1. Introduction

The range of applications associated with the submillimeter wave and terahertz bands (300 GHz–3 THz) is very extensive, such as spectroscopy, imaging and communications^[1]. However, it has historically been extremely difficult to access this band due to a lack of high frequency transistors with bandwidths of above 300 GHz. The bandwidth of InP-based transistors, such as InP HBTs and InP HEMTs, has increased rapidly in recent years. To date, InP HBTs and InP HEMTs have both been demonstrated with maximum oscillation frequency exceeding 1 THz^{[2, 3]}. Compared with InP HEMTs, InP HBTs have some key advantages in submillimeter and THz applications, such as a high breakdown voltage, high threshold uniformity, low 1/$f$ noise, and high digital speed ^[1], which make them very promising for submillimeter and future THz electronics.

For submillimeter and THz applications, the circuits operate near the cutoff frequency of the DHBT, where the power gain of the DHBT decreases sharply with increasing frequency, so the power gain is very precious^[4]. InP DHBT is commonly used in common-emitter configuration. However, common-base DHBT yields a much higher gain than common-emitter DHBT at a high frequency, especially near the cutoff frequency, because the maximum stable gain (MSG) of the common-base DHBT extends into higher frequencies than that of a common-emitter DHBT^[5]. In addition, to increase the output power, the breakdown voltage, the current and the emitter area should be as high as possible. Considering these above factors, a common-base four-finger InGaAs/InP DHBT has been demonstrated in this paper. The InGaAs/InP DHBTs were fabricated with a triple mesa process and a benzocyclobutene (BCB) planarization technique. All processes were carried out on 3-inch wafers. The area of each emitter finger is 1 $\times$ 15 $\mu$m$^{2}$. The maximum oscillation frequency ($f_{\rm max})$ is 325 GHz and the breakdown voltage BV$_{\rm CBO}$ is 10.6 V.

2. Growth and fabrication

The layer structure of the InGaAs/InP DHBTs was grown by molecular-beam epitaxy on a 3-inch semi-insulating InP substrate. The layer sequence is shown in Fig. 1. The DHBT structure includes an InGaAs cap layer (200 nm, 3 $\times$ 10$^{19}$ cm$^{-3})$, an InP emitter (200 nm, 2 $\times$ 10$^{17}$ cm$^{-3})$, a carbon-doped InGaAs base (50 nm, 3 $\times$ 10$^{19}$ cm$^{-3})$ and a compositionally step-graded InGaAs/InGaAsP/InP collector (200 nm, 1 $\times$ 10$^{16}$ cm$^{-3})$. A composite collector with an InGaAs spacer and an InGaAsP quaternary layer was used to eliminate the conduction band spike at the B–C interface and thus the collector current blocking effect was minimized^[6].

The geometry parameters of the devices are similar to those of Ref. [4]. In contrast to most recent reports in China^[7-9], the InP DHBTs in this work were designed and fabricated with standard manufacturing techniques such as i-line stepper lithography, self-aligned contact and selective dry/wet etching, etc. All InP DHBT processes were on 3-inch wafers. The InP DHBTs were fabricated with conventional wet etching and metal deposition with triple mesa design. Non-alloyed ohmic Ti/Pt/Au was used as the n-type ohmic contact and Pt/Ti/Pt/Au was used as the p-type contact. After device isolation, BCB was used for device passivation and planarization. Subsequently, an RIE etch-back step was performed to expose the tops of the device contacts and then the first-level metal was deposited to form the probe pads.

3. Measurements and results

The InP DHBTs were measured on-wafer at room temperature. The DC characteristics of the DHBTs were measured by an Agilent 1500A semiconductor parameter analyzer. The common-base $I$–$V$ characteristics of the DHBT with an emitter area of 4 $\times$ 1 $\times$ 15 $\mu$m$^{2}$ are shown in Fig. 2. The offset voltage is about $-0.6$ V. The common-base breakdown voltage is 10.6 V which is defined at an emitter current density of $J_{\rm e}$ $=$ 10 $\mu$A/$\mu$m$^{2}$ and the common-base breakdown voltage is higher than that of a DHBT in common emitter configuration. As shown in Fig. 2, the knee voltage increases from $-0.5$ to 0.1 V as the emitter current increases from 2 to 100 mA. The small knee voltage and sharp current rising indicate that the current blocking effect is successfully suppressed with the composite collector^[10].

The microwave performance of the fabricated InP DHBTs was characterized by on wafer $S$-parameter measurements from 100 MHz to 40 GHz using an HP8510C network analyzer. On-wafer open and short pad structures identical to those used by the devices were used to de-embed the pad parasitics. There is no current gain cutoff frequency $f_{\rm t}$ for the InP DHBT in common-base configuration because the current gain is always close to but less than unity. Figure 3 shows the maximum stable gain/maximum available gain (MSG/MAG) and Mason's unilateral gain (U) as a function of frequency at the collector-base junction voltage $V_{\rm CB}$ $=$ 1.1 V and collector current $I_{\rm C}$ $=$ 60 mA. Generally, there are two methods to define the maximum oscillation frequency ($f_{\rm max})$. One is to use MSG/MAG and the other is to use U. The expression for Mason's unilateral power gain in terms of the transistor $Y$-parameters is as follows:

$$\begin{equation} U=\vert Y_{21}-Y_{12}\vert ^{2}/[4(G_{11}G_{22}-G_{21}G_{12})], \end{equation}$$

(1)

where $G_{11}$, $G_{12}$, $G_{21}$ and $G_{22}$ are the real parts of the networks $Y$-parameters. For the first method, MSG rolls off at $-10$ dB/decade, while MAG has no fixed slope, so $f_{\rm max}$ can't be accurately extrapolated by MSG/MAG^[11]. Unlike MAG, $U$ has a constant slop of $-20$ dB/decade^[11], so $f_{\rm max}$ was obtained by extrapolation of $U$ with $-20$ dB/decade slope line in this paper.

4. Conclusion

In summary, a common-base four-finger InGaAs/InP DHBT has been successfully demonstrated with standard manufacturing techniques on 3-inch wafers. The area of each finger is 1 $\times$ 15 $\mu$m$^{2}$. The maximum oscillation frequency is 325 GHz and the breakdown voltage BV$_{\rm CBO}$ is 10.6 V, which are to our knowledge both the highest $f_{\rm max}$ and BV$_{\rm CBO}$ ever reported for InGaAs/InP DHBTs in China. The high speed InGaAs/InP DHBT with a high breakdown voltage is very suitable for submillimeter-wave and THz electronics.