Depletion Type N-channel MOSFET

In the depletion type n-channel MOSFET (D-NMOS):

an n-type layer is added to the MOSFET structure

a channel is present even with no voltage applied

to the gate,

p-type

substrate

Oxide

n+

n+

n- type Channel

S

G

D

B

Operation of D-NMOS

1. Operation with 𝑣𝐺𝑆 > 0:

𝑣𝐺𝑆 increased it attracts more e- to channel

Channel become more conductive: enhanced

It conducts higher current iD,

2. Operation with 𝑣𝐺𝑆 < 0:

𝑣𝐺𝑆 decreased (becoming negative) it repels

e- from the channel

Channel becomes depleted

It conducts lower current iD,

As 𝑣𝐺𝑆 is further decreased it reaches a

negative value where the channel becomes

completely depleted:

This value is the threshold voltage 𝑽𝒕

p-type

substrate

Oxide

n+

n+

n- type Channel

S

G

D

B

𝑣𝐺𝑆 > 𝑉𝑡 ⇒ a channel is present it conducts a current iD > 0

𝑣𝐺𝑆 ≤ 𝑉𝑡 ⇒ the channel is fully depleted no current can pass and iD = 0.

Identical to the enhancement type NMOS

I-V Characteristics of D-NMOS

Circuit Symbol D

S

BG

D

S

G

iD

iS = iD

iGvDS

vGS

D

S

iD-vDS characteristics of D-NMOS

At vGS = 0V NMOS passes a current

For vGS = 0V, iD = IDSS

D-NMOS

I - vGS ≤ Vt No channel cut-off Region

𝑖𝐷 = 0

II - vGS > Vt Induced channel

1) vDS ≤ vGS - Vt; ( vGD ≥ Vt ) (continuous channel) Triode Region

𝑖𝐷 = 𝑘𝑛′𝑊

𝐿 𝑣𝐺𝑆 − 𝑉𝑡 ∙ 𝑣𝐷𝑆 −

1

2𝑣𝐷𝑆2

2) vDS ≥ vGS - Vt; ( vGD ≤ Vt ) (pinched-off channel) Saturation Region

𝑖𝐷 =1

2𝑘𝑛′𝑊

𝐿 𝑣𝐺𝑆 − 𝑉𝑡

2

These relations are identical to those of the E-NMOS, except that the value of Vt appearing in the

relations is a negative number.

Saturation Region

𝑖𝐷 =1

2𝑘𝑛′𝑊

𝐿 𝑣𝐺𝑆 − 𝑉𝑡

2

At 𝑣𝐺𝑆 = 0; 𝑖𝐷 = 𝐼𝐷𝑆𝑆

𝐼𝐷𝑆𝑆 =1

2𝑘𝑛′𝑊

𝐿𝑉𝑡2

Usually this value is used for design purpose