Designing GaN HEMT for Receiving Devices
In the past few years, the main focus of developers and manufacturers of electronic components for ground and space wireless communications has been directed at the research and use of AlGaN/GaN heterostructure-based devices. Such interest in devices of this type of material is due to the record values of breakdown voltage, specific output power, electron saturation rate and sufficient thermal conductivity of SiC substrates [1,2]. The above characteristics make it possible to create microwave transistors with specific output power about 10W/mm (at f = 40GHz) and more than 2W/mm (at f = 80.5GHz), which is several times greater than the values achieved with Si, GaAs, InP and other materials [3,4,5]. High breakdown voltage level allows achieving record values of specific output power, as well as developing and producing devices resistant to the effect of continuous input power by several orders of magnitude higher than the values achieved with traditional low-noise amplifiers (LNA) based on GaAs pHEMT .
Table 1 shows the characteristics of devices based on AlGaN/GaN and AlGaAs/GaAs heterostructures. These values show some advantages of GaN HEMT used as part of low-noise amplifiers.
Important parameter for electronic equipment is the resistance to external effects, especially to special factors and operating temperature limits. Microwave AlGaN/GaN HEMTs could be used in a wider operating temperature range, as their channel operating temperature could be up to 200 °C, while GaAs pHEMT channel temperature is 125 °C. Also due to the larger band-gap, GaN has better resistance to special external factors compared to GaAs. Based on the above comparison, we have concluded the feasibility of GaN HEMT use in low-noise amplifiers.
GAN HEMT BASED LNA DESIGN AND NF MEASUREMENTS
We have created microwave HEMT models in order to compare the results of simulations and measurements of GaN HEMT-based low-noise amplifiers’ noise parameters, for amplifiers developed and produced by PULSAR S&PE. Equivalent circuit (Fig. 1) has been used as a HEMT model. Equivalent circuit can be divided into two parts: external, which comprises parasitic elements connected with transistor contacts and substrate; and internal, which comprises voltage controlled current supply and heterostructure parasitic capacitances.
To create transistor model in computer-aided design software AWR Microwave Office, we have carried out equivalent circuit extraction divided into three consecutive stages:
1. Extraction of transistor current-voltage curve;
2. Extraction of equivalent circuit external part;
3. Extraction of equivalent circuit internal part.
Extraction of the CV curve has been carried out by measuring it in pulsed mode. In the course of microwave HEMT measurements, the pulse width has been experimentally chosen as 1µs. For a given pulse width, the effects of self-heating and deep level recharging are minimal. Parameters of HEMT operating point have been defined from the conducted experiments. For the extraction of external and internal parts of equivalent circuit, GaN HEMT S-parameters have been measured at different power supply modes:
1. Ugs = 0V; Uds = 0V (“cold” mode);
2. Ugs = –4V; Uds = 0V (“closed” mode);
3. –3V < Ugs < 0V; Uds = 10V (operating point).
Linear equivalent circuits have been extracted for 2 types of AlGaN/GaN HEMT (Fig. 2a,b). Gate width (Wg) of each type of HEMT is 300µm, drain-source distance being 4µm. Two types of the GaN HEMT differ in their design: HEMT of the 1st type (“U-type”-layout) has through-vias in the source region and the field plate; HEMT of the 2nd type (“fishbone”-layout) has no through-vias and no field plate.
To evaluate the developed HEMT models, we have designed test low-noise amplifier to compare its simulated and measured performance parameters. Measurements have been carried out in a signal path with 50Ohm impedance. Test LNA (Fig. 3) is a hybrid integrated circuit (HIC) with adder on Lange bridges chosen in order to decrease VSWR at devices’ input and output. Mounting of microwave HEMT dies on СuMo test-substrate has been made with silver-epoxy adhesive, and LNA IC performance parameters have been measured in pulsed mode. Measurements have been carried out at 10GHz with 100µs pulse width and Q = 10. Microwave HEMT operating point has been set at: Uds = 10V, Ids = 50mA. Noise figure for microwave HEMT of type 1 has been 3.8dB, for type 2–3.5dB. These results are in a good agreement (within 10 % error) with the results simulated in AWR Microwave Office.
In addition to performing measurements of the LNA HIC parameters under normal conditions, we have studied the behavior of noise figure and gain values at different ambient temperatures. Corresponding measurements have been carried out at the same HEMT operating points, but in continuous mode, temperature range being from 10 to 300K. The results are given in Fig. 4.
Curves show that noise figure of the 1st type of GaN HEMT is 1.15dB and of the 2nd type is 1.04dB at 10K ambient temperature. Based on the conducted research on NF and gain performance, we can conclude that through-vias and field plate in GaN HEMT structure negatively affect the parameters that are critical for LNA.
In the course of the research on the applicability of GaN HEMT in a new generation of LNAs, relevant scientific publications have been reviewed and small-signal microwave transistor model has been extracted. However, small-signal model created on the basis of S-parameters does not give a clear understanding of the value of the optimal input reflection coefficients. To address that issue, we are planning to conduct further measurements using automated impedance tuners in the signal path. The internal part of equivalent circuit needs to be refined, achieving more detailed reflection of the sources of noise in GaN HEMT, for a more accurate creation of an equivalent model in Silvaco TCAD.
Measurement results have shown that GaN HEMT LNA ICs have the same parameters as GaAs pHEMT LNA HIs, since AlGaN/GaN ICs do not require input protection circle. Transistor noise performance has been investigated at ambient temperatures down to 10K. While measuring NF and Gain parameters temperature behavior, the effect of through-vias and the field plate on the noise performance of the devices has also been investigated.
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