Page 57 - ITU Journal Future and evolving technologies Volume 2 (2021), Issue 7 – Terahertz communications

ITU Journal on Future and Evolving Technologies, Volume 2 (2021), Issue 7

The saturated output power is plotted over the desired collector bias shifts towards class-A. The
UmmW band in Fig. 6. The highest output power is gain at 220 GHz is 7 dB higher for the CB compared
demonstrated with GaN HEMTs up to 140 GHz and to the CE stage.
InP HBTs above 140 GHz. The rapid drop in power
for GaN is due to the limited fmax of the technology.
Optimizing GaN HEMTs for high-breakdown tends
to also compromise the gate-drain capacitance
parasitics that impact millimeter-wave
performance. Recent work on N-polar GaN may
offer new device physics for millimeter-wave
operation [21]. Above 250 GHz, the InP HBT and
GaAs mHEMT stand out as the only technologies
that generate reasonable output power. SiGe has
provided competitive performance in bands
between 100 and 200 GHz [9]. Silicon technologies
have to date only offered very limited power
above 200 GHz. Recent work on Gmax-boosted
approaches has pushed the output power
towards 10 dBm with limited efficiency [22]. Fig. 7 - Common-base versus common-emitter for a constant
collector-emitter voltage for a 0.25um InP HBT process.
6. AN INP UMMW POWER AMPLIFIER The CB HBT provides higher MAG over the

To demonstrate a high-efficiency PA design above millimeter-wave band since the feedback parasitics
100 GHz, we have demonstrated circuit design in the CB amplifier are due to the collector-emitter
techniques to maximize efficiency based on the capacitance, CCE, compared to the larger collector-
250-nm InP HBT process with fmax of approximately base capacitance, CCB. Feedback current in the CB
600 GHz. As described previously, this InP HBT HBT is therefore much smaller than in the CE
process offers a PAE as high as 45% due to 1) the topology and the PA is unconditionally stable
high fmax and 2) load-line matching conditions without additional stabilization. Base inductance
close to 50 Ohms for an output power of 15 dBm. typically impacts the stability of the CB
configuration; however, the InP HBT process allows
Theoretically, the fmax is invariant to the choice of that the base can be directly connected to ground to
CE or CB configuration. While Common-Emitter (CE) eliminate any bypass capacitance requirement to
amplifiers are conventionally used in PA design, we produce an AC ground at the base node and the
compare the MAG of CE and Common-Base (CB) potential base inductance to connect to the bypass
amplifiers in the InP HBT process in Fig. 7 over a capacitor.
range of collector bias conditions at 140 GHz and The InP HBT offers a physics-based scalable model
220 GHz. Generally, we observe that the gain that allows accurate load pull simulation of the CB
remains relatively high in all cases but sharply device. A 4-finger by 4um (16 um total) CE or CB
reduces below a certain current density threshold. HBT emitter length produces a 100-Ohm load-line
This substantial reduction in gain occurs when impedance for maximum gain and efficiency. Based
the K stability factor becomes imaginary. Over a on this transistor periphery, the output power, PAE,
range of collector biases between 0.1 mA/um and and gain at peak PAE for the CE and CB amplifier are
3 mA/um (Imax), the CB provides 5dB higher than plotted at 140 GHz in Fig. 8 as a function of the
the CE transistor at both frequency bands. At quiescent collector current (normalized by length)
140 GHz, the CB provides higher gain close to the under the condition of fixed 2.5-V collector-emitter
class-B biasing condition (Ic of zero) and the gain voltage. Note that the DC current differs
increases slightly as we shift to class A. significantly from the quiescent current as the
The additional gain allows optimization for PA shifts from class-B to class-A where the ratio of
class-B operation in CB that would not be possible IDC/IQ approaches unity.
in CE.
The peak output power is 16 dBm at class A for CE
Examining the 220 GHz operation, the CB gain and drops to around 14 dBm under the class B bias.
drops 2 dB relative to 140 GHz. However, the gain The CB configuration produces similar or slightly
of the CE transistor drops substantially (4 dB) as the lower output power than the CE amplifier. Both

© International Telecommunication Union, 2021 45