GaN FETs have a conduction mode similar to a Silicon MOSFET's inherent reverse diode. In both cases (GaN and Silicon FETs), conduction typically happens when the Drain terminal's voltage is lower than the Source. In other words, when the Drain has a negative voltage on it with respect to the Source terminal. For the case of a Silicon MOSFET, there is indeed an inherent physical diode present that needs about -1V to turn on (Vds = -1V). Typically, the gate will be off (Vgs = 0) when this happens. Since, if the gate voltage is greater than a few volts (typically), the MOSFET will be on and it would be difficult to build up a negative Vds of 1 V in that case. The Silicon MOSFET's inherent diode, being a minority carrier device, has a slow turn-off as the stored charges must be swept out of the diode's junction before it can turn off. This is called the Reverse Recovery time. The Silicon MOSFET also has a slow turn-on, which is called the Forward Recovery time. The slow turn on and turn off when the diode conducts leads to high switching losses as the FET is neither fully on, nor fully off leading to a simultaneous high voltage and high current, causing high power dissipation for that time period.
With the GaN FET, also called a High Electron Mobility Transistor (HEMT), there is no inherent physical reverse diode and therefore, there are no minority carriers to slow things down nor to cause diode switching losses. The GaN FET, however, will behave like a Silicon MOSFET's inherent diode in that when the Drain is negative (VDS= negative voltage > Vth), and the Gate is off (Vgs = 0), the GaN FET will conduct as the function of the Drain and the Source effectively switch places, causing a positive Vgd and GaN FET turn-on. The GaN FET can conduct pretty well with both positive bias (Vds = positive) as well as with negative bias (Vds = negative) so long as the gate voltage is greater than the FETs threshold voltage.
Although the GaN FET conducts with a negative Vds, and it therefore, acts like a diode, there is no actual diode, the effective forward voltage drop in the reverse conduction mode will be about 1.7 V or greater, depending on current and temperature. Additionally, when the negative Vds voltage is removed, the effective diode turns off instantly, with zero reverse recovery time, which is a real advantage in DC-DC Converter circuits.
The property of GaN FETs is important in circuits like a synchronous rectifier in a Buck converter, where the Vds of the lower FET goes negative during the deadtime. This is the time when the top and bottom FETs in a half-bridge structure are both off. The switching losses become much less with the GaN FET due to the zero reverse recovery time property.
For readers interested in more details, here's a link to EPC Co's, paper on GaN FET Characteristics: https://epc-co.com/epc/Portals/0/epc/documents/papers/eGaN%20FET%20Electrical%20Characteristics.pdf
