Active Components

D-Mode HEMT

Q: What gate voltage turns off a depletion-mode HEMT?

A depletion-mode HEMT is switched off by driving the gate-source voltage below its pinch-off (threshold) voltage, which is negative. For a typical GaN-on-SiC power HEMT, V_p sits between -2.5 V and -4 V; for a GaAs pHEMT it is usually -0.6 V to -1.5 V. Because the device conducts at V_GS = 0, RF power-amplifier bias circuits must establish a negative gate supply and apply it before the positive drain voltage, otherwise the transistor draws large drain current at turn-on and can be destroyed. Sequencing controllers and gate-bias clamps enforce this negative-before-positive order, and a quiescent current of 10 to 25 percent of I_DSS is then set by trimming V_GS slightly above pinch-off for Class AB operation.

Q: How does a D-mode HEMT differ from an E-mode HEMT?

The difference is the sign of the threshold voltage and therefore the default channel state. A depletion-mode (D-mode) HEMT has a negative threshold, so the AlGaN/GaN or AlGaAs/GaAs heterojunction already supports a two-dimensional electron gas at zero gate bias and the device is normally-on. An enhancement-mode (E-mode) HEMT has a positive threshold (typically +1 V to +2 V for GaN), so the 2DEG under the gate is depleted at V_GS = 0 and the device is normally-off until a positive gate voltage forms the channel. E-mode parts are preferred for power-switching converters because they fail safe and need only a single positive supply, while D-mode parts dominate microwave low-noise and power amplifiers where their higher transconductance and lower gate resistance give better RF gain and noise figure.

Q: Why must the negative gate bias be applied before drain voltage on a D-mode HEMT?

Because a D-mode HEMT is normally-on, at V_GS = 0 the channel is fully open and presents only its low on-resistance. If the drain supply is energized first, the saturated drain current I_DSS flows immediately, often several amperes for a 100 W GaN device, driving dissipation past the safe operating area and causing thermal runaway or gate-junction failure within milliseconds. Applying the negative pinch-off voltage first holds the channel off, then ramping the drain voltage and finally raising the gate toward the chosen quiescent point brings the device up safely. Power-down reverses the order: drain off first, gate bias removed last. This is why D-mode amplifier modules include dedicated bias-sequencing logic.

/dee mohd HEM-tee/ (depletion-mode high-electron-mobility transistor)

A normally-on member of the HEMT family in which the AlGaN/GaN or AlGaAs/GaAs heterojunction already supports a two-dimensional electron gas at zero gate-source bias, so the channel conducts drain current with V_GS = 0. To switch the device off, the gate is driven below a negative pinch-off voltage that fully depletes the 2DEG. Depletion-mode HEMTs have negative thresholds (roughly −0.6 V to −1.5 V for GaAs pHEMTs and −2.5 V to −4 V for GaN-on-SiC power devices), high transconductance, and low gate resistance, which makes them the default choice for microwave and millimeter-wave low-noise and power amplifiers, with InP variants reaching well into the sub-millimeter-wave band above 300 GHz.

Category: Active Components

Default state: Normally-on

Pinch-off V_p: −0.6 to −4 V

How Normally-On Channel Operation Works

In a depletion-mode HEMT the polarization fields and band offset of the heterojunction populate the channel with a high-density electron sheet before any gate voltage is applied. In GaN-on-SiC devices the spontaneous and piezoelectric polarization of the AlGaN barrier induces a 2DEG sheet charge of roughly 0.8 to 1.2 × 10¹³ cm⁻², giving sheet resistances near 300 to 500 ohm per square. Because the channel is already formed at V_GS = 0, the transistor conducts its full saturated drain current I_DSS at turn-on, and the gate must be biased negative to remove carriers and reach the pinch-off (threshold) voltage where conduction stops.

This normally-on behavior is the defining property that separates D-mode parts from their enhancement-mode counterparts. It also dictates the supporting circuit design: every D-mode amplifier needs a negative gate supply, and the bias must be sequenced so the negative voltage is present before drain voltage is applied. A typical Class AB microwave amplifier sets the gate just above pinch-off to draw a quiescent current of 10 to 25 percent of I_DSS, balancing linearity, gain, and efficiency. The high transconductance and short gate length (often 0.15 to 0.25 micrometres) give these devices f_T values from 30 GHz to well over 200 GHz.

Threshold Voltage and Channel Charge

The pinch-off voltage is governed by the barrier thickness, its doping or polarization charge, and the Schottky barrier height at the gate. Thinning the barrier or reducing its charge shifts the threshold toward zero and eventually positive, which is exactly how foundries build enhancement-mode variants. For a D-mode device the designer keeps the barrier thick enough that the 2DEG persists at V_GS = 0, accepting a negative threshold in exchange for higher channel charge, lower on-resistance, and stronger RF gain.

Governing Equations

Pinch-off (threshold) voltage:
V_p = φ_B − ΔE_C/q − (q × N_D × d²) / (2 × ε) (V_p < 0 for D-mode)

2DEG sheet charge:
n_s ≈ (ε / (q × d)) × (V_GS − V_p)

Saturated drain current:
I_DSS ≈ q × n_s × v_sat × W (at V_GS = 0)

Where φ_B = Schottky barrier height, ΔE_C = conduction-band offset, N_D = barrier donor density, d = barrier thickness, ε = barrier permittivity, v_sat ≈ 1.5 to 2.5 × 10⁷ cm/s, W = gate width. Example: GaN device with V_p = −3.2 V, W = 1 mm, n_s = 1 × 10¹³ cm⁻², v_sat = 1.5 to 2.5 × 10⁷ cm/s → I_DSS ≈ 2.4 to 4.0 A (this velocity-saturated ceiling is an upper bound; real 1 mm GaN HEMTs deliver roughly 0.8 to 1.2 A because of access resistance and field-dependent mobility).

Depletion-Mode Device Comparison

Device	Mode	Pinch-off V_p	f_T	P_out density	Typical use
GaAs pHEMT	Depletion	−0.6 to −1.5 V	80 to 150 GHz	0.5 to 1 W/mm	LNAs, mmWave front ends
GaN-on-SiC HEMT	Depletion	−2.5 to −4 V	30 to 90 GHz	4 to 8 W/mm	High-power PAs, radar
InP HEMT	Depletion	−0.3 to −0.8 V	200 to 400 GHz	0.2 to 0.5 W/mm	Sub-mmWave LNAs
GaN E-mode HEMT	Enhancement	+1 to +2 V	20 to 60 GHz	3 to 6 W/mm	Power switching, normally-off

Common Questions

Frequently Asked Questions

What gate voltage turns off a depletion-mode HEMT?

It switches off when V_GS drops below its negative pinch-off voltage: about −0.6 to −1.5 V for a GaAs pHEMT and −2.5 to −4 V for a GaN-on-SiC power device. Since it conducts at V_GS = 0, the amplifier needs a negative gate supply applied before drain voltage. Class AB bias then sets I_DQ at 10 to 25 percent of I_DSS by trimming V_GS just above pinch-off.

How does a D-mode HEMT differ from an E-mode HEMT?

The threshold sign differs. A D-mode device has a negative threshold and is normally-on, with a 2DEG present at zero gate bias. An E-mode device has a positive threshold (about +1 to +2 V for GaN) and is normally-off until a positive gate voltage forms the channel. E-mode parts fail safe and suit power switching; D-mode parts give higher transconductance, lower gate resistance, and better RF gain and noise figure, so they dominate microwave amplifiers.

Why must the negative gate bias be applied before drain voltage on a D-mode HEMT?

Because the device is normally-on, at V_GS = 0 the channel is fully open. If drain voltage comes up first, the saturated current I_DSS, often several amps for a 100 W GaN part, flows instantly and exceeds the safe operating area, causing thermal runaway in milliseconds. Setting the negative pinch-off voltage first holds the channel off; on power-down the order reverses, with the drain off first and gate bias removed last. Dedicated sequencing logic enforces this.

Related Terms

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