GaN HEMT

1. Theory

Definition: A GaN HEMT is a heterojunction field-effect transistor whose channel is a two-dimensional electron gas (2DEG) at the interface of an AlGaN barrier on a GaN layer.
Polarization-induced 2DEG: Wurtzite AlGaN and GaN have strong spontaneous polarization, and the strained AlGaN adds piezoelectric polarization; the difference leaves a fixed positive sheet charge at the interface that attracts electrons.
No intentional doping needed: An undoped AlGaN/GaN stack still forms a dense 2DEG (about $1 0^{13} c m^{- 2}$ ), with electrons supplied mainly by donor-like surface states.
Wide bandgap: GaN’s bandgap ( $\approx 3.4$ eV) and critical field ( $\approx 3.3$ MV/cm) allow high breakdown voltage with short drift lengths.
High saturation velocity: Electrons reach about $2$ to $2.5 \times 1 0^{7}$ cm/s, giving high current density and high-frequency gain.
Normally-on by default: Because the 2DEG exists at zero gate bias, the basic HEMT is depletion mode; normally-off devices use a p-GaN gate, a recessed gate or a cascode with a Si MOSFET.

2. Working Principle

Gate control: A Schottky (or MIS) gate on the AlGaN barrier depletes the 2DEG beneath it when made negative, turning the channel off.
On-state: Current flows laterally through the high-density, high-mobility 2DEG, so $I_{D S} = q n_{s} W_{G} v_{e f f}$ .
High-voltage blocking: The wide-bandgap material and field plates spread the gate-drain field, supporting hundreds of volts in a short lateral distance.
Fast switching: Lateral majority-carrier conduction with no body diode gives no reverse-recovery charge and very low switching loss.
Reverse conduction: With no body diode, the device conducts in reverse through the channel itself once the gate-drain voltage exceeds threshold.
Trapping and current collapse: Surface and buffer traps capture electrons under high drain bias, temporarily reducing current; passivation and field plates reduce this.

3. Applications

RF power amplifiers: Cellular base stations, AESA radar, electronic warfare and satellite uplinks, where high power density at GHz frequencies is needed.
Compact fast chargers and adapters: 650 V GaN switches in high-frequency, high-density USB-C power adapters.
Data-centre and telecom power: High-efficiency PFC stages and 48 V DC-DC converters.
LiDAR laser drivers: Nanosecond high-current pulses for time-of-flight ranging.

Name	Formula	Related Formulae	Variables/Constants
Polarization induced sheet charge density for an undoped AlxGa(1-x)N/GaN heterostructure	$∣ σ (x)∣ = ∣ P_{PE} (Al_{x} Ga_{1 - x} N) + P_{sp} (Al_{x} Ga_{1 - x} N) - P_{sp} (GaN)∣ C/ m^{2}$	$P_{sp} (x) = (- 0.52 x - 0.029) C/ m^{2}$ $P_{PE} = 2 (\frac{a ( 0 ) - a ( x )}{a ( x )}) [e_{31} (x) - e_{33} (x) \frac{C _{13} ( x )}{C _{33} ( x )}] C/ m^{2}$ $e_{31} (x) = - 0.11 x - 0.49$ $e_{33} (x) = 0.73 x + 0.73$ $a (x) = (- 0.077 x + 3.189) * 1 0^{- 10}$ $C_{13} (x) = 5 x + 103$ $C_{33} (x) = - 32 x + 405$	$x : A l co m p os i t i o n$ $e : P o l a r i z a t i o n co n s t an t s$ $e_{31} : co u pl in g b e tw ee n in - pl an e s t r ain an d p o l a r i z a t i o n a l o n g t h e c - a x i s$ $e_{33} : co u pl in g b e tw ee n s t r ain a l o n g t h e c - a x i s an d p o l a r i z a t i o n a l o n g t h e c - a x i s$ $a : l a tt i ce co n s t an t$ $C : E l a s t i c s t i f f n ess co n s t an t s$ $C_{13} : co u pl in g b e tw ee n in - pl an e s t r ain an d t h e c - a x i s d i r ec t i o n$ $C_{33} : s t i f f n ess a g ain s t s t r ain a l o n g t h e c - a x i s$
2DEG sheet carrier concentration	$n_{s} (x) = \frac{σ ( x )}{e} - \frac{ε _{0} ε ( x )}{d e ^{2}} [e ϕ_{b} (x) + E_{F} (x) - Δ E_{C} (x)] m^{- 2}$	$ε (x) = - 0.3 x + 10.4$ $e ϕ_{b} (x) = 1.3 x + 0.84 e V$ $E_{F} (x) = E_{0} (x) + \frac{π ℏ ^{2}}{m ^{} ( x )} n_{s} (x) e V$ $E_{0} (x) = [\frac{9 π ℏ e ^{2}}{8 ε _{0} 8 m ^{} ( x )} \frac{n _{s} ( x )}{ε ( x )}]^{2/3} e V$ $m^{*} (x) = 0.22 m_{e}$ $Δ E_{C} (x) = 0.7 [E_{g} (x) - E_{g} (0)] e V$ $E_{g} (x) = x E_{g} (AlN) + (1 - x) E_{g} (GaN) - x (1 - x) b e V$	$ε : d i e l ec t r i c co n s t an t$ $ϕ_{b} : sc h o tt k y ba r r i er h e i g h t$ $E_{F} : F er mi E n er g y$ $E_{0} : G r o u n d s u bban d e n er g y$ $ℏ : R e d u ce d P l an k^{'} s co n s t an t (\frac{h}{2 π})$ $m^{*} : e f f ec t i v e ma ss$ $m_{e} : ma ss o f e l ec t r o n$ $e : e l ec t r o n c ha r g e$ $ϵ_{0} : p er mi tt i v i t y o f f r ee s p a ce or t h e e l ec t r i c co n s t an t$ $Δ E_{C} : C o n d u c t i o n ban d o f f se t$ $E_{g} : B an d g a p e n er g y$ $b : B o w in g p a r am e t er (1.0 e V)$
Drain to source current	$I_{D S} = q n_{s} W_{G} v_{e f f}$	$v = μ_{n} E$	$W_{G} : g a t e w i d t h$ $v_{e f f} : e f f ec t i v e e l ec t r o n v e l oc i t y in t h e 2 D E G c hann e l$ $E : e l ec t r i c f i e l d$ $μ_{n} : e l ec t r o n m o bi l i t y$

Important Questions

Ques: Where do electrons in the 2DEG come from?

The electrons forming the 2DEG can originate from several sources, depending on the HEMT structure: intentional donor impurities such as Si in a doped AlGaN barrier, donor-like surface states in an undoped AlGaN barrier, and defects such as nitrogen-vacancy-related donor states. Other proposed mechanisms involve unintentional impurities and defects within the heterostructure. In the undoped AlGaN/GaN structure, surface donor states have been experimentally and theoretically identified as an important electron source. The polarization-induced electric field then drives these electrons toward the AlGaN/GaN interface, where they accumulate to form the 2DEG. Reference: https://pubs.aip.org/aip/apl/article/77/2/250/113393/Polarization-effects-surface-states-and-the-source, https://pubs.aip.org/aip/jap/article/108/6/063719/348498/Distribution-of-donor-states-on-etched-surface-of

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GaN HEMT

1. Theory

2. Working Principle

3. Applications

Important Questions

Ques: Where do electrons in the 2DEG come from?

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