Toward THz transistor: Pseudomorphic Heterojunction Bipolar Transistors (PHBT)

Terahertz (THz) communication systems require the development of ultra-fast transistor technologies for VCO sources with power amplification (>50mW) and low noise receiver (NF<8dB). InP-based HBTs are the most promising technology for achieving THz operation based on the current gain cutoff frequency versus breakdown voltage trend for various transistor technologies and material systems¹. It is the only known transistor technology can deliver THz bandwidth with output breakdown voltage greater than 1V, hence, a potential higher power transistor amplifier can be realized. Earlier work on InP/InGaAs SHBTs has demonstrated f_T values of 550 GHz with a current density of 20 mA/μm², corresponding to junction temperatures beyond 200 °C. Further vertical scaling of the SHBT toward THz frequencies will require current densities exceeding 100 mA/μm², which is unacceptable as junction temperatures would surpass 500 °C. Recently, we proposed and experimentally demonstrated that pseudomorphically grading the collector of an SHBT showed an increase in device speed, with a reported f _T =604 GHz at a reduced current density and junction temperature ². Lately, we further optimize the PHBT layer structure by implementing a 21% indium compositional grade from the base to the subcollector of the PHBT. A 25% increase in the effective electron velocity in the collector is estimated when compared to a lattice matched In_0.53Ga _0.47As collector, resulting from the increased overshoot distances due to the larger Γ-L spacing in the graded collector and the increased mobility of the indium-rich crystal. The base thickness is reduced to 12.5 nm, implementing both compositional and dopant grading, and aggressive lateral scaling was employed to take advantage of the short transfer length of the base contact (L_T < 0.1 μm) to achieve cutoff frequencies above 700 GHz. (Figure Presented) A scanning electron micrograph (SEM) PHBT cross section is shown in Fig. 1. Cross section of a vertically scaled pseudomorphic heterojunction bipolar transistor with a base-collector mesa width of 0.55 ,tm and an emitter width of 0.25 μm. The collector-emitter breakdown, determined when I_C = 50 μA, is measured at BV_CEO = 1.75 V, and the DC gain β is 115. High frequency measurements were performed at room temperature using an off-wafer short-open-load-through (SOLT) calibration on an HP8510C vector network analyzer (VNA) from DC to 50 GHz. Open and short on-wafer standards were subsequently de-embedded from the measured devices to remove capacitances and inductances associated with the pads. All high-frequency measurements were verified on an Agilent E8364A PNA with nearidentical results. A plot showing h₂₁, Mason's U, and the maximum stable gain/maximum available gain (MSG/MAG) is shown in Fig. 2 for a 0.25 x 3 μm² PHBT. (Graph Presented) Cutoff frequencies f_T = 710 GHz and f _MAX = 340 GHz were determined by extrapolating the -20 db/decade roll-offs of h2l and U (inset), and are in excellent agreement with the single-pole transfer function represented by the dashed lines in the Fig. 2. Peak f_T is achieved at I_C = 15.3 mA (J_C = 20 mA/μm²) and a power dissipation of P_diss = 14 mW (V_CE = 0.92 V, ΔT_j = 155 °C). Because of the high current density necessary to achieve peak f_T, the associated f_MAX is considerably lower than at lesser current densities; operating the same device at I_C = 5.5 mA (J_C = 7.5 mA/μm², ΔT_j, = 55 °C), the cutoff frequencies are more balanced at 540/407 GHz for f_T/f_MAX. While this higher f_MAX value is more desirable for high-speed analog circuit design, the remainder of this letter will investigate the device operation at peak f_T with the intent of addressing issues to enable THz-bandwidth operation.