1. Theory

Definition: ASM-HEMT (Advanced SPICE Model for HEMTs) is a surface-potential-based compact model for GaN HEMTs in RF and power applications, developed at IIT Kanpur and Macquarie University (ASM-HEMT manual; Ghosh et al.).
Core: The Fermi level at the AlGaN/GaN interface is found from the 2DEG charge, using the first two subbands, and the surface potentials at source and drain give the drain current and charges in closed form (ASM-HEMT manual; Ghosh et al.).
Real-device effects: Short-channel effects, velocity saturation, non-linear access resistance, self-heating, trapping, gate leakage, field plates and noise are added as modules with switches (ASM-HEMT manual).
Status: ASM-HEMT is one of the GaN HEMT models standardized by the Compact Model Coalition, alongside MVSG (background knowledge).

2. Working Principle

Charge control by the barrier: The barrier acts as the gate dielectric, so $n_{s} = C_{g} (V_{g 0} - E_{f}) / q$ , while the 2DEG statistics give $n_{s}$ as a function of $E_{f}$ ; equating the two fixes $E_{f}$ (ASM-HEMT manual).
Current from potentials: With $ψ = E_{f} + V_{x}$ along the channel, the drift-diffusion current integrates to a closed form in $ψ_{s}$ and $ψ_{d}$ , like surface-potential MOSFET models (ASM-HEMT manual; Ghosh et al.).
Access regions: The ungated regions between gate and contacts act as resistors whose value rises as the current approaches their own saturation current (ASM-HEMT manual; Ghosh et al.).
Traps and heat: RC sub-networks shift the cut-off voltage and access resistances with trapping, and a thermal network raises the device temperature (ASM-HEMT manual).

3. Applications

RF power amplifiers: Small- and large-signal design with S-parameters to 50 $GHz$ and load-pull at 10 $GHz$ , as validated by Ghosh et al. (Ghosh et al.).
Power switching: Field-plated power transistors, with dynamic on-resistance and switching capacitances (Ghosh et al.; ASM-HEMT manual).
Noise: Low-noise amplifier design with thermal and flicker noise models (ASM-HEMT manual).
Process kits: GaN foundry design kits using the model with temperature and trap modules (background knowledge).

Core Model

Name	Formula	Related Formulae	Variables/Constants
2DEG charge and Fermi level	$n_{s} = D O S \cdot V_{t v} ln [(1 + e^{(E_{f} - E_{0}) / V_{t v}}) (1 + e^{(E_{f} - E_{1}) / V_{t v}})], n_{s} = \frac{C _{g}}{q} (V_{g 0} - E_{f})$	$C_{g} = \frac{ϵ _{A l G a N}}{T B A R}, E_{0, 1} = γ_{0, 1} E^{2/3}, n_{s} \approx 2 D O S V_{t v} e^{V_{g 0} / V_{t v}} (subthreshold)$	$n_{s} : 2 D E G s h ee t d e n s i t y$ $D O S : tw o - d im e n s i o na l d e n s i t y o f s t a t es (3.24 \times 1 0^{17} in t h e man u a l)$ $V_{t v} : t h er ma l v o l t a g e t im es t h e s u b t h r es h o l d f a c t or$ $E_{f} : F er mi l e v e l r e l a t i v e t o t h e co n d u c t i o n ban d a t t h e in t er f a ce$ $E_{0}, E_{1} : f i r s t tw o s u bban d e n er g i es$ $γ_{0}, γ_{1} : S c h r o d in g er - P o i sso n coe f f i c i e n t s (G A M M A 0 I = 2.12 \times 1 0^{- 12}, G A M M A 1 I = 3.73 \times 1 0^{- 12})$ $E : in t er f a ce f i e l d$ $C_{g} : ba r r i er c a p a c i t an ce p er a r e a$ $ϵ_{A l G a N} : ba r r i er p er mi tt i v i t y (E P S I L O N = 10.66 \times 1 0^{- 11} F / m)$ $T B A R : ba r r i er t hi c k n ess (d e f a u l t 25 nm), V_{g 0} = V_{g s, e f f} - V_{o f f, D I B L} : g a t e o v er d r i v e; A S M - H E M T man u a l : t h ese r e l a t i o n s so l v e d w i t h an in t er p o l a t i o n f u n c t i o n H (V_{g 0}) an d a H o u se h o l d er - t y p e cor r ec t i o n; G h os h e t a l . : t h e u ni f i e d E_{f} = V_{g o} - 2 V_{t} l n [1 + e x p (V_{g o} / (2 V_{t}) - 1/ H) + (C_{g} / q D) e x p (- V_{g o} / (2 V_{t}))] co n s i d er in g t h e f i r s t tw o s u bban d s$ $(co m p u t e d : d e f a u l t E P S I L O N an d T B A R g i v e C_{g} = 4.26 \times 1 0^{- 3} F/ m^{2}; a t V_{g 0} = 2 V, n e g l ec t in g E_{f}, n_{s} = 5.3 \times 1 0^{16} m^{2}, 5.3 \times 1 0^{12} c m^{2})$
Surface potential and drain current	$I_{d} = \frac{W}{L} μ_{e f f} C_{g} (V_{g 0} - ψ_{m} + V_{t h}) ψ_{d s}, ψ_{s} = E_{f} + V_{s}, ψ_{d} = E_{f} + V_{d, e f f}$	$ψ_{m} = \frac{ψ _{d} + ψ _{s}}{2}, ψ_{d s} = ψ_{d} - ψ_{s}, V_{d, e f f} = \frac{V _{d s}}{[ 1 + ( V _{d s} / V _{d s a t} ) ^{D E L T A} ] ^{1/ D E L T A}}$	$I_{d} : d r ain c u r r e n t$ $W, L : g a t e w i d t h an d l e n g t h (d e f a u l t s 200 an d 0.25 μ m)$ $μ_{e f f} : e f f ec t i v e m o bi l i t y$ $ψ_{s}, ψ_{d} : s u r f a ce p o t e n t ia l s a t so u r ce an d d r ain$ $ψ_{m} : m e an$ $ψ_{d s} : d i f f er e n ce$ $V_{t h} : t h er ma l v o l t a g e$ $V_{d, e f f} : e f f ec t i v e d r ain v o l t a g e, V_{d s a t} = (2 V S A T / μ_{e f f}) L V_{g 0, e f f} / [(2 V S A T / μ_{e f f}) L + V_{g 0, e f f}]$ $D E L T A : s m oo t hin g e x p o n e n t (d e f a u l t 2); A S M - H E M T man u a l : t h ese f or m s an d c hann e l - l e n g t h m o d u l a t i o n I_{d s} (1 + L A M B D A (V_{d s x} - V_{d, e f f})); G h os h e t a l . : t h e s am e c u r r e n t w i t h a v e l oc i t y - s a t u r a t i o n f a c t or 1/ (1 + θ_{s a t} ψ_{d s}) an d (1 + λ V_{d s, e f f})$ $(co m p u t e d : f or t h e d e f a u l t 200 μ m w i d t h, C_{g} = 4.26 \times 1 0^{- 3} F/ m^{2}, V_{g 0} = 2 V an d V S A T = 1.9 \times 1 0^{5} m/s, t h e v e l oc i t y - l imi t e d b o u n d W C_{g} V_{g 0} v_{s a t} i s 0.32 A, or 1.6 A/mm, b e f or e a ccess r es i s t an ce an d t h e F er mi - l e v e l t er m)$
Threshold, DIBL and subthreshold slope	$V_{o f f, D I B L} = V O F F - E T A 0 \frac{V _{d s x} V D S C A L E}{V _{d s x}^{2} + V D S C A L E ^{2}}, V_{t v} = V_{T} (1 + N F A C T O R + C D S C D V_{d s x})$	$S S = ln 10 V_{t v}, V_{d s x} = V_{d s}^{2} + 0.01$	$V_{o f f, D I B L} : d r ain - d e p e n d e n t c u t - o f f v o l t a g e$ $V O F F : c u t - o f f v o l t a g e (d e f a u l t - 2 V), E T A 0 : D I B L coe f f i c i e n t (d e f a u l t 0)$ $V D S C A L E : D I B L s a t u r a t i o n v o l t a g e (d e f a u l t 5 V)$ $N F A C T O R : s u b t h r es h o l d f a c t or (d e f a u l t 0.5)$ $C D S C D : i t s d r ain d e p e n d e n ce (d e f a u l t 1 \times 1 0^{- 3})$ $S S : s u b t h r es h o l d s w in g; A S M - H E M T man u a l : t h ese f or m s w i t h t r a p t er m s an d t h e t e m p er a t u r e f or m V_{o f f, D I B L} (T) = V_{o f f, D I B L} - (T / T N O M - 1) K T 1; G h os h e t a l . : V O F F = - 2.3 V e x t r a c t e d f or t h e in t r in s i c t r an s i s t or o f a p o w er d e v i ce$ $(co m p u t e d : N F A C T O R = 0.5 g i v es 89 mV/dec a t 300 K, 90 mV/dec a t V_{d s} = 20 V; an a ss u m e d E T A 0 = 0.05 s hi f t s V O F F b y 0.24 V a t 20 V)$
Mobility and velocity saturation	$μ_{e f f} = \frac{U 0}{1 + U A E _{y, e f f} + U B E _{y, e f f}^{2}}, E_{y, e f f} = \frac{C _{g} ( V _{g 0} - ψ _{m} )}{ϵ _{A l G a N}}$	$μ_{e f f, s a t} = \frac{μ _{e f f}}{1 + T H E S A T ^{2} ψ _{d s}^{2}} (as read), U 0 (T) = U 0 (\frac{T}{T N O M})^{U T E}$	$U 0 : l o w - f i e l d m o bi l i t y (d e f a u l t 0.17 m^{2} / (V s))$ $U A, U B : m o bi l i t y d e g r a d a t i o n coe f f i c i e n t s (d e f a u l t s 0)$ $E_{y, e f f} : e f f ec t i v e v er t i c a l f i e l d$ $μ_{e f f, s a t} : v e l oc i t y - s a t u r a t e d m o bi l i t y$ $T H E S A T : v e l oc i t y - s a t u r a t i o n p a r am e t er (d e f a u l t 1, u ni t g i v e n a s V^{- 2}, ini t ia l i z e d t o μ_{e f f} / (V S A T L))$ $U T E : m o bi l i t y t e m p er a t u r e e x p o n e n t (d e f a u l t - 0.5); A S M - H E M T man u a l : t h ese f or m s (t h e e x t r a c t p u t s t h e s q u a r e r oo t o v er μ_{e f f} a s w e l l, see t h e Q / A); G h os h e t a l . : 1/ (1 + θ_{s a t} ψ_{d s})$ $(co m p u t e d : U T E = - 0.5 l o w er s U 0 b y 13 p er ce n t f r o m 300 t o 400 K)$

Access Regions, Charges and Field Plates

Name	Formula	Related Formulae	Variables/Constants
Access-region resistance	$R_{s} = \frac{R S C}{W \cdot N F} + \frac{L S G}{W \cdot N F \cdot q N S 0 A C C S \cdot U 0 A C C S} [1 - (\frac{I _{d s}}{I _{s a t, s}})^{M E X P A C C S}]^{- 1/ M E X P A C C S}$	$I_{s a t, s} = W \cdot N F \cdot q N S 0 A C C S \cdot V S A T A C C S$	$R_{s} : so u r ce a ccess r es i s t an ce (R_{d} ana l o g o u s w i t h R D C, L D G, N S 0 A C C D, U 0 A C C D, M E X P A C C D)$ $R S C : co n t a c t r es i s t an ce (d e f a u l t 1 \times 1 0^{- 4} Ω m)$ $N F : n u mb er o f f in g er s$ $L S G : so u r ce - g a t e l e n g t h (d e f a u l t 1 μ m), N S 0 A C C S : a ccess 2 D E G d e n s i t y (d e f a u l t 5 \times 1 0^{17} m^{2}), U 0 A C C S : a ccess m o bi l i t y (d e f a u l t 0.155 m^{2} / (V s))$ $M E X P A C C S : e x p o n e n t (d e f a u l t 2)$ $I_{s a t, s} : a ccess s a t u r a t i o n c u r r e n t$ $V S A T A C C S : a ccess s a t u r a t i o n v e l oc i t y (d e f a u l t 5 \times 1 0^{4}); A S M - H E M T man u a l : t hi s f or m f or R D S M O D = 1, w i t h t e m p er a t u r e f a c t or s an d t r a p t er m s, an d b o t h r es i s t an ces se t t o 1 \times 1 0^{- 6} f or R D S M O D = 0 (q in I_{s a t, s} r es t or e d, m y r e a d in g); G h os h e t a l . : R = R_{0} / [1 - (I_{d} / I_{a cc, s a t})^{γ}]^{1/ γ} w i t h R_{0} = L_{a cc} / (Q_{a cc} μ_{a cc})$ $(co m p u t e d : f or t h e d e f a u l t s an d W = 200 μ m, t h e co n t a c t p a r t i s 0.50 Ω an d t h e 2 D E G p a r t 0.40 Ω (0.08 Ω mm), I_{s a t, s} = 0.80 A, an d t h e 2 D E G p a r t r i ses b y 15 p er ce n t a t ha l f o f I_{s a t, s})$
Intrinsic charges	$Q_{g} = - \frac{C _{g} L W}{V _{g 0} - ψ _{m} + V _{t v}} [V_{g 0}^{2} + \frac{ψ _{d}^{2} + ψ _{s}^{2} + ψ _{d} ψ _{s}}{3} - V_{g 0} (ψ_{d} + ψ_{s} - V_{t v}) - V_{t v} ψ_{m}]$	$Q_{s} = - Q_{g} - Q_{d} (Ward-Dutton), C_{a cc d} = \frac{C J 0}{( 1 + V _{d s} / V B I ) ^{M Z}}$	$Q_{g}, Q_{d}, Q_{s} : g a t e, d r ain an d so u r ce c ha r g es$ $C_{a cc d} : d r ain a ccess c a p a c i t an ce, C J 0 : i t s z er o - bia s v a l u e (d e f a u l t 0)$ $V B I : b u i l t - in p o t e n t ia l (d e f a u l t 0.9 V)$ $M Z : e x p o n e n t (d e f a u l t 0.5); A S M - H E M T man u a l : t h ese f or m s (s i g n o f Q_{g} a s r e a d), w i t h a q u an t u m c ha r g e - ce n t r o i d cor r ec t i o n C_{g} = ϵ / (T B A R + A D O S / (1 + C D O S Q_{g} / QM 0)^{B D O S}) an d o v er l a p c a p a c i t an ces C GS O, C G D O, C D S O; G h os h e t a l . : Q_{g} = - \int q W C_{g} (V_{g o} - ψ) d x w i t h W a r d - D u tt o n p a r t i t i o nin g; p a r t i t i o nin g o n t h e C ha r g e an d C a p a c i t an ce M o d e l l in g p a g e$ $(n o co m p u t e d v a l u e)$
Field plates	$V_{o f f, F P} \approx - \frac{q n _{s} d _{F P}}{ϵ _{F P}} (order of magnitude)$	$I_{d} = I_{d, in t r in s i c} = I_{d, F P 1} = \dots (series transistors)$	$V_{o f f, F P} : p in c h - o f f v o l t a g e o f t h e 2 D E G u n d er a f i e l d pl a t e$ $d_{F P} : d i e l ec t r i c t hi c k n ess u n d er t h e pl a t e$ $ϵ_{F P} : i t s p er mi tt i v i t y (es t ima t e, ba c k g r o u n d k n o w l e d g e); A S M - H E M T man u a l : u p t o f o u r f i e l d pl a t es F P 1 t o F P 4 m o d e l l e d a s a dd i t i o na l t r an s i s t or s in ser i es, e a c h g a t e - or so u r ce - co nn ec t e d (M O D 1 or 2) w i t h i t s o w n D F P, L F P an d V O F F F P, an d cr oss - co u pl in g an d s u b s t r a t e c ha r g es sc a l e d b y C F P x S C A L E an d C S U B S C A L E x; G h os h e t a l . : a T os hiba p o w er d e v i ce w i t h g a t e - an d so u r ce - co nn ec t e d f i e l d pl a t es, 33 p er ce n t A l, V O F F = - 2.3 V f or t h e in t r in s i c t r an s i s t or an d - 50.5 V f or t h e g a t e f i e l d pl a t e, f i e l d - pl a t e c a p a c i t an ces co n t r o l l in g s w i t c hin g$ $(co m p u t e d : - 50.5 V w i t h an a ss u m e d n_{s} o f 1 \times 1 0^{13} c m^{2} an d S i N (ϵ_{r} = 7.5) im pl i es ab o u t 210 nm o f d i e l ec t r i c u n d er t h e pl a t e)$

Thermal, Trapping, Gate and Noise

Name	Formula	Related Formulae	Variables/Constants
Self-heating and temperature	$T_{d e v} = T + Δ T, Δ T = R T H 0 \cdot P_{d} (static)$	$V S A T (T) = V S A T (\frac{T _{d e v}}{T N O M})^{A T}, R S C (T) = R S C [1 + K R S C (\frac{T _{d e v}}{T N O M} - 1)]$	$T_{d e v} : d e v i ce t e m p er a t u r e$ $Δ T : se l f - h e a t in g r i se, R T H 0 : t h er ma l r es i s t an ce (d e f a u l t 5, l i s t e d in o hm s f or t h e e l ec t r i c a l ana l o g u e, in K / W), C T H 0 : t h er ma l c a p a c i t an ce (d e f a u l t 1 \times 1 0^{- 9})$ $P_{d} : d i ss i p a t e d p o w er, A T, K R S C$ $K T V B I : t e m p er a t u r e coe f f i c i e n t s (d e f a u l t s 0)$ $T N O M : 300 K; A S M - H E M T man u a l : an R C t h er ma l n e tw or k w i t h S H M O D = 1 b y d e f a u l t; G h os h e t a l . : g a t e c u r r e n t v a l i d a t e d f r o m 333 t o 453 K in 30 K s t e p s$ $(co m p u t e d : t h e d e f a u l t s g i v e 5 K p er w a tt an d a t h er ma l t im e co n s t an t o f 5 ns)$
Trapping and dynamic on-resistance	$TRAPMOD = 1 : V_{t r a p} \to V O F F, E T A 0, R_{S}, R_{D}; TRAPMOD = 3 : R_{T R A P 3} C_{T R A P 3} \to R_{D}$	$τ_{t r a p} = R T R A P 3 \cdot C T R A P 3$	$V_{t r a p} : t r a p - n e tw or k v o l t a g e, R T R A P 3, C T R A P 3 : t r a p n e tw or k e l e m e n t s (d e f a u l t s 1 Ω an d 1 \times 1 0^{- 4} F)$ $τ_{t r a p} : t r a p t im e co n s t an t; A S M - H E M T man u a l : T R A P M O D 0 o f f, 1 an R F t r a p s u b c i r c u i t u p d a t in g V O F F, E T A 0, R S an d R D, 2 a p u l se d - I V m o d e l w i t h tw o R C n e tw or k s, 3 a ba s i c m o d e l a c t in g o n l y o n t h e d r ain r es i s t an ce, r eco mm e n d e d f or d y nami c o n - r es i s t an ce in p o w er d e v i ces, w i t h coe f f i c i e n t s A T R A P V O F F, B T R A P V O F F, A T R A P R S, B T R A P R S, A T R A P R D, B T R A P R D; G h os h e t a l . : p u l se d an d d u a l - p u l se d m e a s u r e m e n t s s h o w in g t h r es h o l d s hi f t an d o n - r es i s t an ce in cr e a se f or v a r i o u s q u i esce n t bia ses$ $(co m p u t e d : t h e T R A P M O D = 3 d e f a u l t s g i v e τ_{t r a p} = 0.1 m/s)$
Gate current, gate resistance and noise	$I_{g s} = I GS D I O \cdot A (e^{V_{g s} / (N J GS V_{T})} - 1), R_{g} = \frac{R S H G ( X G W + W / ( 3 N GC O N ) )}{N GC O N \cdot N F \cdot L}$	$S_{i d, t h} \propto \frac{4 k _{B} T _{d e v}}{I _{d} L ^{2}} (μ_{e f f, s a t} W C_{g})^{2} [V_{g 0} ψ_{d s} + \frac{ψ _{d}^{3} - ψ _{s}^{3}}{3} - V_{g 0} (ψ_{d}^{2} - ψ_{s}^{2})]$	$I_{g s} : g a t e - so u r ce d i o d e c u r r e n t (I_{g d} ana l o g o u s)$ $I GS D I O : s a t u r a t i o n c u r r e n t d e n s i t y (d e f a u l t 1 A / m^{2})$ $A : d i o d e a r e a$ $N J GS : i d e a l i t y (d e f a u l t 2.5)$ $R_{g} : g a t e r es i s t an ce$ $R S H G : g a t e s h ee t r es i s t an ce (d e f a u l t 0.1 Ω/ s q)$ $X G W : o f f se t$ $N GC O N : n u mb er o f g a t e co n t a c t s$ $S_{i d, t h} : d r ain t h er ma l - n o i se d e n s i t y$ $k_{B} : B o l t z mann co n s t an t; A S M - H E M T man u a l : tw o g a t e d i o d es w i t h G A T E M O D = 1, t h e R_{g} f or m (l ab e l l e d G_{g a t e} in t h e e x t r a c t an d se t t o 1 \times 1 0^{3} i f n o t p os i t i v e), t h er ma l n o i se in t hi s f or m sc a l e d b y T N S C an d f l i c k er n o i se w i t h N O I A, N O I B, N O I C an d E F; G h os h e t a l . : S - p a r am e t er s f r o m 500 MHz t o 50 GHz an d l a r g e - s i g na l p o w er a t 10 GHz f or a Q or v o 10 b y 90 μ m, 125 nm d e v i ce$ $(co m p u t e d : t h e d e f a u l t s g i v e R_{g} = 26.7 Ω f or o n e 200 μ m f in g er o f 0.25 μ m)$

Important Questions

Ques: How does ASM-HEMT find the 2DEG charge and the surface potential?

By solving two relations for the Fermi level. The 2DEG density follows the quantum statistics of the first two subbands, $n_{s} = D O S V_{t v} ln [(1 + e^{(E_{f} - E_{0}) / V_{t v}}) (1 + e^{(E_{f} - E_{1}) / V_{t v}})]$ , with subband energies proportional to $E^{2/3}$ , and it is also set by the barrier capacitance, $n_{s} = C_{g} (V_{g 0} - E_{f}) / q$ (ASM-HEMT manual). Ghosh et al. give the closed-form unified solution for $E_{f}$ in terms of $V_{g o} = V_{g s} - V O F F$ , and the manual refines it with an interpolation function and a Householder-type step. The surface potential is then $E_{f}$ plus the channel potential. With the default 25 $nm$ barrier, $C_{g} = 4.26 \times 1 0^{- 3}$ $F/ m^{2}$ , so 2 $V$ of overdrive gives about $5.3 \times 1 0^{12}$ $c m^{2}$ (computed). Because the model is built on these potentials rather than on a threshold voltage, the current, charges and their derivatives stay continuous from subthreshold to strong accumulation (my reading). Reference: S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, ‘ASM-HEMT 101.0.0 Technical Manual’, Macquarie University and IIT Kanpur (16 March 2018) (equations partly garbled in the extract) (fetch-tool summary); S. Ghosh, S. A. Ahsan, A. Dasgupta, S. Khandelwal and Y. S. Chauhan, ‘GaN HEMT Modeling for Power and RF Applications using ASM-HEMT’, IIT Kanpur and UC Berkeley (venue and year not stated; references to 2016) (fetch-tool summary).

Ques: Is the ASM-HEMT density of states consistent with GaN’s effective mass?

Not obviously. The manual lists DOS = $3.24 \times 1 0^{17}$ , which, read as $q m^{*} / (π ℏ^{2})$ per square metre per volt including spin, implies an effective mass of 0.078 times the free-electron mass (computed). The electron effective mass in GaN is about 0.2 (background knowledge), which would give $8.35 \times 1 0^{17}$ (computed), 2.6 times larger. Dropping the spin factor would raise the implied mass only to 0.155 (computed). The value may be a fitted default, or the manual may define DOS in different units (my reading). In strong inversion the error matters little, because $n_{s}$ is then set mainly by $C_{g} (V_{g 0} - E_{f})$ , but it changes the subthreshold current $2 D O S V_{t v} e^{V_{g 0} / V_{t v}}$ and the Fermi-level offset. Users should treat DOS as a parameter to check against Schrodinger-Poisson results. Reference: S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, ‘ASM-HEMT 101.0.0 Technical Manual’, Macquarie University and IIT Kanpur (16 March 2018) (equations partly garbled in the extract) (fetch-tool summary).

Ques: Which velocity-saturation form does ASM-HEMT use?

The two sources give different forms. Ghosh et al. multiply the current by $1/ (1 + θ_{s a t} ψ_{d s})$ , while the extract of the 101.0.0 manual divides the mobility by $1 + T H E S A T^{2} ψ_{d s}^{2}$ and initializes THESAT to $μ_{e f f} / (V S A T L)$ . Both reduce to the low-field mobility at small $ψ_{d s}$ and limit the current at high fields; at $θ ψ_{d s} = 1$ they give factors of 0.5 and 0.71 (computed). The manual is the later and normative document, so its square-root form is more likely the current implementation (my reading). Its table gives THESAT in units of V^{-2}, but THESAT times $ψ_{d s}$ must be dimensionless, so the unit should be V^{-1}, consistent with the $μ / (v_{s a t} L)$ initialization (computed). The extract also places the square root over $μ_{e f f}$ , which is probably a transcription slip (my reading). Reference: S. Ghosh, S. A. Ahsan, A. Dasgupta, S. Khandelwal and Y. S. Chauhan, ‘GaN HEMT Modeling for Power and RF Applications using ASM-HEMT’, IIT Kanpur and UC Berkeley (venue and year not stated; references to 2016) (fetch-tool summary); S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, ‘ASM-HEMT 101.0.0 Technical Manual’, Macquarie University and IIT Kanpur (16 March 2018) (equations partly garbled in the extract) (fetch-tool summary).

Ques: Why is the access resistance of a GaN HEMT current dependent?

Because the ungated access regions also saturate. ASM-HEMT writes the source resistance as a contact part plus a 2DEG part $L S G / (W N F q N S 0 A C C S U 0 A C C S)$ multiplied by $[1 - (I_{d s} / I_{s a t, s})^{M E X P A C C S}]^{- 1/ M E X P A C C S}$ , where $I_{s a t, s}$ is set by the access charge and saturation velocity; Ghosh et al. give the same form with $R_{0} = L_{a cc} / (Q_{a cc} μ_{a cc})$ . For the defaults and a 200 $μ m$ device, the contact part is 0.50 $Ω$ , the 2DEG part 0.40 $Ω$ and the access saturation current 0.80 $A$ (computed, with q restored). At half of that current the 2DEG part rises by 15 per cent (computed), and near it the resistance diverges. This explains the drop in transconductance at high gate voltage in GaN HEMTs, which a constant resistance cannot reproduce. Reference: S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, ‘ASM-HEMT 101.0.0 Technical Manual’, Macquarie University and IIT Kanpur (16 March 2018) (equations partly garbled in the extract) (fetch-tool summary); S. Ghosh, S. A. Ahsan, A. Dasgupta, S. Khandelwal and Y. S. Chauhan, ‘GaN HEMT Modeling for Power and RF Applications using ASM-HEMT’, IIT Kanpur and UC Berkeley (venue and year not stated; references to 2016) (fetch-tool summary).

Ques: How does ASM-HEMT model trapping and dynamic on-resistance?

With selectable RC sub-networks. TRAPMOD = 1 adds an RF trap subcircuit whose voltage shifts VOFF, ETA0 and the source and drain resistances; TRAPMOD = 2 uses two RC networks for pulsed-IV behaviour; TRAPMOD = 3 acts only on the drain resistance and is recommended for dynamic on-resistance in power devices (ASM-HEMT manual). Its defaults, 1 $Ω$ and $1 \times 1 0^{- 4}$ $F$ , give a 0.1 $m/s$ time constant (computed). Ghosh et al. validate the approach against pulsed and dual-pulsed measurements, which show a threshold shift and an increase in on-resistance that depend on the quiescent drain voltage and current. Real traps have a spread of time constants from microseconds to seconds (background knowledge), so a single RC captures only one dominant trap level, which is usually enough for switching-loss estimates. Reference: S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, ‘ASM-HEMT 101.0.0 Technical Manual’, Macquarie University and IIT Kanpur (16 March 2018) (equations partly garbled in the extract) (fetch-tool summary); S. Ghosh, S. A. Ahsan, A. Dasgupta, S. Khandelwal and Y. S. Chauhan, ‘GaN HEMT Modeling for Power and RF Applications using ASM-HEMT’, IIT Kanpur and UC Berkeley (venue and year not stated; references to 2016) (fetch-tool summary).

Ques: How does ASM-HEMT model field plates?

As extra transistors in series with the intrinsic device. The manual allows up to four field plates, each gate-connected or source-connected, with its own dielectric thickness, length and cut-off voltage, and with cross-coupling and substrate charges that set the switching capacitances. Ghosh et al. apply this to a Toshiba power HEMT with gate- and source-connected plates, where the intrinsic transistor has VOFF = -2.3 $V$ and the gate field plate -50.5 $V$ . A plate pinches off the 2DEG beneath it at about $- q n_{s} d_{F P} / ϵ_{F P}$ (background knowledge), so -50.5 $V$ with an assumed $1 \times 1 0^{13}$ $c m^{2}$ and silicon nitride implies about 210 $nm$ of dielectric (computed). The plate transistor stays on in normal operation and pinches off at high drain voltage, which spreads the field and raises breakdown, and its capacitance appears as a step in the C-V curve. Reference: S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, ‘ASM-HEMT 101.0.0 Technical Manual’, Macquarie University and IIT Kanpur (16 March 2018) (equations partly garbled in the extract) (fetch-tool summary); S. Ghosh, S. A. Ahsan, A. Dasgupta, S. Khandelwal and Y. S. Chauhan, ‘GaN HEMT Modeling for Power and RF Applications using ASM-HEMT’, IIT Kanpur and UC Berkeley (venue and year not stated; references to 2016) (fetch-tool summary).

Ques: Does ASM-HEMT use the correct Boltzmann constant?

Nearly. The manual’s process constants list KB = $8.636 \times 1 0^{- 5}$ $eV$ per kelvin and q = $1.6 \times 1 0^{- 19}$ C, whereas the exact values are $8.617333 \times 1 0^{- 5}$ $eV$ per kelvin and $1.602176634 \times 1 0^{- 19}$ C (background knowledge). The KB value is 0.22 per cent high (computed), giving a thermal voltage of 25.91 $mV$ instead of 25.85 $mV$ at 300 $K$ (computed). Because the subthreshold factor NFACTOR and the cut-off voltage are fitted, the difference is absorbed into the extracted parameters and has no practical effect on a fitted model, but parameters moved between ASM-HEMT and another model can shift slightly; 89 $mV/dec$ of subthreshold swing for NFACTOR = 0.5 at 300 $K$ (computed) changes by the same 0.22 per cent. It may be a typographical slip or an older value (my reading). Reference: S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, ‘ASM-HEMT 101.0.0 Technical Manual’, Macquarie University and IIT Kanpur (16 March 2018) (equations partly garbled in the extract) (fetch-tool summary).

References

S. Khandelwal, S. Ghosh, S. A. Ahsan, A. Dasgupta and Y. S. Chauhan, “ASM-HEMT 101.0.0 Technical Manual,” Macquarie University and IIT Kanpur, Mar. 2018. Available: https://www.iitk.ac.in/asm/asm_package/ASMHEMT101.0.0_Manual.pdf
S. Ghosh, S. A. Ahsan, A. Dasgupta, S. Khandelwal and Y. S. Chauhan, “GaN HEMT Modeling for Power and RF Applications using ASM-HEMT,” IIT Kanpur and UC Berkeley (venue and year not stated). Available: https://iitk.ac.in/stc/Publications/YSC/4-GaN_HEMT_Modeling_for_Power.pdf

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