mini-Circuits ZFBT-4R2GW-FT+. For higher frequency measurements such as lasers, the bias-tee from Prof. Christodoulou’s lab can be used which goes up to 12 GHz (Picosecond Pulse Lab bias-tee). For pulsed-RF measurements to avoid the reflection of the pulse from the CW bias-tees, a 12 GHz, 3 A current, pulsed bias-tee 8860SMF3-02 from API Technologies Corp is used. This bias-tee ensures the minimum reflection of pulses with frequencies above 5 kHz. The lower RF frequency limit of this bias-tee is 30 MHz. The pulse input of this bias-tee is coaxial standard to maintain the quality of the pulses. Figure 55 shows the image of the pulsed bias-tee.

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