# High Linearity DC-38 GHz CMOS SPDT Switch
###### Abstract  
In this paper, we demonstrate a low-loss and high-linearity DC-38 GHz CMOS SPDT switch for 5G multi-band communications in 0.18 \(\upmu\)m CMOS. Traveling-wave matching (CLCL network) is used for the output-port (ports 2 and 3) matching and isolation enhancement, while \(\uppi\)-matching (CLC matching) is adopted for the input-port (port 1) matching. Positive/negative gate-bias is adopted for linearity enhancement because larger \(\mathrm{P_{in}}\) (i.e., AC signal with larger negative \(\mathrm{V_{in}}\)) is required to conduct the off-state series switch transistor. Negative-body bias is used for insertion-loss reduction because the off-state series switch transistor is closer to an open state. The SPDT switch achieves insertion loss of 0.4-1.4 dB, 3.6-4.3 dB, and 4.5-5.9 dB, respectively, for DC-6 GHz, 21-29 GHz, and 31-38 GHz. Moreover, the SPDT switch achieves isolation of 37.5-59.4 dB, 25.7-28.7 dB, and 24.3-25.2 dB, respectively, for DC-6 GHz, 21-29 GHz, and 31-38 GHz. At 28 GHz, the SPDT switch achieves remarkable input 1-dB compression point (\(\mathrm{IP_{1 dB}}\)) of 25.6 dBm, close to the simulated one (28 dBm). To the authors' knowledge, this is one of the best \(\mathrm{IP_{1 dB}}\) results ever reported for millimeter-wave (mm-wave) SPDT switches. CMOS; millimeter-wave; SPDT; switch +
Footnote †: journal: Applied Sciences  
On-chip SPDT switch normally exhibits decent insertion loss, isolation, and power-handling capability (in the transmit mode). For the SPDT switch in Figure 1b, in the condition of port-1-to-port-2 (P1-to-P2) being on and port-1-to-port-3 (P1-to-P3) being off, the signals from the transmitter (at port 2) are sent to the antenna (at port 1) for transmit. Decent P1-to-P3 isolation (antenna-to-RX isolation) and P2-to-P3 isolation (TX-to-RX isolation) (i.e., small S31 and S52) are required. Moreover, in the condition of P1-to-P2 being off and P1-to-P3 being on, the signals received by the antenna (at port 1) are sent to the receiver (at port 3). Decent P1-to-P2 isolation (antenna-to-TX isolation) and P2-to-P3 isolation (i.e., small S21 and S32) are required. Overall, in addition to small chip area, the basic requirements for an SPDT switch include small input return loss (for port-1 and the on-state port, and almost total reflection for the off-state port), low insertion loss, good isolation, and decent power handling capability (normally evaluated by the input 1-dB compression (IP1 dB)) over the band of interest. For sub-6 GHz, SPDT switch with series-shunt transistor structure is commonly used due to its low insertion loss, high isolation, good power performance, and small area [6]. This SPDT switch structure normally exhibits small input and output matching bandwidth from DC to several GHz, so it is not applicable to the mm-wave band. For the 24.25-52.6 GHz (mainly 28/39 GHz) FR2 5G band, \(\lambda\)/4-transmission-line (TL)-based SPDT switches are commonly used due to its low insertion loss, decent isolation, and good power performance [9; 10]. However, this SPDT switch has the disadvantage of occupying a relatively large chip area. To reduce the chip area, lumped-LC-based or transformer-based SPDT switches have been proposed [11]. These SPDT switch structures normally exhibit medium insertion-loss (or gain) bandwidth around the center frequency, not including the sub-6 GHz band. Clearly, the abovementioned SPDT switches are not suitable for dual band, such as the sub-6 GHz and the 28 GHz, 5G applications. A wideband SPDT switch structure, which covers the sub-6 GHz and the 28 GHz 5G bands, is desirable. Several excellent mm-wave SPDT switches have been reported (see Table 1) [1; 2; 3; 4; 5; 6]. However, the overall performance still has room for improvement. For instance, in [4], a 15-25 GHz SPDT switch with alternate nMOS and pMOS as series and shunt transistors in 28 nm CMOS process is demonstrated. Low insertion loss of 0.6-2 dB is achieved. However, isolation of 15-31.5 dB and IP1 dB of 22.1 dBm are not good enough.