GaN HEMTs for Pulsed Radar Applications

The defense sector continues to grow, with various countries increasing their defense budgets and contributing to between 3% and 4% growth in 2020 to reach $1.9 trillion, according to consultancy Deloitte¹. This growth in defense spending will be a key driver for the overall RF gallium nitride (GaN) market, pushing it, according to Yole Développement, toward the $2 billion mark by 2024².

A significant part of the demand for GaN components will come from radar, both in defense and civil applications. For GaN on SiC, high-electron-mobility transistors (HEMTs) offer advantages of high gain, high switching speed, and high-power density.

These advantages are behind the current trend toward replacement of the high-power, large-bandwidth travelling wave tube (TWT) amplifiers that radar commonly uses. GaN HEMTs eliminate the inherent TWT amp limitations of relatively short operating life due to cathode exhaustion, risk of TWT damage at turn-on after extended storage, and lower mean time between failure (MTBF) because of all components in the tube being potential single points of hard failure.

Key consideration

At the heart of most GaN HEMT-based radar designs is a pulsed amplifier that uses a pulsed RF signal, with the transistor being powered by a DC bias. Pulsing the DC bias as well for some applications reduces both interference and power consumption. Pulsed operation, as opposed to continuous wave (CW) mode, has the advantage of reducing external DC power needs and heat generation.

The main challenges with such operation are achieving high current on the drain side along with short switching times, both of which can be achieved with GaN devices.

However, GaN HEMTs, such as those from Wolfspeed, do require careful consideration in terms of biasing. They are depletion-mode components that are normally on when the gate-to-source voltage is zero. With that in mind, a negative voltage must first be applied to the gate to turn it off.

Switching for drain pulsing

To achieve drain pulsing, a P-channel or N-channel pass MOSFET can be used in a load-switch configuration. When using a P-channel MOSFET (Figure 1A), the source is connected to the input voltage rail and the drain to the load. To turn the switch on, the gate-source voltage V_GS must be greater than the threshold voltage V_TH. In other words:

V_IN ≥ V_GS + V_TH

When using an N-channel MOSFET, the drain is connected to the input voltage rail and the source to the load. The output voltage is the voltage across the load. To turn the switch on, the gate-source voltage V_GS must be greater than V_TH. In other words:

V_GS ≥ V_OUT + V_TH

To achieve this, the N-channel load switch requires a second voltage rail to control the gate (Figure 1B). The input voltage rail can be considered independent of the pass transistor and the N-channel load switch can be used for very low input voltage rails or for higher voltage rails as long as V_GS remains higher than V_TH.

When the switching time is not critical, the P-channel MOSFET offers simplicity of circuit design.

Pulsing Circuit

The following circuit (Figure 2) achieves a 1.9-μs turn-on time to deliver up to 20 A on the drain side using the simpler P-channel load switch.

With the 100-V P-channel MOSFET SQM120P10, the following specification is achieved:

• V_DS (V) = –100 V
• R_DS(ON)(Ω) = 0.0101 @ V_GS = –10 V
• R_DS(ON)(Ω) = 0.0150 @ V_GS = –4.5 V
• Drain Current (I_D) = –120 A

The inputs and outputs of the circuit are:

• DC_IN: connected to the DC power supply (50 V)
• PA_DRAIN: connected to the drain of the RF transistor
• J1: connected to the pulse control signal
• J2: shorted to bypass Q2 when setting the desired I_DQ

The Zenner diode protects the MOSFET so that Q2 V_GS does not exceed 14 V.

R2 and R5 form a voltage divider that provides the control voltage for the Q2 gate. A voltage >2 V at Pin 1 of J1 turns the N-channel FET Q4 on, and the V_GS for Q2 changes from 0 V to –8 V, thereby turning it on. The output voltage then becomes:

PA_DRAIN = DC_IN – (R_DS(ON)× I_D)

When the pulsing input signal at Pin 1 of J1 is low (>0.7 V), Q4 is off and Q2 V_GS = 0, Q2 is off, and output voltage at PA_DRAIN is 0 V.

Selection of R2 and R5 values is based on the current through the divider and the MOSFET gate control voltage. For high-current applications, R2 and R5 values can be selected to set Q2 V_GS such that it reduces Q2 turn-on resistance RDS(ON). This may, however, delay turn-off.

Setting V_GS at –8 V at turn-on allows for a 28-V to –50-V wide DC range. In this range, V_GS ranges from –4.5 V to –8 V.

During testing at Wolfspeed, the drain current, I_D, was 18 A and R_DS(ON)≈ 0.01 Ω, resulting in an output voltage of 49.82 V, which is close to DC_IN (50 V). With a 1-ms 10% pulse test signal, a turn-on time of 1.88 μs was observed when the output DC voltage reached 90% of the DC_IN.

Wolfspeed for GaN applications

As the defense sector consolidates several RF functions like radar, electronic support, electronic attack, and communications within shared electronics and antenna apertures using active electronically scanned array (AESA) technology³, the GaN HEMT advantages of size, weight, power, and robustness will become even more compelling.

And Wolfspeed, with over two decades of experience in this area starting with the industry’s first GaN-on-SiC HEMT, has many reference designs as well as expertise to offer. Contact us to find out more.

References

1. Deloitte, 2020 Global Aerospace and Defense Industry Outlook https://www2.deloitte.com/content/dam/Deloitte/global/Documents/Manufacturing/gx-global-outlook-ad-2020.pdf

2. Yole Développement, "RF GaN Market: Applications, Players, Technology And Substrates 2019" https://www.i-micronews.com/products/rf-gan-market-applications-players-technology-and-substrates-2019/

3. IHS Markit, "Top Defence Trends to Watch in 2019", https://ihsmarkit.com/research-analysis/top-defence-trends-to-watch-in-2019.html