Mechanism of Forward Bias Assisted Drain Hot Electron Injection in

The concept of forward bias assisted programming is proposed by Z. Liu in 1999 [13].

The sample is N-Channel floating gate cells. This program scheme is called “Pulse Agitated Substrate Hot Electron Injection” (PASHEI). The illustration of Program setup is shown in Fig. 4.6. The programming pulse is divided into two parts, emitting time and collect time. The emitting is used to make the substrate-drain diode in forward bias (a negative drain voltage Ve

is applied) and the diffusing current is taking place. Then, changing the diode to reverse bias mode (a positive drain voltage Vwr is applied), and the electrons which are flowing into substrate will be injected into floating gate. The gate voltage VG keeps positive during emitting and collecting time. Please refer to Fig. 4.7 for the injection mechanism. The advantage of this programming technique is only a small programming voltage (~5V as gate voltage and 4V as drain writing voltage) is needed. The disadvantage is a multiple pulse is needed in a program procedure, such as: 100 pulse per programming. It makes the long period of programming time and a complicated circuit design is needed in order to apply this continuous pulse.

We modify the pulse pattern in this study. As shown in Fig. 4.8. We call the new scheme

“Forward Bias Assisted Substrate Electron Injection” (FBASEI). The difference is that the gate voltage is zero during the emitting period and is positive during collecting period. The mechanism is similar as PASHEI, so the injection current is from the substrate too. The performance comparison of CHE, PASHEI and FBASEI is shown in Fig. 4.9. The characteristic of FBASEI is also pulse count dependent, pulse period independent, and low operating voltage is achieved [13]. The reason of poorer programming speed of FBASEI (comparing with PASHEI) is that the less gate pulse period makes fewer electrons injected into the floating gate.

0 V

_G

emitting collecting

floating

S D

FG

CG

V _G

V _G

B

V _S

(b)

V

_wr

V

_e

(a)

Fig. 4.6 The programming setup and pulse pattern of PASHEI in n-channel flash cells.

-D (n+)

(b) V

_D

=V

_e

=-1

h

⁺

e

-D (n+)

(c)V

_D

=V

_wr

=6 e

-+

D (n+)

(a) V

_D

=0

B (p)

Fig. 4.7 The injection mechanism of PASHEI in n-channel flash cells. Substrate electron current inject to floating gate by the effect of vertical electric field.

floating

S D

FG

CG

V _G

B

(b) (a)

0 V

_G

emitting collecting

V _G V _S

V

_D-High

V

_D-Low

Fig. 4.8 The programming setup and pulse pattern of FBASEI in n-channel flash cells. The substrate (p)-drain (n) diode in emitting period is in forward bias and is in reverse bias in collecting time.

1E-7 1E-6 1E-5 1E-4 1.0

1.5 2.0 2.5 3.0 3.5

4.0 CHE

PASHEI FBASEI

Threshold Voltage, V

th

(V)

Program Time (sec)

Fig. 4.9 The characteristics of transient program of CHE, PASHEI and FBASEI in n-channel flash cells.

4.4 Mechanism of Forward Bias Assisted Drain Hot Electron Injection in P-channel Flash Cell

In this section, we will first discuss the difference of mechanism of forward bias assisted hot electron injection in P and N Channel cells. Second, we define the gate current components under different program schemes. Finally, we compare the characteristics with the assistance of forward bias at different operation conditions.

The injection mechanism and performance of the forward bias assisted programming scheme in P-channel cells is quite different from the in N-Channel cells. Fig. 4.10 (a) shows the programming setup and pulse pattern of this program scheme. The concept is similar to N-Channel ones. First, a moderate positive bias is applied on the P-type drain side, and the junction is under forward bias. When changing the mode to reverse bias, deep depletion region is created and carriers in this region suffer strong electric field. Impact Ionization occurs and much energetic carrier is generated and injected into the floating gate. Please refer to Fig. 4.10 (b) for the detailed mechanism. The difference of the scheme in N-Channel ones is the carrier generated in the drain depletion region, so it is Drain Electron Injection and can be called Forward Bias Assisted Drain Hot Electron Injection (FBADHE).

Fig. 4.11 shows the operational region (the combination of gate voltage and drain voltage) with different programming schemes. Before discussing the injection mechanism of the above schemes, we should first clearly define the regions of breakdown, sub-breakdown and Band-to-Band tunneling region. The I-V characteristic of drain-substrate diode is shown in Fig. 4.12. When the drain voltage is larger than 0.7V, the junction is in forward bias (drain current >0); on the contrary, when the drain voltage is below than 0.7V, the junction is in reverse bias (drain current <0). Significant reverse bias leakage current rises as VD= -5.5V (~10^-11 A), and increases rapidly to ~10^-4 A at VD= -7V. The point (I) (VD= -5.5V) is called Zener Breakdown. This is due to Band-to-Band tunneling which electrons tunnel from the

p-side valance band to n-side conduction band [23]. The point (II) (VD= -7V) is avalanche breakdown. It is due to impact ionization induced generated electron-hole pair. The original and generated electrons are both swept to the n side of the junction, and the holes are swept to the p-side [23]. The drain voltage in FBADHE (~6V) is between (I) and (II), both breakdown mechanism may also occur. Avalanche breakdown is the main breakdown mechanism in p-n junction [24], so in this region, we believe the avalanche breakdown dominates. In the following description of this thesis, we call this region sub-breakdown region. It is noted that the breakdown voltage is related to the doping concentration of the drain and substrate.

Next, we discuss the mechanism of injected electrons in different operational conditions.

In FN-PGM regions, of course the injected current is F-N tunneling current; If the device operates in the Band-to-Band tunneling region (i.e. VG-VD= 15V, VD= -5V, VG= 10V), the Band-to-Band tunneling current dominates; If the device operates in the drain avalanche breakdown condition (i.e. VD = -7V and VG= 0V), the injection charge is drain avalanche induced hot electrons.

The drain leakage current at VG= 0V and VD in sub-breakdown region in MOSFETs is realized as Band-to-Band tunneling current from the gate-drain overlap region [12] [19]. But in flash EEPROMs, the thicker oxide makes the Band-to-Band tunneling current injection at zero gate voltage impossible. So the applied voltage must be adjusted, i.e., applying an appropriate gate voltage to achieve enough band bending. Furthermore, if we operate the device in the sub-breakdown region plus a small gate voltage (i.e. VD = -6V and VG= 6V), the composition of gate current is more complicated. The amount of drain breakdown (involves Zener breakdown and avalanche breakdown and avalanche breakdown dominates) induced carriers in sub-breakdown region is small but some of them may gain sufficient energy from vertical field. Besides, with the influence of vertical electric field caused by the positive gate voltage, the potential difference between gate and drain is large enough to induce more Band-To-Band Tunneling current. Thus, the injection charge will involve drain avalanche

induced hot electrons current and Band-To-Band Tunneling current.

It is hard to separate the gate current components in different operational conditions accurately by measurement only because the current component substantially changes as the change of the combination of the drain voltage and gate voltage. Fig. 4.13 shows the relationship of gate current and drain current in p-channel dummy cells. By varying the value of gate voltage, we can roughly separate the gate current components. The gate current difference between VG= 0V and VG= 9V at VD= 0V is F-N tunneling current. The triangle line (VG= 9V) rises as VD= 5.5V is due to Band-to-Band tunneling. The square line (VG= 0V) rises sharply at VD= 7V is drain avalanche breakdown current. The gate current of circle line (VG= 5V) rises at VD= 5V is initially Band-to-Band tunneling current. And then as the increasing of drain voltage (to ~6V), some of the avalanche breakdown induced electrons gain enough energy from vertical field and become a part of gate current.

Another simple method to determine the gate current components is shown in Fig. 4.14.

The potential difference of gate and drain is fixed as 12V (solid circle) and 7V (hollow circle) respectively. It represents the band bending is the same at the whole line so that the amount of injection carrier due to BBHE is the identical. It helps us to find the other gate current components with different combinations of gate/drain voltage. When the drain voltage is -2V, electron F-N tunneling current is the main component. At drain voltage equals to -6V, sub-DAHE generates, as previous description. At drain voltage equals to -7V, there is a lot carriers injecting due to drain avalanche breakdown. Besides, comparing the two spot at VD= -7V, we can see the small gate voltage (5V) enhance the injection of electrons by an order.

This is because the injection of Band-to-Band hot electrons and additional drain avalanche breakdown hot electrons.

Fig. 4.15 shows the transient programming characteristics of BBHE, DAHE and

self-convergent. So, in these 2 conditions the Drain Avalanche breakdown current is the main injection component. The injection component of BBHE is Band-To-Band Tunneling current.

The threshold voltage of sub-breakdown B is almost fixed. This is because both the band bending and drain avalanche is not large enough to generate apparent injected current. The measurement result conforms to our explanation.

In summary, the injected gate current under sub-breakdown region is drain avalanche induced hot electrons and Band-To-Band tunneling current, and from Fig. 4.15, drain avalanche induced hot electrons is the main component.

Now we investigate the influence of the drain forward bias in the above operational regions. Table 4.2 shows the operational condition of P/E schemes in this study. Fig. 4.16 shows the original and forward-bias-assisted characteristics of transient program under different operating conditions. From this measurement, we clearly identify the mechanism of the assistance of forward bias. At BBHE (circle line), there is no difference after assistance of forward bias, this is because the forward bias doesn’t influence the Band-to-Band tunneling current. The performance with forward bias shows the emphatically difference on DAHE (square line) and sub-breakdown region (triangle line). This is because the forward bias makes more impact ionization at the Drain-Substrate junction so that more energetic carriers surmount the oxide barrier and inject into floating gate. This result is consistent with our proposal at the second paragraph of this section.

Since the assistance of forward bias works in drain avalanche breakdown and sub-breakdown region, considering the reliability issue: drain disturb, the larger drain bias during programming causes more serious drain disturb, so we select the sub-breakdown region as our operational condition in this thesis..

0

Fig. 4.10 The (a) program setup, pattern mode and (b) injection mechanism of PASHEI in p-channel flash cells.

(a)

(b)

V _D (-)

V _G (+)

DAHE

BBHE

FN-PGM

Subbreakdown

Fig. 4.11 The combination of gate voltage and drain voltage of different programming schemes.

-8 -6 -4 -2 0 2 4 1E-14

1E-12 1E-10 1E-8 1E-6 1E-4 0.01 1

(II)

(I)

P-N Diode

Forward-Bias Breakdown

Breakdown

Zener Avalanche

I

_D

<0 I

_D

>0 D rain Cur rent, | I

D

| (A)

Drain Voltage, V

_D

(V)

Fig. 4.12 I-V characteristics of drain (p)-substrate (n) diode.

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 10

^-12

10

^-11

10

^-10

10

^-9

10

^-8

10

^-7

BBHE

BBHE+ DAHE DAHE

F-N Tunneling

V

_G

= 0V V

_G

= 5V V

G

= 9V

G ate Curr ent, I

G

(A)

Drain Voltage, V

_D

(V)

Fig. 4.13 The IG-VD characteristics with different gate voltage in p-channel dummy cells.

From this figure, we can roughly separate the gate current components by the difference of gate voltage.

-7 -6 -5 -4 -3 -2 10

^-12

10

^-11

10

^-10

10

^-9

10

^-8

10

^-7

DAHE

Fixing Band Bending V

_G

-V

_D

= 12V V

_G

-V

_D

= 7V

Gate Current, I

G

(A)

Drain Voltage, V

_D

(V)

DAHE

Sub-DAHE

F-N Tunneling

Fig. 4.14 The gate current components by fixing the potential difference between gate and drain terminal.

10

^-7

10

^-6

10

^-5

10

^-4

10

^-3

-5

-4 -3 -2 -1 0 1 2

3

BBHE (10/-5)

Subbreakdown A(6/-6) DAHE (0/-7)

Subbreakdown B(2/-6) Note: (V_G/V_D)

Thres hold Voltage, V

th

(V)

ProgramTime (sec)

Fig. 4.15 Transient program characteristics under different operational conditions.

Operate V

_G

V

_D

V

_B

V

_S

BBHE 11 -5 0 f

DAHE 0 -7 0 f

PGM

FBADHE 6 ^V

^D-High

⁼¹

V

_D-Low

=~ -6* 0 f

ERS Channel-FN -10 f 5 f

Read -3 -1 0 0

Table 4.2 Operational table of P/E schemes in this study.

10

^-7

10

^-6

10

^-5

10

^-4

10

^-3

10

^-2

-5

-4 -3 -2 -1 0 1 2

Triangle:V

_G

= 6 V

_D-Low

= -6 Square: V

_G

= 0 V

_D-Low

= -7

V_D-High= 0 V_D-High= 1

Thres hold Voltage, V

th

(V)

ProgramTime (sec)

V

_G

= 10 V

_D-Low

= -5

Fig. 4.16 Transient program characteristics with the assistance of drain forward bias under different operating conditions.

4.5 Study of FBADHE at Sub-breakdown Region in P- Channel Flash Cell

In this section, we will investigate the characteristics of Forward Bias Assisted Drain Hot Electron Injection at sub-breakdown region. The related symbols are shown in Fig. 4.10 (a).

First, we discuss the dependence of drain emitting voltage (drain forward bias, VD-High), as shown in Fig. 4.17 (a). The threshold voltage shift has a maximum at VD-High= 1V. It can be explained that the large forward bias (> 1V) will increase the momentum of diffusing carriers and reduce the effect of impact ionization. Fig. 4.17 (b) shows the impact of the drain collecting voltage (drain reverse bias VD-Low). As predicted, the larger drain bias makes the more threshold voltage shift. This is because the larger drain bias causes more impact ionization in the deep depletion region.

Fig. 4.18 shows the dependence of drain emitting time. We can find that there is no dependence of emitting time since the time to drive P-N diode in active area is very short (<1 μs). For convenience, we select emitting time =1μs on following measurement. The dependence of collecting time (pulse width) is shown in Fig. 4.19. This figure shows that the FBADHE is pulse width independent, because the same program time with different width shows the different result. The characteristic is different from other programming schemes, i.e., CHE, F-N and BBHE. Let me redraw this figure by replacing the X axis to programming pulse count, as shown in Fig. 4.20. We find that FBADHE is indeed pulse width dependent.

This characteristic is due to a large amount of carriers are generated by the path of Sub-drain avalanche breakdown and Band-to-Band tunneling in a very short time (<1μs), and increasing the programming width is useless to enhance the amount of generated charges.

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Threshold Voltage s hift, ∆ V

th

(V)

Drain Forward Bias, V

_D-High

(V)

Threshold Voltage shift, ∆ V

th

(V )

Drain Reverse Bias, V

_D-Low

(V)

Fig. 4.17 The relationship of drain (a) forward bias (VD-High) and (b) reverse bias (VD-Low) on FBADHE in p-channel flash cells.

10

^-1

10

⁰

10

¹

10

²

2.0

2.5 3.0 3.5 4.0 4.5 5.0

VG= 6V, VD-Low= -5.6V Colleting Width= 1us

T hres hold Voltage shift, ∆ V

th

(V )

Emitting Time (us)

Fig. 4.18 The relationship of drain emitting time on FBADHE in p-channel flash cells.

10

^-7

10

^-6

10

^-5

10

^-4

2

3 4

V

_G

= 6V V

_D-High

= 1V V

_D-Low

= -5.6V Delta Threshold Voltage, ∆ V

th

(V)

Program Time,(sec)

Fig. 4.19 The relationship of pulse count and threshold voltage shift in p-channel flash cells.

0 2 4 6 8 10 12 2.0

2.5 3.0 3.5 4.0 4.5

V

_G

= 6V V

_D-High

= 1V V

_D-Low

= -5.6V

Width= 20 µ s Width= 2 µ s Width= 0.1 µ s

Delta Threshold Voltage, ∆ V

th

(V)

Pulse Count

Fig. 4.20 The relationship of pulse count and threshold voltage shift in p-channel flash cells.

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