Well-posedness of the hydrodynamic model for semiconductors

Mathematical Methods in the Applied Sciences, Vol. 19, 1489-1507 (1996) MOS subject classifications: 35 Q 35, 35 L 60, 35 M 20

Well-posedness

of the Hydrodynamic

Model

for

Semiconductors

Li-Ming Yeh

Department of Applied Mathematics, National Chiao Tung University, Hsinchu, Taiwan, R. 0. C

Communicated by B. Brosowski

This paper concerns the well-posedness of the hydrodynamic model for semiconductor devices, a quasi- linear elliptic-parbolic-hyperbolic system. Boundary conditions for elliptic and parabolic equations are Dirichlet conditions while boundary conditions for the hyperbolic equations are assumed to be well-posed in L 2 sense. Maximally strictly dissipative boundary conditions for the hyperbolic equations satisfy the assumption of well-posedness in L2 sense. The well-posedness of the model under the boundary conditions is demonstrated.

1. Introduction

This paper addresses the well-posedness of the hydrodynamic model for semiconduc- tors. The model is derived from moments of the Boltzmann’s equation, taken over group velocity space. When coupled with the charge conservation equation, it de- scribes the behaviour of small semiconductor devices and accounts for special features such as hot electrons and velocity overshoots. The model consists of a set of non-linear conservation laws for particle number, momentum, and energy, coupled to Poisson’s equation for the electric potential. It is a perturbation of the drift diffusion model [7]. We consider a ballistic diode problem which models the channel of a MOSFET, so the effect of holes in the model can be neglected. The model is [4,11]

a,n

+

V . ( n V ) = 0, _(1.1) 1

4

I/

a,v

+

( V . V ) V

+

- V(nT) - - VY = --, m 2 P mn 1 2 2mV2 m V 2 T - T , a,T - - V . ( k n V T )

+

VVT

+

- T V . I/ - -~

+-

+--

- 0, (1.3) n 3 32, 32, 7, ( 1.4)

where

k

is a constant, and t E [0, tM], x E

a.

The electron density is labeled n, I/ is the

average electron velocity, T the temperature in energy units (Boltzmann’s constant

4

AY = - (n - Z ( x ) ) ,

0

CCC 0170-4214/96/181489- 19

0 1996 by B. G. Teubner Stuttgart-John Wiley & Sons Ltd.

Received 14 December I994 Revised 2 February I996

has been set to l), m the effective electron mass, q the electron charge, Y the

electrostatic potential, T, the surrounding temperature, Z ( x ) the prescribed ion

background density, o the dielectric constant, and R c R 3 the bounded semiconduc-

tor domain. Electric field E is given by E =

-VY.

The collision terms of equations

(1.2-1.3) are approximated in terms of momentum and energy relaxation times, respectively, [2,4]:

TS Ts T Ts

T ' L V = B 2 - + 8 3 - - -

2T T

+

Ts

TP = B 2 -

for some constants

B2,B3.

The initial conditions of system (1.1-1.4) are

V ( 0 , x ) = Vo, n(0,x) = n o > 0, T ( 0 , x ) = T o . (1.6)

In addition, system (1.1-1.4) is supplemented by the following boundary conditions:

where ep = n and the admissible form of M will be explained in section 3. The

hydrodynamic model couples a hyperbolic system of two equations (equations (1.1-1.2)), a parabolic equation (equation (1.3)), and an elliptic equation (equation For steady-state solutions of Euler-Poisson model, a simplified version of the hydrodynamic model, we refer the reader to the works [4,7,11,13,16], and references therein. The well-posedness of the transient model in the two-dimensional space was discussed in [ 131, where maximally strictly dissipative boundary conditions for hyperbolic equations were mainly concerned. For non-linear hyperbolic initial boundary value problems, we refer to [9, lo].

We consider the transient model in three-dimensional space. Boundary conditions for elliptic and parabolic equations are Dirichlet conditions while boundary condi-

tions for the hyerbolic equations (1.1-1.2) are assumed to be well-posed in L 2 sense.

Maximally strictly dissipative boundary conditions for the hyperbolic equations are shown to satisfy the assumption of well-posedness in L 2 sense. The well-posedness of

the model under these boundary conditions is then demonstrated. To d o this, we first

establish an existence theorem for a linear hyperbolic system by extending the results of [8,17]. Then the existence and the uniqueness of the solution of the transient hydrodynamic model are proved by employing contractive map and interpolation theorem [6,12]. More precisely, we prove the following:

Theorem 1.1. Under the assumptions A1-8 (see section 3), system (1.1-1.7) has a unique

classical local-in-time solution. (1.4)).

This paper is organized as follows: In section 2, pertinent notation is reviewed. In section 3, we reformulate the differential equations (1.1-1.7) and present the assump- tions A1-8 of Theorem 1.1. In section 4, maximally strictly dissipative boundary conditions for hyperbolic system are shown to satisfy the assumption of well-posed- ness in L 2 sense. In section 5, we establish an existence theorem of a linear hyperbolic system for our application. The existence and the uniqueness of a local-in-time solution of (1.1-1.7) are proved in section 6.

Well-posedness of the Hydrodynamic Model for Semiconductors 1491

2. Notation

Summation convention is used. c denotes various constants, which may not be the

same. That z is a tangential vector field in R means a vector field

z

in Q satisfies

z . 3 = 0 on aR (where 3 = (nl, n 2 , n 3 ) is the unit outward normal vector on aQ).

52' = [O, t M ] x R, aR' = [O, t M ] x

aQ.

For a system

the partial derivative "a:U(O)" of (2.1) is defined by formally taking i - 1 time

derivatives of the system, solving for 3: U , and evaluating at time t = 0, e.g. "U(0)"

=f,

"at

U(0)" = ( A O ( O ) ) - ' [ F ( O ) - Aj(O)a,f - B ( O ) f ] , etc. That compatibility conditions

for (2.1) hold up to m - 1 means that, on aR for 0

<

p

<

rn - 1,

Let Y * be, the formal adjoint of

9,

9* = -AOa,

-

~ j a ,

+

B* - ~ , A O - a , A j ,

where B* is the conjugate transpose of the matrix B. The kernel of

A

is the boundary

subspace denoted by N. The adjoint boundary subspace is N

*

= (AaN)', where

Ail :=

C

Ajnj. Let

A*,

adjoint boundary operator, be a matrix-valued function on

R x aR whose kernel is N * .

For p 3 1, ( I f P ) ' is the dual space of H P . For a matrix A, IAl denotes its

operator norm. Suppose A is a positive definite matrix, we define the norm

/I

U

by

3. Statement of the problem

We rewrite the system (1.1-1.4). Letting e P = n, (1.1-1.4) can be written as

T v 9 1 m T P m

a,v

+

( v q v

+

-

v p

+

- = -

v~

- -

v ~ ,

2mV2 mV2 T - T , a,T

-

kAT = kVpVT

-

VVT -

5

TV. V

+

-

-

__

-

____ 3 (3.3) 3Tp 32, 2, (3.4) 9 AY = - ( e p - Z ( x ) ) . 0

The new system (3.1-3.4) is equivalent to the old system (1.1-1.4).

To solve (3.1-3.4), we uncouple the system into a Poisson equation, a parabolic equation, and a linearized symmetric hyperbolic system. Next we show a fixed point of the uncoupled system exists in some weak spaces. Then by interpolation theorem, we show the fixed point is a classical solution of system (3.1-3.4).

System (3.1-3.4) is uncoupled as follows: Given Q, q, S, find Y by solving

9

AY = - (eq - Z(x)), w

Y l l a O = y b .

Then determine T by solving

2mQ’

mQ’

S - T ,

a , T - kAT = kVqVS

-

QVS

-

:SV.Q

+-

-~

- _ _ _

3TP(S) 3TW(S) TW(S) ’

Tlant = Tb,

T(0,x) = To.

Finally, we solve the following linearized system for V and p

T T T

m m m

- a , p + - ~ ~ p + - ~ . ~

= o ,

V(0, x) = V,, p(0, x) = In no.

Obviously, a smooth fixed point of system (3.5-3.9) is a classic solution of the system (3.1-3.4) and vice versa. Therefore, we can proceed to work with system (3.5-3.9).

We note that if a smooth solution of the system (1.1-1.4) exists, the following

compatibility conditions hold, for p 2 0,

Well-posedness of the Hydrodynamic Model for Semiconductors 1493

where derivatives are obtained by differentiating (1.1-1.4) with respect to t and setting t = 0.

Define

(q,

p i , Ti)

as follows: When i = 0, it is the initial condition (Vo,lnno, To);

When i = 1,2, it is obtained by formally taking i - 1 time derivatives of the system

(1.1-L3), solving for (aiV,afp,afT), and evaluating at time t = 0. Define K6,', by

Let us make the following assumptions

(Al) $2 is open, bounded, and smooth in R3,

(A21 0 <

(Vo,ii)21an

and 0

<

l(V0,ii)' - (To/m)I in 82,

(A3) 0 < no, To, Tb and Vo,no E H4(R),

(A5) To: I f 5 @ ) ,

Tl

E _{H4(R), T z} E H2(R), and there is a

T

satisfying

TIanG

= Tb, (A6) compatibility conditions (3.10-3.11) for system (1.1-1.4) hold up to 2. ( ~ 4 ) T, E

c3(cit),

z ( x ) E H ~ ( R ) , y b E ~ ~ ( a ~B E t )~ 3 ( a q , , M E H ~ ( R ' x

129,

a f ~

E L ~ ( H ~ - ' ) , i = 0,1,2,3,

a,4T

E L ~ ( H - ' ) ,

Remark 1. B y A3-5, we know

( q , p i ,

Ti)

E H3-i(R), i = 0,1,2. B y trace theorems [3],

we see K6,t, is not an empty set for some 6.

Given (Q, q, S ) E K d , t M , we are able to solve (3.5-3.6) to obtain T. Then by (Q,q, T ) , we

want to solve (3.7-3.9). For convenience, (3.7-3.9) will be written as Y W := Ao(T)atW

+

A j ( Q , T ) a , W

+

B(T) W = F in R',

M ( t , X,

Q,

q, T) W = on aR', (3.13)

W ( 0 , x ) = (Vo,lnno)' in R,

where

W : = ( V , p ) ' , F : =

Let a,, a , , a2, a3 be constants satisfying

(1) 0

<

a l < Ao(To)

<

a. in R',

(2) n j A j ( V o , To) is non-singular and 0 < a3 < InjA'(Vo, To)/ < az on aR'.

(A7) There are two constants M 1 , M z such that, for any ( Q i , q i , S i ) E K 6 , t M ,

B y A2-3, above four constants exist. T w o more assumptions for (3.13). i = 1,2,3, the following hold on boundary aRt,

and

*(lVolMl

+

IlnnoIM2) < min{l,c(R)k).

(A8) There is a positive constant a. such that, for any a 2 ao, W E H ' ( R x

n),

(a) the followings hold:

(Q,

Y], T ) E &,t,

(3.14)

(3.15)

where 0

<

t l < t2

<

tM, 9, M are the operators in (3.13), and 9*, M* are the adjoint

operators of 9, M (see section 2),

2a1 2

+

a0

II

w e IIL2(n)(t2),

(b) there are Bk,Mk, and symmetric A;,A: c P ( @ ) satisfying

k - + c o ,

{AkO,Ai,Bk}

-,

{AO(T),Aj(Q, T ) , B ( T ) } in

x3(Qt),

Mk + M ( t ,

x,

Q,

?, T ) on ff3(aR'),

and inequalities (3.14-3.15) hold for the following

y k w : =

&a,w

+

A : a , W

+

BkW = F

M k W = g on aRr, (3.16)

W(0,x) = (Vo,lnno)' in R,

in

a',

where 9, M , 2'*, M* in (3.14-3.15) are replaced by y k , M k r 9 t , M:, respectively,

(T;, M: are the adjoint operators of

zk,

Mk).

Actually, A8 assume that boundary condition M(t, x, Q, q, T ) for the hyperbolic

system (3.13) is well-posed in L2 sense. In section 6 we will show that if A1-8 hold,

then a fixed point of system (3.5-3.9) exists uniquely in a short-time period, which

implies Theorem 1.1.

4. Example

We give one example for M of (1.7) such that assumptions A1-8 hold. Multiplying

(3.2) by T/m and combining it with equation (3.1) along with initial and boundary

conditions, we obtain

Ao(T)a, W

+

Aj(V,7')ax,W

+

B ( T ) W = F in R',

W(0,x) = (Vo,lnno)' in R,

Well-posedness of the Hydrodynamic Model for Semiconductors 1495

where W := (V,p)'. Assume that the matrix A-,(=Ajnj) of (4.1) on

ant

has negative

eigenvalues

A,,

p = 1

...

t,

and positive eigenvalues

A:,

v =

t

+

1

...

4, and, corres-

ponding to them, orthonormal eigenvectors rp-,r:, p = 1

...

t,

v = t?

+

1

...

4. The

boundary condition M is taken as

c c 4

M W =

2

( r ; . W)ap,,rp-

+

c

1

(r,?. W)bp,,rp-

p, Y = I p = l v = c + 1

where up,", bp,,, are smooth functions and (ap,") is an invertible matrix in

an'.

If bp," = 0

for all p, v, then M is maximally strictly dissipative boundary condition [ 171.

Remark 2. T h e eigenvalues of A-, for (4.1) are A1 = A2 = V,( := V

.G),

(1

+

T/m)V-,

+

J [ V i ( l

+

T/m)' - 4T/m(V$ - T / m ) ]

2 3 = 9

(1

+

T/m)V-, - J[V,'(1

+

T/m)'

-

4T/m(Vi - T/m)]

2

A4 =

Let z1 := ( T ~ ~ , ~ ~ ~ , T ~ ~ ) , z2 := ( T ~ ~ , T ~ ~ , T ~ ~ ) be two orthonormal tangential vectorsin 0,

and [ be dejned by

T h e corresponding eigenvectors of

A1,A2,A4

are ( T ~ ~ , T ~ ~ , T ~ ~ , O ) , ( ~ ~ ~ , ~ ~ 2 , ~ 2 3 , 0 ) , and (nl , n 2 , n 3 , - m [ / T ) , respectively.

Therefore on inflow boundary {(t,x) E aR') Vs(O,x)

<

0}, (1) if V$(O, x) < T,/m, we specify three boundary conditions (because A1, A 2 ,

L4

are negative and l 3 positive), ( 2 ) if

T,/m < V$(O, x), M is the identity matrix (since all of the eigenvalues are negative). T h e former corresponds to subsonic case, and the latter to supersonic case. However an outfrow boundary { ( t , x ) E

ant/

V-,(O, x )

>

0}, (3 ) if V$(O, x) < T,/m, we need one bound- ary condition (because only

A,

is negative), (4) i f T o / m < V,"(O,x), M is the zero matrix (because no boundary condition is needed). Similarly, ( 3 ) and (4) correspond to subsonic and supersonic cases, respectively.

For example, M W = 39 of (1.7) can be: in case (1)

which means ( W r t , W r 2 , W-, - p m [ / T ) = B f on a0, where Wr, := W . z i , i = 1,2, in case (2) ( W , p ) = 99' on aR, in case ( 3 ) M = (nl ,n2, n 3 , - m [ / T ) , i.e. W, - pmi/T = 99 on

80, and in case (4) M = 0.

If A1-6 and (4.2) hold, and if ( P ( ( O , ~ ) ( ~ ~ , { b p , v ) of (4.2) are sufficiently small, then

they imply assumptions A1-8 hold. A7 holds obviously if JPl(0,x)lan is small. The

Lemma 4.1. Consider (1.1-1.7). Besides assumptions A1-6, ifthe following holds: M of

(1.7) is constructed as (4.2), { a , , } and { b p , v } are in H3(aRt), and they satisfy

max laibl < bC1, max Ibp,J

<

bo, V P , ~ ,

4(1/a2

+

l/a3)b;'b;

<

l/a2,

an1 anc

for some constants b l, b o , then A8 holds for any 6 as long as tM is small enough. Pro05 (3.14) is proved by energy L 2 estimate. By means of a local co-ordinate change

and a partition of unity, it is sufficient to prove (3.14) in a half space. The estimate in

a half-plane can be obtained as follows: Multiplying (3.13) by W integrating the

resulting equation over Rt, and employing assumptions of boundary conditions of this

lemma.

(3.15) can be proved in a similar way as (3.14). A8(b) holds by using Friedrichs' mollifiers and arguing as the proof for (3.14).

5. Existence results for linear problems

In this section, we present existence results for a parabolic equation (Lemma 5.1)

and for a linear symmetric hyperbolic system (Lemma 5.4). These are used to show the existence of a fixed point of system (3.5-3.9) in section 6. Lemma 5.1 is a standard

result. Lemma 5.4 is established by Lemmas 5.2, 5.3. Lemma 5.2 gives an a priori

estimate for a linear symmetric hyperbolic system (5.3) by energy method. Lemma 5.3

is to present approximate systems of (5.3) so that the solutions of these approximate

systems exist. Lemma 5.4 is to show that the solutions of the approximate systems converge (by a priori estimate obtained in Lemma 5.2). Furthermore, we show the

limit is a solution of (5.3). Domain R will be assumed in Rd.

Lemma 5.1. Consider the following system

a,U - aAU = F in R',

u = g

on aR', U ( 0 , X )

=f

in R,

where a is a constant. I f F E H2(Rt), a:F E L 2 ( H - ' ) and if there exists a function

@ satisfying

@laat

= 9,

a;@

E L ~ ( H ~ - ~ ) , i = 0,1,2,3,

a;'@

E L ~ ( H - l),

af(U -@ ) ( O , X ) E H ; ( R ) , i = 0 , 1 , 2 ,

a:(u

- @ ) ( O , X ) E L ~ ( R ) , then there is a U 6 X3(R') such that

af

U E L2(H4-'), i = 0,1,2,3, and

i = O L i = O

Well-posedness of the Hydrodynamic Model for Semiconductors 1497

Lemma 5.2. Consider the system

(5.3)

I f the following conditions hold:

1. R is open, bounded, and smooth in Rd,

2. A', A', B E X,(R') are j x j matrices, A%' E H"(aR'), and s 2 [d/2]

+

2,

3. A', A' are symmetric and 0 < a, < A'

<

a. in R',

4. Aa :=

1

A j n j is nonsingular on aR' and 0 < a3

<

I

Ah

I

<

a2 on

aR',

5. F E Hm(R'), f E Hm(R), g E Hm(aR'), 1 G m d S,

6. for any U E H'(R x

n),

0 d t l < t 2

<

t ~and , alcl 2 ~ ( l l L ( ( ( ~ , ~ ~ ~ ) , (3.14) holds for

(5.3), i.e.

(5.4)

Proof: By means of a local co-ordinate change and a partition of unity, it is sufficient

to prove (5.5) in a half-space. The estimate in a half plane will be obtained by

differentiating the equation (5.3), to estimate tangential derivatives and then, using det Aa # 0 to solve for the normal variables in terms of the tangential ones.

For convenience, we assume (5.4) holds in a half plane. (5.5) is proved by induction.

Proof of this lemma is close to that of Lemma 3.2 in [17], so similar arguments will

not be repeated here. The main difference between the two proofs is the estimate for

/I

Aa,Tx,u

- a Y x , ( A U ) IIO:anr,n.

where

a/,x,

is the y derivative with respect to t , x' = (x2,

.. .

, xd) and I y ( = 0,1,

.. . ,

s. By

(5.41, (5.6)

+

a0

II

a : x J J ll;n(O). (5.7)

By Holder inequality, embedding theorem, using det Aa # 0 to solve for the normal

variables in terms of the tangential ones, and an argument similar to (3.20-3.28) of

[17], one can show that, for 0 6 t 6 t M ,

Next we assume the following inequality holds:

+

a0

I1

u

Il;n(O). (5.10)

By Holder inequality, (5.9), and arguing as (3.31-3.38) of [17], one can show that

11

~ a T x ~ u

-

a y x , z u

ll;nh

Well-posedness of the Hydrodynamic Model for Semiconductors 1499

m - 1

<

c

C

I I a ~ ~ p ~ I I t ~ ~ ( a n ~ ) I / a [ x , U e - * ' IItyant), (5.11)

where for (i) m = s, p = 0 case we pick p = 1, q = co in (5.11), and for (ii) m < s, or for (iii) m = s, p > 0 case we pick p , q 2 1 in (5.1 1) in such a way that

p = o

1 1 1 d - 2 ( s - m + p ) 1 d - 2 ( m - p - l / 2 )

d - 1

I = - + - , - 2 >

P 4

P

d - 1

For case (i), inequality (5.1 1 ) implies that

llAa?x,U - aG,.,d~llo2,an~,, G max lU12

lla?x~~llo2,iJnh

n'

(5.12)

For cases (ii), (iii), inequality (5.1 1 ) implies that

By using det A, # 0 to estimate

lla~x,,x,

U Il&e-2"' as (3.39-3.42) in [17] and combin-

ing (5.10-5.13), we complete the proof.

Remark 3. Consider system (5.3). Besides the assumptions of Lemma 5.2, we assume: 1. compatibility conditions for (5.3) hold up to m - 1, 2

<

m

<

s,

2. there are B k , A k , and symmetric { A t , A : } c C m ( n f ) such that, as k -+ co,

{ A ~ , A : , B ~ } -+ ( A O , A ~ , B } in

x,(cY),

A k+ 4 on Hs(aQt).

B y {A:, A:, B k , A k } above we consider the following:

(5.14)

I t is clear that if k is large, assumptions 2-4 of Lemma 5.2 hold in (5.14), where

(A', A', B , 4 , A,} are replaced by { A t , A { , Bk,J%Lk, Ak,,( : = A [ n j ) } . Let S?:,

A:

be the

3. for U E H'(R x

Q,

0

<

t l < t2

<

t M , and alcl 3 C ( ( ~ A O , A ~ , B ~ ~ ~ ~ , ( ~ I , , (5.4) and the following hold for (5.14):

Zat 2

+

a0

II

u

e llL2(n,(tz).

Assumption 3 above is similar to A8(b) but for (5.14).

(5.15)

Lemma 5.3. Under assumptions of Lemma 5.2 and Remark 3, there exist a sequence of functions {Fk,fk,gk} such that Fk E H"+'(R'),fk E

ff"+2(n),

g k E Hrn''(afif),

Fk -+ F in H"(R')

fk+f in H"(R) as k - i c o ,

g k -+g on Hm(aRt)

and,

if

we consider the following (replacing F 7 J g of (5.14) by F k , f k , g k )

z k uk= A j a t u k f Akjax, u k

+

Bk

uk

= Fk in

a',

A k uk= g k on aili (5.16)

u k (07 = f k ( x ) in R,

then (5.4), (5.15), and compatibility conditions up to m also hold in (5.16).

Remark 4. First let us give definition (for (5.3)) for Gi,f,:

Gi := -(ai[(Ao)-1AJ])8j - a:[(Ao))- ' B ] , i 3 0,

fo := U ( 0 , x ) =f;

f p : =

pi'

( p

7

l ) G i ( O , x ) f p - l - i

+

(ap-'[(Ao)-'F])(O,x), p 2 1. (5.17)

i = O

By definition, f , corresponds to "apU(0,x)". W e define B,, E, as Bo f := f o = f and B, f

+

E,(A0)-'F := f,, p 2 1, where

B, f := G t ( 0 , x ) f

+

term of the form GG G I ,

. . .

GZq(O, x ) f (5.18) w i t h y + i , +

...

+ i , d p - l a n d

E,(Ao)-'F := sum ofthe terms of the form GLG,,

...

Glq(0)C$'[(Ao)-lF](O)

(5.19) with y

+

i,

+

...

+ i,

+

p b p - 1

Proofi ( 1 ) That (5.4), (5.15) hold in (5.16) is because (5.4), (5.15) hold in (5.14). By assumption 2 of Remark 3, there are smooth functions A;,Akj,Bk,Ak which converge to A o , A J , B , A . Consider system (5.14) for each k . Let us use f k $ Bk,p,Ek,p to

Well-posedness of the Hydrodynamic Model for Semiconductors 1501

denote the f p , B,, E , of (5.14) (see (5.17-5.19)). By the assumption of compatibility

conditions for (5.3), on boundary asZ, for 0

<

p

<

rn - 1,

where f p - p is the ( p - p)th derivative, "af-"U(O,x)", of (5.3). Since

we may choose k such that

Let q 2 rn

+

2 be an integer. For fixed k, we choose sequences F: E H"+q(sZ'),

4;

E H"+q(R), and g: E Hm+q(aC2f) such that

(5.21)

Then we want to find i ( k ) and such that, for 0

<

p

<

rn,

(5.22) 1 2k' hi'k' E H"+2(R),

jl

h p

IIHyn)

d -

i

( ~ ) a : & k ( B k , p - p ( - h i ( k '

+

4;'")

+

E k , p - p ( A t ) - l F L ( k ) ) -

a

p

g k i'k) = 0 9 p = O (5.23)

for ( t , x) E {O] x

an.

Suppose is available, we define

f k i ( k ) = - h l ( k ) +

4 i ( k )

Then we see, when i(k) is large,

I/

F f ( k ) - F

I I H ~ R ' )

< c / k

II fki(k)

-f

Ilwyn) <

c / k II

g:'k' - IIHyaR') <

c / k

and system (5.16) satisfies compatibility conditions up to m. So Lemma 5.3 holds true.

Next step is to search for

(2) Equation (5.23) can be written as, for 0

<

p

<

m, ( t , x ) E (0) x

an,

Let Rk be the inverse of d kwhen it is restricted to the orthogonal complement of the

kernel of d k . Note a:gf(o,x) is in the image of &k. For

o <

p

<

m on (t,x) E ( 0 ) x

an,

to solve (5.24) is sufficient to solve

(5.25) The left-hand side of equation (5.25) can be written as

P - 1

p = 1 y = o

Bk,ph:

+

R k (;)ardkBk,p-uhL =Dkqaa:hf

+

ck,p,p-ya$hi,

where Dk,ii:= -(A;)-'A{nj, a;h: denotes the normal derivative of h: on

an,

and

C k , p , p - an operator of order ( p - y) which only contains derivatives tangential to

an.

So equation (5.25) becomes

(5.26) where aqhh := bp,i,k. Since a p . I , . k E Hmtq-p-1/2 (an), it follows that

(an). b . E ~ m + q - ~ - 1 / 2

p . i , k

Also by (5.20-5.21) and (5.25-5.26), there is a large i ( k ) such that IIbp,i(k),k I l H m - p - 1 / 2 ( a q < c/k, 0 d

P

d m - 1.

Furthermore, by a trace theorem [lo], there is a u k E Hm+q(Q) such that

= bp,i(k),k, 0 d p d m - 1,

//

u k / / H y q

d

c/k.

Finally, we write hLfk' = u k

+

wk, where wk is required to satisfy wk E H~(R), and,

/I

wk

/IH"(R)

<

c / k ; arwklan = bm,i(k),k

-

arVk; a $ W k l a f i = 0, 0

<

p

<

m - 1.

To find such a wk, we refer the reader to the proof of Lemma 3.3 of [8]. Therefore the

Lemma 5.4. Under the assumptions of Lemma 5.3, system (5.3) has a unique solution U E X,(n') n Hm(i3Qt) a n d there is a polynomial P such that

desired hi(k' can be obtained.

I//

uIII&(Qfi

+

II

u

II&aoll

e 2 a h

<

~ ~ ( l l l ~ l I l X S ( * ~ ) 9 l l ~ l l ~ ~ ~ ~ ~ ~ ) ~

+

~lIglI~-(aa) 2 ~ ~ l l

+

~

llflli~i*)l~

l l i ~ i * ~ )

a1

(5.27)

Well-posedness of the Hydrodynamic Model for Semiconductors 1503 Proof: (1) By Lemma 5.3, there exist

&

and symmetric matrices A f , A : E Cm(Cl'),

d kE

cm(aQ'),

Fk E H""(a'),f, E H"+'(R), and g k E f f m + 2 ( 8 Q * ) such that

( A : , A:,

&}

-

{A', A',B} in X,(CY)

d k--t &? on Hs(aR')

Fk --* F in H"(Rt) as k +co.

.ti

-f

in Hm(R)

gk

-

on Hm(aRt)

Furthermore (5.4), (5.15), and the compatibility conditions up to m hold for system

(5.16) for each k . Consider the system (5.16). By Proposition 5.1 of [ 8 ] , one can show

that system (5.16) has a solution

uk

EX,, By Lemma 5.2, { U k ) is bounded, i.e.,

]/I

U k l l l X ,

+

11

Uk lIH"(af2') 6 c, v k . (5.28)

Let us consider the following:

p(vk -

u/)

= Fk - FI

+

(9 - 9 k ) U k - (9 - 9 l ) U l in R',

d ( U k - U , ) = ( A - A k ) uk

+

(Ai

-

d )

u,

+

gk - gl on aiz',

( U k - U/)(o,x) = f k

-fi

in

iz.

(5.29)

Inequality (5.4) of Lemma 5.2 applied to { Uk - U , } shows that {

uk}

converges in

C ( L 2 ) . Let U be the limit of { U , } , then U E C(L'). By (5.28),

I1

U IIHm(an~) 6 c.

By (5.28) and the Sobolev's interpolation theorem [l, 61, we have for arbitrary v

>o,

1 - v i m

11

Uk - U / llHm-*'(Q)(t) 6

/I

Uk - U /

11

Uk - uI \/H"(R) ( t ) ?

so

Uk + U in C ( H m - " ) as k -+a.

By (5.3) and (5.16), we see that

m - 1

Uk -+ U E C i ( H m - i - v ) as k +a,

i = O

a#

E L " ( H m - i ) , i = 0

...

m.

By

( 5 . 3 ,

we see U satisfies the estimate (5.27). Equation (5.27) implies solution of (5.3) is unique. So the proof is complete as long as we know

af

U E C ( H m P i ) , i = 0

...

m.

( 2 ) That a f U E C ( H m - i ) , i = 0

...

m, is proved by three steps:

Step 1: As t -+to, afU(t;) -+ a l U ( t 0 ; ) weakly in H m - i ( Q ) for all i 6 m.

Step 2: a r x U ( t ; ) is continuous at t = 0 in L2(iz). Step 3: U E Xm(Cl').

Proofof Step 1. If q < m and € ( H q ( Q ) ) ' , by (5.30), $ l ( u k ( t , . ) ) - + $ t ( U ( t , . ) ) uni- formly in t as k -+ co. Now if $ E (H")', there is a $ I E ( I f q ) ' , for some q < m, such that

1)

$ - $1

Il(Hm),

d 1/2k for any k . By (5.28), for all t,

/ $ ( U k ( t ? ' ) ) -

$('(t,'))I

6

l$(uk)

-

$l(uk)l

+

l $ l ( u k )

-

$ l ( U ) l

+

1$1(U) - $ ( U ) /

6 C / 2 k - '

+

I $ l ( U k )

-

$ l ( U ) l .

This implies that $(&) -+ $ ( U ) uniformly in t. Because $(&) is continuous in t, $( V )

is also continuous in t for all $ E (H"(R))'; that is, as t --+ t o , V ( t ; ) -+ LJ(t0;) weakly in

H"(R). Similarly, one can show that if t + to and 1 d i < m, then

af

U ( t ) -+

af

U ( t o )

weakly in H"-'(R) by (5.30). Then, by (5.3), we see that a?U(t;) - a ~ U ( t 0 ; ) weakly

in L2(Q) as t -+ to.

Proof of Step 2. To prove this step, we will show the convergence result in an

equivalent norm

// .

I/O,AO(0).

By differentiating (5.16) with respect to tangential vectors

or time a:,,, we have

- % k a ~ , t U k = a ~ , t - % k U k + ( - % k a ~ , t u k - a y , t - % k U k ) in R',

&ka;.tuk =a:,t(&kUk)

+

( A h a y , t u k - a y , t ( & k u k ) ) on (5.31)

a;,, U k ( 0 , x) =

a:,,

uk(o,

x)

in R.

Multiplying above by a;,, V k , and integrating the resulting equation over R, we derive,

for

I y I

d m,

(5.34) Now, since U k ( t , ' ) converges weakly to U ( t ; ) in H"(R) for each t, U k ( t ; ) converges

weakly to U(r;) in H"(Q) in an equivalent norm for each t. So for all I y I d m,

(5.35)

Well-posedness of the Hydrodynamic Model for Semiconductors 1505

Let RE be the intersection of R and a E-neighborhood of a n , and let $& be a half bell

function satisfying cl/"(x) = 1 for x E aR and with support in

a'.

Multiplying (5.31) by

I,P, then applying (5.4) to the resulting equation, one can show that, for any E,

lim

1's

(e-2af$Ea;,,Uk(2dodt

k - m 0 an

<

c

(il$%

u

IILa(n)(0)

+

1' 1

le-2afa;,,g12 d a d t

+

R(z)dz , (5.38) for some constant c. To obtain the result (5.38), we need to control the boundary term

11

uk

- a;,r(Ak

uk)

/ 1 , & 1 , ~ . It can be estimated by the same technique as

(5.1 2-5.13). (5.38) implies that

o an

l

)

_ -

lim lim

1' 1

la:,

ukAk,aay,t

ukl

d o d t

<

cjl

pay,,

u

i ~ ~ ~ ( ~ , ( o ) .

5-0' 5-0' n!

(5.39)

5 + 0 + k + ~ o an

The right-hand side goes to 0 as E approaches 0. Since the left-hand side is independent

of E, it equals 0. By the continuity of A', we get, for JyI Q rn,

lim

In

a;,tuAOa;,,u(~,X)dx =

lim

a~,tu(r,x)~O(o,x)ay,,u(~,X)dx.

(5.40) By (5.37-5.40), we have, for J y J Q rn,

lim

S,

a:,

t ~( (7x ) ~ o ( ~ x ) ~ , t

~ ( 5 ,

dx Q

11

a:,,

~ ( 0 ) 1102,Ao(o).

5-0'

(5.41)

Combining the result of Step 1, Theorem 3.8 in [l], and (5.41), we see

a:,,

U ( t ; ) is

continuous at t = 0 in L2(R) for all / y J = m.

Next we show that a?,U(t;) is also continuous at t = 0 in L2(R). By partition of

unity, we can reduce the problem to the boundary aR. By using det Aa # 0 on aR', we

can solve for the normal variables in terms of the tangential ones and time. Because

a:,,

U(t;) is continuous at t = 0 for ( y I = rn, a;,U(t;) is also continuous at t = 0 in L 2

around the boundary aR. Therefore, we conclude that U(t,. ) is continuous at t = 0

in L2(R).

ProofofStep 3. By the same argument as that in Step 2, one can prove strong right

continuity at any point t E [O,t,). Note that equation (5.3) and the argument of

Step 2 are reversible in time, so the proof of strong right continuity in [0, t M ) implies

strong left continuity on (0, t M ] . Therefore, we conclude that U E X,(R').

6. Existence of a unique local-in-time solution

In this section we prove the existence of a unique local-in-time solution of the system (1.1-1.7) under assumptions Al-8 of section 3. That is equivalent to showing the existence of a fixed point of the system (3.5-3.9). By A3-5, we know K6,1, (see (3.12)) is not an empty set for some 6. Given ( Q , q , S ) E K 6 , t y , we are able to solve

n(Q, q, S) = ( V , p , T ) . The existence of a unique local-in-time solution of the system (1.1-1.7) is proved as follows: First we show that, as tM is small enough, there exists

a 6 such that

II

is a map from K 6 , [ , to itself (Lemma 6.1). Next we prove that

n

is

a contractive map in some weak spaces. So we obtain a fixed point in these weak spaces. Then we show the fixed point is a smooth solution of the system (3.5-3.9) (Lemma 6.2). The existence and uniqueness of classical solution of system (1.1-1.7) follows the result of Lemma 6.2.

Lemma 6.1. Under assumptions A1-8 of section 3, as tM is small enough, there exists

6 such that (3.5-3.9) is uniquely solvable and the solution of (3.5-3.9) is in K6,[, for all

ProoJ: Given (Q, q, S) E we solve the system (3.5-3.9) to obtain ( V , p, T ) by A1-8

of section 3 and Lemmas 5.1, 5.4. By Lemma 5.1, the solution of (3.6) satisfies

(Q, V , S) E Kd,r,.

T E X ~ ( C F ) , afTEL2(H4-i), i=O,1,2,3,

afT(o,x) =

Ti,

i = 0,1,2.

By (5.2),

111

T -

T o ,

T ,

,

T 2 ,

T,, no, Vo but is independent of 6.

+

11

T - To ( I H 3 ( a n t ) d dl. If tM is small enough, d1 depends on

By Lemma 5.4 for d = m = s = 3, we see that the solution (3.7-3.9) satisfies

( ~ , p ) ~ x ~ ( n I ) n ~ ~ ( a n I ) , (a:v,a:p)(o,x)

=(c,pi),

i = 0,1,2.

BY (5.271,

IIt(v,p)

-

VO,1nno)III&

+

II(V,p) - (~O,lnno)li&an~, G 6 2 . If tM is small

enough, we see that d2 is dependent on no, Vo, To,"* but independent of 6. If we take

do := max{6,61,62}, repeat the above procedure, and let tM smaller, we see

( V , p, T ) E KGo,tm for all (Q, q, S) E KdOlfy. That is,

I

I

is a map from K d , , t M to itself.

W

Lemma 6.2. Under assumptions A1-8 of section 3, system (3.5-3.9) has a uniquefixed

point

if

tM is small enough. Moreover, the solution satisfies

(afv,ajp,afT)E L " ( H ~ - ~ ) , a f T E L ~ ( H ~ - ~ ) ,

afw

E L " ( H ~ - ' ) , (6.1) for i = 0,1,2,3.

Proof. By Lemma 6.1, II(Q,y, S) = ( V , p , T ) is a map from Kd,t, to for some 6.

Given ( Q l , q l , S l ) and ( Q 2 , q 2 , S 2 ) , by solving (3.5-3.9) we get two solutions

( V l , p l , T l , Y l ) and ( V 2 , p z , T 2 , Y 2 ) . Subtracting one solution from the other, multi-

plying the difference of (3.6) by ( T I - T2)e-2"', integrating the resulting equations

over Q', using (5.4) for the difference of system (3.7-3.9), we see, if c( is large and t M is

small enough,

II

is a contractive map in V , p E C(L2)nL2(aR'), T E L2(Q'). That is,

/I

Ti -

GIl;n',.

+

Il(v1

- V2,PI - P2)e-2a'11Z~cn,

+

IlVI

- V29Pl

-

Pz)ll;ant,. 2at 2

< c

/I

Si - S 2 Il;n',,

+

II

(Qi - Q 2 , ~1 - V Z ) ~ - 1 / ~ 2 ( n )

2

+

ll(Qi

- Q 2 , ~ 1 - ~ 2 ) 1 / 0 . a n ~ , ~ ~

for some constant c < 1.

By the above, if we define n(Vk- 1 , pk- 1, Tk- 1 ) = ( v k , pk,

Tk),

then sequence { Vk, P k , Tk} converges to the unique fixed point { V , p , T } where

v,pEc(L2)nL2(af21), TeL2(fit). Since {Vk,pk,Tk} c ( V , ~ , T ) E

Well-posedness of the Hydrodynamic Model for Semiconductors 1507 a f T E L 2 ( H 4 - i ) , i = 0,1,2,3. By Sobolev's interpolation theorem [1,6], we have for arbitrary v

>

0 (set w k := ( V k , P k ) )

11

w k

-

IIH"-'(12)(t)

I/

wk

- wi

IIti:l2)(s

11

wk

-

w i i i , $ i A { 3 ( c t ) 7

which implies that I/, p E C ( H 3 - " ) . By equation (3.7), we see

a:

V , alp E

C(H3-v-i).

Therefore { V , p , T ) is the unique classical solution of system

(3.5-3.9). Moreover, by (3.1-3.4), we see the fixed point satisfies, for i = 0,1,2,3,

(afv,afP,a:T)EL")(H3-'),

a f ~

€ L ~ ( H ~ - ~ ) ,

a f y

P(H~-').

Theorem 1.1 is a direct result of Lemma 6.2.

Acknowledgement

The author would like to thank Professors Jim Douglas Jr., Peter Markowich, and Anton Arnold for their valuable suggestions while preparing this paper.

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