What is the difference between enhancement mode and depletion mode GaN transistors?

GaN transistors are available in two fundamental operating modes, defined by their threshold voltage (V_th): (1) Depletion mode (D-mode): the transistor is ON (conducting) at V_GS = 0 V. The gate must be biased negative (V_GS < 0 V) to turn it off. Threshold voltage: V_th = -1 to -4 V (negative). Bias: requires a negative gate supply voltage (typically -2 to -5 V). The drain is connected to a positive supply (+28 to +50 V). This requires a dual-polarity power supply (positive for drain, negative for gate). Sequencing: the negative gate voltage must be applied BEFORE the positive drain voltage is turned on. If VDD is applied without V_GS: the transistor is fully on (V_GS = 0), drawing maximum current. This can exceed the safe operating area and destroy the device. After operation: VDD must be turned off BEFORE the gate voltage is removed. This power supply sequencing is a critical design requirement. Most RF GaN HEMTs are depletion mode because the AlGaN/GaN 2DEG (two-dimensional electron gas) naturally forms at the interface without any bias. (2) Enhancement mode (E-mode): the transistor is OFF at V_GS = 0 V. The gate must be biased positive (V_GS > 0 V) to turn it on. Threshold voltage: V_th = +0.5 to +2 V (positive). Bias: requires only a positive gate voltage (+1 to +5 V). The drain supply is still positive (+28 to +50 V). Advantages: no negative supply needed (simplifies the power supply design). Normally-off operation (the device is safe if the gate control fails; the transistor turns off and does not pass current). No power supply sequencing required. Disadvantages: current E-mode GaN processes have lower performance than D-mode: lower transconductance (g_m), lower maximum current (I_DSS), and lower fT/fmax. The gate voltage swing is limited (must stay below the Schottky diode forward voltage, approximately +2 V). This limits the maximum current and output power compared to D-mode.