Terahertz Emission from Collapsing Field Domains during Switching of a Gallium Arsenide Bipolar Transistor

Broadband pulsed THz emission with peak power in the sub-mW range has been observed experimentally during avalanche switching in a gallium arsenide bipolar junction transistor at room temperature, while significantly higher total generated power is predicted in simulations. The emission is attributed to very fast oscillations in the conductivity current across the switching channels, which appear as a result of temporal evolution of the field domains generated in highly dense electron-hole plasma. This plasma is formed in turn by powerful impact ionization in multiple field domains of ultrahigh amplitude.

Figure 1

(a) measured (curves 1,2) and simulated ($1^{\prime }$, $2^{\prime }$) voltage and current waveforms across the collector contact, and current across the switching channels (curve $3^{\prime }$ in the inset); (b) time-resolved spectrum of near-infrared (optical) emission from the switching channels recorded from the gap in the emitter and base metallization using a spectrograph-equipped streak camera.

Figure 2

Electric field (curves 1–8) and donor density (9) profiles along the switching channels. The field profiles correspond to transient instants in Fig. 1: 1–2.78 ns, 2–3.179, 3–3.579, 4–3.979, 5–3.980, 6–3.981, 7–3.982, 8–3.983.

Figure 3

Measured and simulated peak power in the THz range. (a) schematic presentation of the transistor chip and experimental setup; the emitter and base metal contacts of size $\mathit{d}$ provide undesirable shielding of the emission source from the bolometer. (b)-oscillograms of Si bolometer responses scaled by the known bolometer responsivity ($4.4\mathrm{nW}/\mathrm{mV}$); the curves correspond to different bands of the band-pass filters used in the experiment: 1–$25\mathrm{GHz}<f<0.3\mathrm{THz}$; 2–$25\mathrm{GHz}<f<1\mathrm{THz}$; 3–$25\mathrm{GHz}<f<9\mathrm{THz}$; 4–$25\mathrm{GHz}<f<25\mathrm{THz}$. (c) measured (1,2) and simulated (3–5) peak power for a “low voltage” transistor: (1) and (3) correspond to the spectral band $f<0.3\mathrm{THz}$, (2) and (4) to $f<1\mathrm{THz}$, and (5) to $f<3\mathrm{THz}$. Curves 6–7 show the simulated emission power in different spectral bands for a “high-voltage” (300 V) transistor analogous to that in Refs. 4, 5: (6) corresponds to band $f<0.3\mathrm{THz}$, (7) to $f<1\mathrm{THz}$ and (8) to $f<3\mathrm{THz}$. (d) simulated spectra for a “low voltage” transistor (black) averaged over the transient time interval 3.1–6.3 ns and for a “high-voltage” transistor (gray) averaged over the interval 3–5.1 ns.